Understanding mechanical concepts

[Claude Bile]

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.

 

[Maxwell's Beard]

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

 

[Orson1981]

I'm going to second the explanation 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.

 

[Skhandelwal]

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.

 

[Orson1981]

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.

 

[Skhandelwal]

You told me how it happens...proving by experiments and theories...but i was looking for why.

 

[Orson1981]

No, I told you why:
It takes more Energy to overcome an increase in inertia (mass).

As to why that is:
it is because inertia opposes any change in direction: Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

Or nothing moves on its own.

As to why that is:
When the universe cooled after the big bang and could finally poke our heads up this was the universe in place

As to why that is:
Well people are still working hard on this one, the laws of physics where codified long before (between 10^-43 s to 10^-35 s after the big bang) they came into exsistance due to initial conditions of this universe.

As to why this is:
God was arm wrestling his roommate Steve one Friday night...

 

[russ_watters]

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.

Technically speaking, the mass of the train matters and you can easily calculate the final speed of the train using the momentum equations. What you will find, however, is that the change in speed is so small you can ignore it. In other words, a person hit by a heavier train will not fly (measurably) further than one hit by a lighter train.

This is anothe great example of why the math is essential for understanding the concepts.

 

[Skhandelwal]

If a bullet goes through a person perfectly, he stands still, but if it bounces off him, he bounces too...why?

What is velocity? I understand that w/o velocity, a collision won't take place...but when 2 objects collide, why do they bounce off? Why don't they transform into 1 object speeding up the chemical reaction?

 

[TVP45]

If a bullet goes through a person perfectly, he stands still, but if it bounces off him, he bounces too...why?

This does not happen. Why do you think bullets bounce off people?

 

[Skhandelwal]

I am talking about negligable affect...the bullet does impose that kinda affect...it just isn't great enough to actually bounce the man off. Ex. fighting w/ a sharp sword vs. an iron rod.

 

[TVP45]

I am talking about negligable affect...the bullet does impose that kinda affect...it just isn't great enough to actually bounce the man off. Ex. fighting w/ a sharp sword vs. an iron rod.

Please explain. Can you give an example, perhaps with some measurements?

 

[Dale]

You told me how it happens...proving by experiments and theories...but i was looking for why.

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.

Hi Skhandelwal, TVP45 is 100% correct IMO. Essentially you are asking a "why" question. "Why" is generally the purview of priests, not scientists. The scientific method is a method for improving and refining testable predictions (hypotheses) about how the universe behaves. It fundamentally can not determine why the universe behaves that way.

If you do not want an explanation involving experiments and theories then you do not want a scientific explanation.

 

[russ_watters]

If a bullet goes through a person perfectly, he stands still, but if it bounces off him, he bounces too...why?

Again, the only way to really understand this is to apply math to the problem.

If the bullet goes through a person, it will impart some nonzero momentum on him. He won't stay still, but he will be affected less than if it bounces off him.

Lets say you have a 10g bullet traveling at 300m/s and it hits a person who weighs 50kg.

Case 1: The bullet goes through the person, losing half of its speed.
Case 2: The bullet bounces off the person at half the speed it hits him with.

Apply the momentum equation and find out what happens to the person!
[assume no momentum is lost to heat or permanent deformation of the person or bullet]

What is velocity? I understand that w/o velocity, a collision won't take place...but when 2 objects collide, why do they bounce off? Why don't they transform into 1 object speeding up the chemical reaction?

Velocity is just distance over time in vector form. What exactly happens in a collision depends on the properties of the materials.

 

[codec9]

I don't think I saw an explanation for KE apart from those of the form "since KE = 1/2mv^2, ..."

From the very basics, we know matter has inertia. This can proved (informally I guess) by considering a ball going up and down a V shape and one side gets progressively flatter. I won't explain all that though.

So to have a change in velocity, we posit a thing called force. The larger the change in velocity, the larger the force. In other words, F is proportional to a. There is no reason to think it is exactly a, so we tack a "coefficient of acceleration" on there called m, the object's mass. F = ma.

I don't know understand work as anything but the integral of Force applied over a distance. Maybe someone can explain further. But that's how we get KE.

We can't take the integral of F over distance yet because all we know is F = ma. But we can substitute a = dv/dt = d^2s/dt^2.

Integral (ma)dt = Integral (m d^2s/dt^2) dt

the dt's cancel...

Integral (m ds/dt)

which is back to velocity

Integral (mv)
= 1/2 mv^2

So at least you know where that comes from.

 

[PatPwnt]

If a bullet goes through a person perfectly, he stands still, but if it bounces off him, he bounces too...why?

What is velocity? I understand that w/o velocity, a collision won't take place...but when 2 objects collide, why do they bounce off? Why don't they transform into 1 object speeding up the chemical reaction?

Think of dropping a bowling ball on your bed. It bounces off because the blankets absorbs some energy and the spring will compress then expand throwing the ball back upward. This is because the ball has relatively large surface area in contact with the bed compared to say a sword. If you drop a sword tip down on the bed, blankets will rip because the sword has a very small area on its tip, so the blankets will not absorb much kinetic energy and the sword will keep going through.

A bullet is going to rip through your skin because your skin does not absorb most of the kinetic energy of bullet. If the bullet was nearly flat and larger but same mass and speed, more of its energy will be absorbed by the skin because the energy is not focused on one point.

If you landed on a trampoline from sky diving, you would rip right through it and splatter about 10 ft into the ground, ruining the trampoline. But if you jump onto the trampoline from about 10 ft up, you have less potential energy converted to kinetic energy and will be falling slower when you hit the trampline thus the fabric you land on has more time to absorb your momentum and then launch you back up. Same with you skin. The bullet is just going to damn fast to bounce it off. It's going to tear through your skin. Your skin's tensile strength is too small I guess you could say.

Velocity is a difference in position over a time period. When objects collide, they bounce off because... quit simply, they can't go through each other, indirectly due to a fundamental force such as the electromagnetic force.

 

[kaotak]

I disagree with people saying that physics only addresses the question of "how". I agree that on its most fundamental level, there is no answer to "why". But I think that why everything else behaves the way it does can be explained by citing certain postulates or empirical laws. These postulates or empirical laws are the fundamentals that cannot be explained themselves.

If a bullet goes through a person perfectly, he stands still, but if it bounces off him, he bounces too...why?

What is velocity? I understand that w/o velocity, a collision won't take place...but when 2 objects collide, why do they bounce off? Why don't they transform into 1 object speeding up the chemical reaction?

This is a good example. We do not know why momentum is conserved; it could work differently in a different universe. But we do know why the person bounces too, based on our knowledge of the conservation of momentum.

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

Thx.

As for your question about friction, I had a similar question. (Skip the first three paragraphs until the boldface.)

 

[Orson1981]

Codec9,

your right I did not put any further explanation of energy in my previous post.

My aim with that post was to phrase the question in a way that it could be intuitively understood without any background in mathematics, as I feel is the intention of this thread. In my experience tutoring I have found most students don't have an intuitive sense of momentum. Usually I find when first broaching a subject with them it helps to frame questions that way, then build upon that understanding later.

Talking about energy is a good intuitive tool, because even with a lack of correct understanding, people understand some things have more energy than other things.


kaotak,

I fully agree with you, this whole last week I have been thinking.

"Man I wish I would have worked more about electromagnetism into the post rather than just jumping to god."

Unfortainitly, I just don't feel like I know enough about it to comment on the topic with any type of authority.

 

[TVP45]

I disagree with people saying that physics only addresses the question of "how". I agree that on its most fundamental level, there is no answer to "why". But I think that why everything else behaves the way it does can be explained by citing certain postulates or empirical laws. These postulates or empirical laws are the fundamentals that cannot be explained themselves.



This is a good example. We do not know why momentum is conserved; it could work differently in a different universe. But we do know why the person bounces too, based on our knowledge of the conservation of momentum.



As for your question about friction, I had a similar question. (Skip the first three paragraphs until the boldface.)

This goes a long way toward explaining science's inability to address "why". The OP starts by postulating that bullets bounce off people and then asks "why..." Well, bullets don't bounce off people and people don't bounce off bullets. The OP was asked to cite an example and simply restated the general question.

I might as well post this question: If Posh Spice wears a red dress, why doesn't that violate Euclid's proof?

 

[kaotak]

The bullet example was misconstrued, but I was addressing elastic collisions in general. It's kind of pedantic to detract from my argument just because of this one example, when it can easily be seen that I could be talking about a tennis ball bouncing off a tennis racket or any other kind of elastic collision.

 

[Kilian]

to energy stored in an atom

it is stored on the bonds with the electrons releasing photons and jumping energy levels

 

[Ashwath]

You should also note that friction is a very complicated phenomenon. All our theories and explanations concerning it are mere derivations from the experimental world. And they involve significant approximation. People still do research on it.

 

[TVP45]

The bullet example was misconstrued, but I was addressing elastic collisions in general. It's kind of pedantic to detract from my argument just because of this one example, when it can easily be seen that I could be talking about a tennis ball bouncing off a tennis racket or any other kind of elastic collision.

If pedantic means insisting on following physical evidence from experiment rather than speculation, I plead guilty.