Understanding mechanical concepts

Orson1981

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 seperate 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

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

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

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

DaleSpam

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

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] 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

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.

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.)

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

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

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