Physics Game

[Duki]

Ok, so here are the rules:

-Keep the questions very basic (PHYS 101 & 102); this is, afterall, a computer forum.
-Wait for confirmation that your answer is correct before posting your question.
-If 24 hours passes and still no confirmation, post a new question to keep the game going.


** Please follow the rules! **

I'll start:

Question:

A car (mass = 1100kg) is traveling at 32 m/s when it collides head-on with a SUV (mass = 2500kg) traveling in the opposite direction. In the collision, the two vehicles come to a halt. At what speed was the SUV traveling?

[Rashakil Fol]

That depends: did the passengers come to a halt too, or did one or more of them fly out the windshield?

[christina>you]

That depends: did the passengers come to a halt too, or did one or more of them fly out the windshield?

Does that even matter?

[joshSCH]

haha no..

[christina>you]

lol ok, just checking...

[joshSCH]

The answer, of course, is 14.08 m/s in the opposite direction of the first car.

[joshSCH]

Isn't that right? It better be.. I'm taking the Physics 2 "Mechanics w/ Calculus" AP test next month..

[christina>you]

You're right... I'm talking to Duki on the phone and he said you are correct.

[joshSCH]

yay! My question is:

The position vector of a particle of mass 3kg is given by r(vector)= 4i+3t^2j, where r is in meters and t is in seconds. Determine the angular momentum and torque acting on the particle about the origin.

[Infarction]

Does that even matter?

If you were worried about details, it would matter. The passengers were part of the energy system, and stopping them would require energy as well. If passengers from one vehicle were to continue flying through the windshield, it would change the amount of energy involved in the collision, and would change the result a bit.

[joshSCH]

Yes, but the change you are talking about is irrelevant. It would not change the SUV's velocity by much.

[Infarction]

Hm... I got 21.22m/s... then again, I haven't done physics in a long time and may have mis-remembered something along the way... (that was for the first one btw)

[Rashakil Fol]

Infarction: we know the momenta of the cars sum to zero because the combined mass has no velocity. So 32 * 1100 = v * 2500. Then 35200 / 2500 = v, i.e. 352 / 25, which is 14 + 2/25 i.e. 14.08.

[joshSCH]

Hm... I got 21.22m/s... then again, I haven't done physics in a long time and may have mis-remembered something along the way... (that was for the first one btw)

You use conservation of energy.. the fact that the cars will stick after the collision. Therefore you know that the collision is perfectly inelastic, so the maximum kinetic energy was lost. So, m1*V1 + m2*V2 = (m1+m2)*V

You solve the equation 1100kg * 32 m/s + 2500kg * X = [(1100+2500)*0]kg m/s

therefore x=-14.08 m/s.. which means the SUV had a velocity of 14.08 m/s in the opposite direction of car 1.

[joshSCH]

Or that way works too..

[Rashakil Fol]

Your explanation does not use the principle of conservation of energy.

[Infarction]

But isn't kinetic energy 1/2 mv^2?

Maybe this is why I did so bad in physics years ago... :sad:

[Rashakil Fol]

No, you're right Infarction.

[joshSCH]

Your explanation does not use the principle of conservation of energy.

Um.. actually it does. I was referring to the law of conservation of energy.. Kinetic energy is not conserved in inelastic collisions, therefore you know that the maximum kinetic energy is lost. However, The law of conservation of energy is true for the system. The energy is transferred from kinetic energy to different kinds of energy (sound, heat, etc.)

[joshSCH]

But isn't kinetic energy 1/2 mv^2?

Maybe this is why I did so bad in physics years ago... :sad:

This problem does not require you to find the kinetic energy of anything. You are simply looking for the velocity. Since it is an inelastic collision, all the kinetic energy is lost. So if you found the initial kinetic energy and the final kinetic energy (1/2mv^2), you would find the total change in kinetic energy to be (-initial kinetic energy) b/c the final kinetic energy is 0...