# Momentun And Collisions

#### sb

1. An experiment is performd in a physics laboratory to find the mass of a stationary particle, B. A proton, A, of mass 1.7 x 10^-27 kg, travelling at 4.0 x 10^6 m/s, strike B and bounces straight back at a speed of 2.0 x 10^6 m/s. If B moves ahead at 1.0 x 10^6 m/s. calculate B's mass

2. Two, studends, C and D, are facing each other on "friction-less" roller skates. C has a mass of 80kg and D has a mass of 50kg. Now they push each other and D acquires a velocity of 4.4m/s [W]. What velocity does C acquire?

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#### arcnets

Hi sb,
could you please tell us what you have got so far, and where you are stuck.

#### sb

ummmm i am very very very confused. Plz kindly show me how u wud go about solving this question. It wud be very helpful and appreciated and i will be very thankful.

#### Tom Mattson

Staff Emeritus
Gold Member
Originally posted by sb
Plz kindly show me how u wud go about solving this question.
Sorry, but that is not the policy of this Forum. I placed a note at the top of the Homework Help Forum called Read This Before Posting. Please do read it.

It wud be very helpful and appreciated and i will be very thankful.
Actually, it would most likely not be helpful to you at all. If we simply show you how to do the problem, then you really aren't learning anything.

You should have learned about the law of conservation of momentum in your class. Come on, give these a shot. At least try to set them up. They really are very easy problems.

#### sb

atlest give me a clue sir...

#### HallsofIvy

Homework Helper
Okay, here's clue: look up the definition of "momentum" and the law of "conservation of momentum" in your text book

Calculate the momentum of each particle, before and after the collision and post them here. Then we will be able to help you further.

#### Doc

Unless this guy has posted in another thread, I can't say he is even using a textbook (even though it is in homework help). We seem to assume all of the time that these people are students. He might not be. I post here, I'm not a student. Hell, someday I might even post a question!

#### Tom Mattson

Staff Emeritus
Gold Member
Originally posted by Doc
Unless this guy has posted in another thread, I can't say he is even using a textbook (even though it is in homework help).
OK, then he can look up "conservation of momentum" up in HyperPhysics, one of the many helpful resources I posted at the top of the Homework Help Forum.

Hell, someday I might even post a question!
OK, but if you post it in my Forum, then you had better show your work first.

#### sb

One kg of fat is equivalent to about 30 MJ of energy. The efficiency of converting fat to mechanical energu is about 20%.
a. Suppose you lift a mass of 12kg 2.0m vertically, 500 times how much work do you do? (Assume that the work done by mass on you is disepated as heat to the surroundings).
b. If asll the enery used to do the work comes from "burning" fat, how much fat is used up by the expercise

This is what I did:
1kg = 30MJ
percentage efficinecy = 20%
m = 12kg
delta d= 2.0m
w=?

1kg = 1,000,000J
12kg = 12,000,000J
E = 12,000,000J
IMA = 12,000,000J

potencial efficiency = (AMA/IMA) * 100%
20% = (AMA/12,000,000) * 100
AMA = (20/100) * 12,0000
AMA = 24000J

FBD Diagam

Eg = mgh
Eg = (12kg)(9.8m/s^2 [D])(2.0)
Eg = 235.2N

Answers for this problem given at the end of the book are
a. 1.2x10^2 kJ
b. 20g

#### sb

sorry about that message above, clicked the wrong button, thanks a lot for all your help

#### Doc

Originally posted by Tom

OK, but if you post it in my Forum, then you had better show your work first.
That's the thing though. I suspect that MOST of the people that could answer the type of question I would ask hang out here. That includes YOU Tom. The type of question I would ask would most likely concern the type of problem where I couldn't show any work at all. That would be my point in asking. I agree with the policy of showing your work but sometimes it is not possible. Kind of a catch 22. I guess I won't expect much. Too bad, considering the seemingly high level of intelligence in here.

#### HallsofIvy

Homework Helper
Doc said
That's the thing though. I suspect that MOST of the people that could answer the type of question I would ask hang out here. That includes YOU Tom. The type of question I would ask would most likely concern the type of problem where I couldn't show any work at all. That would be my point in asking. I agree with the policy of showing your work but sometimes it is not possible. Kind of a catch 22. I guess I won't expect much. Too bad, considering the seemingly high level of intelligence in here.

Have you considered posting such questions in the "mathematics" and "physics" sections rather than the "Homework Help" section?

#### Tom Mattson

Staff Emeritus
Gold Member
Originally posted by Doc
I agree with the policy of showing your work but sometimes it is not possible. Kind of a catch 22.
It is always possible.

If a student has studied and absorbed the material to any extent, he will at least have an incling as to the starting point of the problem. Homework problems are not designed to be masochistic. Certainly they are meant to be met with some strain, but they can all be done with some effort.

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