Understanding mechanical concepts

In summary, kinetic energy is a term used in physics to describe the change in an object's energy based on its mass and velocity. It is independent of direction and needs a frame of reference. Pressure is the force per unit area and is related to the kinetic energy and density of molecules. Potential energy is the energy an object has based on its position within a gravitational or electrical field. Spin can refer to a property of electrons or the rate at which an object is rotating. Friction is the resistance to movement between two surfaces. Understanding the physical meaning behind equations is important in fully comprehending concepts like pressure and potential energy.
  • #1
Skhandelwal
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3
Hello, I have merely been brainwashed by the formulas and math of AP Physics. Now, I want to actually understand what I was being taught.
I don't really want the answer to my question, but the questions that are in the brackets.

1. What is Kinetic Energy?(I know faster an object, the greater the KE but why? What is KE anyways? Why does it transfer when it collides to another object?)

2. What is pressure?(practically, is there a difference between force and pressure? Ex. that baseball came w/ a lot of force(or pressure).

3. What is potential Energy?(technically, where is it stored inside the atom-if it is in there?)

4. What is spin?(Lets say a ball is falling to the edge of the flat table, 40% of it makes the contact(btw, what would happen if 50% made the
contact?), so that 40% starts rebounding, but it can't, b/c it is attached to the other 60% part of the ball, therefore, it starts rolling in the air as
it is falling by its axis.(why is it spinning by its axis though? shouldn't it be the 40-60 split point?) Is this what spin really is?

5. What is Friction?(what is the cause of the surface being wielded, why isn't it naturally smooth?)

Thx.
 
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  • #2
Skhandelwal said:
1. What is Kinetic Energy?
KE is a physics term to describe how work (another physics term) done on an object results in a change in it's energy in terms of mass and velocity. Work equals force times distance (technically this is an integral of force(s)ds where s is the distanced moved along a line. It turns out that a force applied to an object of mass "m" over a distance will accelerate the object and results in a change in KE that is equal = 1/2 m v^2. Since velocity is squared, KE is independent of direction. The KE of a ball moving left is the same as the KE of a ball moving right at the same speed. Since KE is based on velocity, it needs a frame of reference (the frame of reference would usually have a velocity of zero).

2. What is pressure?
It's force per unit area, such as pounds (force) per square inch. It's related to the kinetic energy and density of molecules "bouncing" off each other.

3. What is potential energy?
A physics term to describe how an object's position within a gravitational or electrical field could be converted into kinetic energy if the object was allowed to move freely. For example, the height of an object above the ground times the force of gravity on the object is it's potential energy, relative to the ground. Using "g" to mean the rate of acceleration of gravity (about 9.8 m/s^2), "m" for mass, and "y" for height, the potential energy = m x g x y.

4. What is spin?
In physics, normally "spin" refers to a property of electrons, but it can also mean angular velocity, the rate at which an object is rotating.

5. What is Friction?
The resistance to movement between two surfaces. In real life, surfaces are rarely "smooth". Even crystals will have valleys and peak at the molecular level.
 
  • #3
Jeff Reid said:
In physics, normally "spin" refers to a property of electrons, but it can also mean angular velocity, the rate at which an object is rotating.

Spin is not a phenomenon solely attributed to electrons. The answer to "what is spin" depends upon the context of the question, as some people talk about classical spin as opposed to the more usual use of the word-- quantum mechanical spin.
 
  • #4
Hmm... the term "energy" denotes a measurable physical quantity which is conversed. (Read Feynman chapter 3 for more... beautifully given).

In mechanics, force and energy are two ways of giving the same concept, in a way. Potential energy is present whenever a body is present in a field of force. The force tries to pull it , if it is at rest, it has a "potential" to move. That is what Potential energy is. If you leave the body, as it moves along the force, it gains velocity. Take a gravitational field. If i leave the ball from a height h 10 meters, it will have a greater velocity v at the ground than if I leave it from h = 5 meters. This is found to obey the law m*g*h = 0.5*m*v^2 .

So, we say that mgh is the potential for it to gain velocity, and since the sum 0.5*m*v^2 - m*g*h is always the same, we say that this sum is the energy of the system. At the top, it has v = 0, so all energy is potential. The the ground, all energy is kinetic, since h=0. At all points along the path, the sum of these two remains constant.

We call the term which is due to the "poential" to gain energy in a field of force as potential energy, and the term which is due to the motion (velocity) as kinetic energy.

What I have said is trivial, I know, and Jeff has already given a nice explanation. :)
 
  • #5
This is so funny...nobody really read my question...guys, I really appreciate the help you are giving but if you could read the whole post, you could have really saved a lot of your time. I made it clear in the beginning that I don't want the answer to the first question after the numbers, but the answer to the questions in the bracket. Thx.
 
  • #6
The problem is that the equations are what describe what is happening, which makes what you are asking nonsensical. What you need is not to discard the equations, but to understand what they mean physically.

The definition of pressure is a very simple example of this. You asked how it is different from force. Look at the equation: f/a is force spread out over an area.
 
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  • #7
Skhandelwal said:
1. What is Kinetic Energy?(I know faster an object, the greater the KE but why? What is KE anyways? Why does it transfer when it collides to another object?)

Consider an object, e.g. a car.
I suppose you agree with me, that when the car is moving it will have more energy than when it's standing still, and the faster it's going the more energy it will have. The reason that we have speed limits, is that crashing your car into a wall (or another car, for that matter) at 50 km/h is not nice, but still better than crashing the same car into the same object at 120 km/h.
The kinetic energy is a measure of this energy, which an object has (additionally, compared to the same object standing still) simply because it's moving. If an object is not moving, it has no kinetic energy. The faster it is moving, the more kinetic energy it has. Just like, the more you move an object against a potential, the more potential energy it will have.
This is about as basic as it gets. If you are still not satisfied with the answers given, you will probably not be satisfied until somebody gives you a definition of energy.

If our object hits another object, it will slow down, hence it loses kinetic energy. The other object will absorb this energy. For example, hitting our car into a brick wall will cause the brick wall to absorb the energy (for example, its temperature is increased a little - though the effect will not be noticeable even when you use a truck). When the car collides with another car, the energy absorbed by the other car can be in the form of kinetic energy - which means that this car will start moving as well.

I don't know if you've ever heard of http://en.wikipedia.org/wiki/Newton's_cradle]Newton[/PLAIN] [Broken] balls (no, it's no a new sex toy). Here the effect is very clear: we start one ball moving, and when it hits the rest of the balls, the energy is transferred all the way to the last one in line. There it is converted to kinetic energy and the ball will start moving. (as it's moving against gravity, finally all the energy will be converted to potential energy, and the ball will come to a stop, and when gravity is still pulling on it, this potential energy will again get converted to kinetic energy when the ball starts to move back). In an ideal situation, this process would go on forever (though with each tick a little energy is lost in practice, due to friction).
 
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  • #8
I have actually seen them in action! However, I am not satisfied. I need to know WHY an object at higher speed has a greater KE.
 
  • #9
Skhandelwal said:
I have actually seen them in action! However, I am not satisfied. I need to know WHY an object at higher speed has a greater KE.

Kinetic energy is the energy attributed to a particle's motion, so by definition, a body moving quickly has more kinetic energy than that same body moving slowly.

Note that, as physicists, we are interested in how things work, not why.
 
  • #10
I've always been interested in why things work the way they do.
 
  • #11
Because the formula for kinetic energy is [tex]\frac12 m v^2[/itex], so if v increases, the KE increases.
Also see the derivation of that formula.
 
  • #12
It seems to be the other way around for me. I haven't a clue from formulae, but seem to have a 'feel' for how things work. Math would be a nice addition to my tool kit.
 
  • #13
Basically, I think you should read chapter 3 from Feynman vol 1. He clarifies the concept of energy very nicely. It is basically a quantity that is observed to be conserved in nature.

Let us consider just KE. I do supply some energy to an object by doing work on it. That is, I act upon it by a force for a certain distance. Now if I let it go, and it strikes a piston (with due apologies to thermodynamics), the piston will move backwards from the impact (and suppose the object then comes to rest). How did the piston now acquire velocity? Apparantly the object gave its velocity to the piston. But did it entirely? No. If the piston is heavier than the object, it'll have a lesser velocity. If one does the actual experiment, one can see that the product m*v^2 is stays constant.
Apparantly the object in motion had a quantity associated with it, due to which it could make the piston move backwards. Now the object is at rest: it has lost this quantity. But the piston can now make something else go, because IT is in motion. Since the quantity m*v^2 decided the fate of the collision, we give it a name: Kinetic energy. It is a property which an object has due to it's motion. Basically, it is constant in collisions because we searched for a quantity that IS constant in collisions, and gave it the name KE.

Now for Potential energy. You ask where it is stored. It isn;t, in fact, stored, but it is also a property of the object, just like KE. Now, the work energy theorem says that the product of the force applied on the system, multiplied by the distance for which it acts, gives the change in the KE for the system. (See the link in Compuchip's post). Suppose I place a body in a gravitational field, at height h. If I leave it, it will gain velocity, because the gravitational force acts on it for a certain distance. This will cause it to gain kinetic energy. Isn't kinetic energy supposed to be conserved? Well, only if no external force acts on the system (like two colliding particles in space). If a force field IS present, the body presumably has another quantity, dependent on it's position in the field, which it can convert to kinetic energy. The total energy then remains constant. We'll call this position dependent quantity "Potential energy". Note that it is not stated where the energy is stored: it is a property of the body. It need not be stored anywhere, it's a property. Like the volume of a body. Where is the volume stored? It manifests itself. (In EM, energy can be taken to be stored in the fields too).

Pressure, well, measures how "intense" the force is. For example, if I apply a force of 10 N uniformly over an object of area A, it will not break. But if I apply it at a point, or a much lesser area, I may cause a hole. The force was the same in both instances; the pressures were different.

Hope I answered some of your bracketed questions. :)
 
  • #14
lol thx. but you seem to answer one thing w/ another question like everyone else. The problem is, you said that the reason something moving has Kinetic Energy is b/c if it doesn't, it shouldn't be moving by a force, However, why does something moves when a force is applied? Why can't something else happen(like a chemical reaction and they mix together, IDK, I am making up stuff here)

Alright I get Potential Energy, personally, it is the easiest concept to understand but the name misled me. However, it arose another question when I understood PE, Why is one thing attracted to another? How does Force(attractive) exist? Let's say one proton is attracted to an electron due to Coulomb's law...why? Besides more math..grrr, what is Coulomb's law conceptually?(its funny that to understand heart of one part of physics, I have to learn hearts of others kind of physics too)
 
  • #15
You have to go to the basic definitin of force for that one. (Newton's 2nd law). A particle can move with constant velocity, but if it is moving with an acceleration (in an intertial frame), that means it is interacting with some object. Basically, all changes in motion are due to interaction. Motion being that along a straight line with constant velocity. Now, the rate of change of velocity at an instant is proportional to the force acting on the body at that instant, and inversely proportional to the amount of matter in a body.

So, basically, forces cause accelerations. If there is no accleration, there is no net force. That is what Newton postulated, and it has been confirmed by experiment. In classical physics, you have no explanation for "why" forces are the way they are... they are simply deductions from experiments.
 
  • #16
I'm going way outside the bounds of normal PF protocol here, but I think that it might be the proper approach in this situation. If that is in error, please (Moderators) feel free to delete it.
Skhandelwal, try looking at the kinetic energy thing this way (as I do).
Say some nasty dude confronts you in an alley. You can clock him with a concrete block, that weighs maybe 10 kgs, with a pretty slow swing. Or, you can throw a 1 kg brick at him at maybe 25 ft/sec. Or, you can throw a 240gr chunk of lead at him with a muzzle velocity of 850 ft/sec. Whichever one you choose, he's going to stop bothering you.
 
  • #17
Skhandelwal said:
lol thx. but you seem to answer one thing w/ another question like everyone else. The problem is, you said that the reason something moving has Kinetic Energy is b/c if it doesn't, it shouldn't be moving by a force, However, why does something moves when a force is applied? Why can't something else happen(like a chemical reaction and they mix together, IDK, I am making up stuff here)

Alright I get Potential Energy, personally, it is the easiest concept to understand but the name misled me. However, it arose another question when I understood PE, Why is one thing attracted to another? How does Force(attractive) exist? Let's say one proton is attracted to an electron due to Coulomb's law...why? Besides more math..grrr, what is Coulomb's law conceptually?(its funny that to understand heart of one part of physics, I have to learn hearts of others kind of physics too)
We don't know why in most cases, we only know how and that it does.

It should be pointed out that momentum and kinetic energy go together.

In some cases kinetic energy can be converted to internal energy (inelastic collision, where kinetic energy is not conserved, but total energy is).
http://hyperphysics.phy-astr.gsu.edu/hbase/elacol.html
A perfectly elastic collision is defined as one in which there is no loss of kinetic energy in the collision. An inelastic collision is one in which part of the kinetic energy is changed to some other form of energy in the collision.

A force will cause an object to move if the object is not subjected to an opposing force. If one unopposed (unbalanced) force acts on an object, it will accelerate. If there is an equal and opposed force (couple), the object will experience a compressive force if the forces act inward, or a tensile force if the force acts outward.

Friction is an example of a shear force. Shear forces arise where equally opposed forces operate off-center.

One has to consider static (equally balanced/opposed) forces and dynamic forces (which result in motion).
 
  • #18
Danger said:
I'm going way outside the bounds of normal PF protocol here, but I think that it might be the proper approach in this situation. If that is in error, please (Moderators) feel free to delete it.
Skhandelwal, try looking at the kinetic energy thing this way (as I do).
Say some nasty dude confronts you in an alley. You can clock him with a concrete block, that weighs maybe 10 kgs, with a pretty slow swing. Or, you can throw a 1 kg brick at him at maybe 25 ft/sec. Or, you can throw a 240gr chunk of lead at him with a muzzle velocity of 850 ft/sec. Whichever one you choose, he's going to stop bothering you.
Ah, the brute force method! It is often applied in solving an intractable problem in engineering or physics. :biggrin:
 
  • #19
Skhandelwal said:
why does something moves when a force is applied? Why can't something else happen(like a chemical reaction and they mix together, IDK, I am making up stuff here)
Other things can happen. If an object isn't strong enough, you could crush it, for example.
 
  • #20
Astronuc said:
Ah, the brute force method! It is often applied in solving an intractable problem in engineering or physics. :biggrin:

:redface: It isn't an accepted teaching practice, I'm sure, but I hope that it conforms to what I percieve as the PF mandate... to help people learn by whatever means seem appropriate. In my everyday life, I find it a lot easier to explain things in 'street terms' that people without formal education (like me) can understand. I know that it wasn't a Politically Correct post, but you know how I feel about that ****...
(Incidentally, I didn't use the short 'PC' for that because up here it means the Progressive Conservative political party. :yuck:)
 
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  • #21
But why bounce? And a question about spin.

Here is what I don't get, when 2 objects collide together, why do they bounce off, instead, why don't they destroy each other.(stronger one survives w/ some damage may be) This leads to another question, what is elasticity. When there is collision, things rebound b/c of their elasticity, but what is stretching anyways?

Another question, if a ball hits the edge of the table as it is falling down(40% of it), what would happen w/ friction and w/o friction?(why won't the ball rebound at 180 degrees?) (Would the ball spin if there was no friction?)
 
  • #22
Skhandelwal said:
Here is what I don't get, when 2 objects collide together, why do they bounce off, instead, why don't they destroy each other.(stronger one survives w/ some damage may be)
Sometimes they do destoy each other. That's what a car accident is. It all depends on the properties of the objects: strength, elasticity, etc.
This leads to another question, what is elasticity. When there is collision, things rebound b/c of their elasticity, but what is stretching anyways?
When things collide and deform elastically, the atoms are litterally pushed closer together.
Another question, if a ball hits the edge of the table as it is falling down(40% of it), what would happen w/ friction and w/o friction?(why won't the ball rebound at 180 degrees?) (Would the ball spin if there was no friction?)
If a force is applied to a spherical object in a direction that does not pass through the center, the object will spin even if there is no friction. The deformation is part of the reason why.
 
  • #23
Skhandelwal said:
Here is what I don't get, when 2 objects collide together, why do they bounce off, instead, why don't they destroy each other
That totally depends upon the properties of the objects. On the macro scale that we all live with every day, similar objects do destroy each other; look at highway accident statistics.
On the bottom end of that, electron degenerancy (sp?) pressure keeps any two atoms from actually contacting each other unless there are very high energies involved.
In between those situations, there are various degrees of interaction between two bodies. (Yeah, I know... that opened up a whole ****load of sexually-based responses.)

edit: Hi, Russ. You sneaked in on me again.

Skhandelwal said:
Another question, if a ball hits the edge of the table as it is falling down(40% of it), what would happen w/ friction and w/o friction?(why won't the ball rebound at 180 degrees?) (Would the ball spin if there was no friction?)
The ball will not deflect at 90* because of the 'angle of incidence' of the impact. Some part of that ball which is at less than its diameter has to hit the intervening body.
 
  • #24
You still didn't tell me why though Danger about the ball...you said that basically happen b/c of this law...but you didn't tell me how is that law correct.(tell me theoretically, not mathematically)

Another question, why does it take more Energy to accelerate an object of a greater mass?
 
  • #25
Skhandelwal said:
Another question, why does it take more Energy to accelerate an object of a greater mass?

Because that is what mass is: it measures the inertia of an object to motion, just as the moment of inertia I measures the inertia of rotation. So the bigger the mass, the harder it is to get it moving.
In physical terms; if the mass increases, you have to apply a greater force to get the same acceleration. Or, put differently, you have to do more work ([itex]F \cdot s[/itex]) to get the same end velocity after a certain distance.
In mathematical terms: [itex]F = m a, W = F s[/itex].

I consider that a physical argument, though it's also just interpreting the mathematics :smile: -- depends on how you look at it.
 
  • #26
Skhandelwal said:
You still didn't tell me why though Danger about the ball...you said that basically happen b/c of this law...but you didn't tell me how is that law correct.(tell me theoretically, not mathematically)

Hmmm... I'm not sure if this is a proper explanation or not, but it's the way that I look at it. For the deflection component, you can think of the ball's radius as a modified inclined plane, like the lobe on a cam. When it hits the edge of the table, it sort of rides along it and gets pushed sideways. I'm ignoring the elasticity for this example; in reality there's 'bounce' involved as well.
As for the spin, there wouldn't be any without friction. With friction, the part that hits the table get decelerated while the rest (including the centre of mass) doesn't. That starts the ball 'rolling' around the contact point.
Again, this is not by any means an acceptable explanation in scientific terms, and ignores some other factors. It's just the most basic way that I can think of to say it.
 
  • #27
About the mass acceleration, you didn't answer why. Btw, I don't like interpreting at things by math pov.(Why is it considered inertia of an object to motion?)

That explanation about the ball is kinda confusing b/c someone earlier said that the ball will spin regardless of friction, and now you are telling it wont, I understand you answer why for this explanation about why it will not spin but could you tell me why the part that hits the table get decelerated?(I think that was a stupid question so sorry for asking) And why would the ball slide across the edge and fall? I mean isn't Coulomb force stronger than gravity?(isn't the ball attracted to the table more than ground?)

btw, since there are only 4 fundamental force, whose part is Coulomb force?)
 
  • #28
Skhandelwal said:
About the mass acceleration, you didn't answer why.
I didn't?

Skhandelwal said:
(Why is it considered inertia of an object to motion?)
You can take that as the definition of mass. Anyway, Newton's second law gives the answer to your question. I cannot help it.
You should perhaps read this and this article at Wikipedia, and then state your objections.
Basically, you're asking why a fundamental quantity is what it is :smile: I'm sorry if that is unsatisfying to you. All I can try to do, is try to justify and possibly give an alternative POV, but in the end - you'll have to do with it.
 
  • #29
Skhandelwal said:
That explanation about the ball is kinda confusing b/c someone earlier said that the ball will spin regardless of friction, and now you are telling it wont

I did specify that I was omitting some factors, such as elasticity. As Russ pointed out, deformation is a large contributor to the spin. Without elasticity, there is no deformation. Sorry about the confusion.
 
  • #30
Physics is the way we describe what we measure (and what we expect to measure) in nature. It tells us nothing at all about why nature is the way it is. Quantities such as kinetic energy are useful measures that we have invented to assist our conversation; we accept them as such and live with our limitations.
 
  • #31
I realize that discussion as progressed beyond the original post, but I'll put my 2c in anyway.

1 & 3. Let's look at KE and PE together. Energy is defined as "the ability to do work". Where is energy "stored"? Well this is where the distinction between different "types" of energy such as KE and PE come in. KE is "stored" as motion, as long as a particle keeps moving, it will retain its KE. Gravitational PE is "stored" as position, as long as a particle doesn't move (more correctly, doesn't move in the direction of the field), it will keep its PE.

One can therefore generally make the statement that the energy of an object is "stored" in it's state, because the state of the particle will determine the ability of the particle to do work.

Why do faster objects have greater KE? Because one needs to do work on an object to speed it up. Since energy is conserved, that work must go into increasing the energy of the object.

Why does KE transfer to another object in a collision? Because the object does work on other objects during the collision, since two objects coming into contact with one another apply forces and work = force x distance.

Onto 2. This has already been answered, but I'll repeat it - Pressure is force per unit area. You can apply much more pressure to a surface with a needle that you can with a cricket bat with the same amount of force, because the needle applies the force over a much smaller area. That's why pointy things are dangerous, and why weapons throughout history tend to be pointy and therefore dangerous.

4. In this scenario, half of the ball wants to go down and the other half wants to go up - but they can't because an object has interatomic forces binding the chunk of matter together. These interatomic forces act to pull the atoms inward, and like any scenario with a force that acts radially inward, it causes the system to rotate.

Objects rotate about their centre of mass because this is the axis of rotation where inertia is minimised. In other words, it is easiest to rotate an object about its centre of mass, therefore without external intervention, that is the axis about which the body will rotate.

5. Friction is a shear-force (i.e. a force that acts parallel to a surface or surfaces) that is a result of electromagnetic interactions between two surfaces. Essentially the outside atoms of each surface cause the outside atoms of the other surface to wiggle around. This causes the surfaces to "couple", in other words, the surfaces no longer move independently to one another, the motion of one surface will induce motion in the other surface. This is what gives surfaces their "stickyness" and why we don't normally see objects sliding all over the place.

Normally we associate friction with retarding motion, but this is a misconception of sorts. Friction can act to speed up a body, indeed if there were no friction we would find it difficult to move around at all. Friction is, however, a dissipative mechanism that removes energy from the system. The use of friction to speed up an object, (or keeping two surfaces moving at different relative speeds with friction present) can only be accomplished if work is imparted into the system, and indeed this is why the world around us gives the illusion that force is required to keep a body in motion.

Claude.
 
  • #32
Skhandelwal said:
1. What is Kinetic Energy?(I know faster an object, the greater the KE but why? What is KE anyways? Why does it transfer when it collides to another object?)

A faster object has more kinetic energy than a slower object of the same mass due to more momentum, that is mass times velocity.

2. What is pressure?(practically, is there a difference between force and pressure? Ex. that baseball came w/ a lot of force(or pressure).

Pressure is force per unit area, so yes there is a practical difference. You have a force of 1 Newton acting on a surface of 1 square centimeter, or one square meter. In each case you have 1 Newton, but the pressures are different by 10,000. So the total force acting on an object would be F = p x A, pressure times area.

3. What is potential Energy?(technically, where is it stored inside the atom-if it is in there?)

Don't think of it as stored in the matter, rather think of it as stored in the field within which the object has a potential. Imagine the field (gravitational, electric, magnetic) as like a spring which can stretch or compress. A bowling ball elevated and held in position has a potential energy, because work was done against gravity to get it there, and the gravitational spring is stretched. Same with charges and magnetic poles. Two north poles fly apart when released because work was done against their repulsive fields to get them there, and the magnetic spring is compressed.

4. What is spin?(Lets say a ball is falling to the edge of the flat table, 40% of it makes the contact(btw, what would happen if 50% made the
contact?), so that 40% starts rebounding, but it can't, b/c it is attached to the other 60% part of the ball, therefore, it starts rolling in the air as
it is falling by its axis.(why is it spinning by its axis though? shouldn't it be the 40-60 split point?) Is this what spin really is?

Because all the particles of the ball are tied to all the other particles, they all pull on each other. The forces acting on the ball at the point of impact try to change the direction of the particles involved, but since those particles are being tugged on by the rest of the ball, the direction of the forces change, and they tend to find a balancing point. In symmetrical objects like spheres, that is usually along an axis.

5. What is Friction?(what is the cause of the surface being wielded, why isn't it naturally smooth?)

Thx.

Objects aren't naturally smooth because lots of different things have occurred during the lifetime of the object in question. Different forces have acted to cause the surface to have differences in elevation. Because solid matter cannot pass through other solid matter, friction is where many microscopic mountains are trying to slide past one another
 
  • #33
I'm going to second the explination of Claude Bile, that was really well written. Only to add, Skhandelwal, trust me, it takes awhile, but the longer you keep with physics, the clearer these ideas become.

Understanding Physics is an evolving practice, if you look at these questions two years from now you will be amazed at how your understanding has deepend. But what is great is, you can continue to come back to this time and time agian (after your batchlors, during grad school, doing your post-doc) and see how your views have grown and changed over time.

Never quit asking questions, just don't give up or turn away because you didn't grasp it in a day.
 
  • #34
What is momentum? I know it is mass times velocity but I can't seem to figure out why a person hit by a train goes farther away than a narrower train in width? I mean on the part that hits the person should matter. Right? If we add another box by the end of train to increase it's weight, I don't how would affect the impact since I don't the vibration will travel down the end the much.

Btw, don't take this technically, pretend when 2 boxes of train are connected, they turn into 1 box which is twice as long to keep things simple.
 
  • #35
What is momentum? I know it is mass times velocity but I can't seem to figure out why a person hit by a train goes farther away than a narrower train in width? I mean on the part that hits the person should matter. Right? If we add another box by the end of train to increase it's weight, I don't how would affect the impact since I don't the vibration will travel down the end the much.

Btw, don't take this technically, pretend when 2 boxes of train are connected, they turn into 1 box which is twice as long to keep things simple.

While you could look at it that way, it is not the way I would view the problem, I think a better understand would come from interperting the energy of the train. In order to overcome inertia (mass) it would take twice as much kinetic energy for the two box train (or twice width train) to reach the same speed (or velocity, whatever you want) as the one box train.

Viewed simply in the language of mathamatics as

KE = (1/2)*m*v^2

so when we double the mass we double the amount of energy it takes to reach the same speed.

This is also something we can view very simply in our everyday world. A simple thought experiment that comes to mind is comparing how hard it is to stop a person running at 5mph compared to how hard it is to stop a car at 5 mph. I'm sure you are a bright kid and can devise some simple at home experiments (that safely) verify that it is harder to stop something that is heavier than it is to stop something lighter when they are both traveling at the same speed.

As for why a train carrying an extra boxcar or two on the end would effect you when you are hit in the front of the train, remember that all the energy to push those cars is generated in the front at the engine. However even if it was generated in the back of the train, the energy to push the train remains the same throughout the whole system.


As for you comment about width of the train making and impact because of the area that is struck narrows:

This deals with pressure, which is a totally separate beast... ok, not totally different, but I think it probably is not exactly what you are looking to discuss, if I am wrong we can go into pressure as well.
 

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