Understanding mechanical concepts

1. Sep 15, 2007

Skhandelwal

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.

Last edited: Sep 15, 2007
2. Sep 15, 2007

rcgldr

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).

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.

A physics term to describe how an object's position within a gravitational or eletrical 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.

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.

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. Sep 15, 2007

cristo

Staff Emeritus
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. Sep 15, 2007

rahuldandekar

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. Sep 15, 2007

Skhandelwal

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. Sep 15, 2007

Staff: Mentor

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.

Last edited: Sep 15, 2007
7. Sep 15, 2007

CompuChip

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).

Last edited by a moderator: May 3, 2017
8. Sep 15, 2007

Skhandelwal

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. Sep 15, 2007

cristo

Staff Emeritus
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. Sep 15, 2007

rewebster

I've always been interested in why things work the way they do.

11. Sep 15, 2007

CompuChip

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. Sep 15, 2007

Danger

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. Sep 15, 2007

rahuldandekar

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. Sep 15, 2007

Skhandelwal

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? Lets 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. Sep 15, 2007

rahuldandekar

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. Sep 15, 2007

Danger

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. Sep 15, 2007

Astronuc

Staff Emeritus
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 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. Sep 15, 2007

Astronuc

Staff Emeritus
Ah, the brute force method! It is often applied in solving an intractable problem in engineering or physics.

19. Sep 15, 2007

Staff: Mentor

Other things can happen. If an object isn't strong enough, you could crush it, for example.

20. Sep 15, 2007

Danger

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:)

Last edited: Sep 15, 2007
21. Sep 15, 2007

Skhandelwal

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. Sep 15, 2007

Staff: Mentor

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.

23. Sep 15, 2007

Danger

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.

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. Sep 17, 2007

Skhandelwal

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. Sep 17, 2007

CompuChip

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 ($F \cdot s$) to get the same end velocity after a certain distance.
In mathematical terms: $F = m a, W = F s$.

I consider that a physical argument, though it's also just interpreting the mathematics -- depends on how you look at it.

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