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[SoulPlaya]

1. A 7 kg glider on an air track starts with an initial velocity of 32 m/s and then runs into a 2 kg glider which is initially at rest. During the collision, which glider has a larger magnitude change in momentum? Explain your answer.

I know that the rate of change in momentum is the same for both, but why? I'm trying to use the idea of impulse, as impulse is the change in momentum, but I'm getting a bit lost, any help would be appreciated.

 

[M.Bluth]

I'm in a bit of a hurry, so IIRC you have to treat them as a single object after collision.
And there's something about conservation of momentum or something

Sorry :/

 

[Dogenzaka]

We need a general Homework Help Thread

 

[SoulPlaya]

At the point of collision, don't the objects exert an equal but opposite in direction force on each other? And if force=rate of change in momentum, then what one object gains in momentum, the other will lose in momentum, but the rate of change is the same, right?

Thanks BTW.

 

[FunkyMunkey]

Can't help much since I've got to go, but since it's frictionless, you could use conservation of momentum to figure out which mass changes speed more drastically (it's magnitude is parallel with the ground).

Edit: I didn't help at all haha.

 

[OxtheMidget]

m1v1 + m2v2 = m1v1 + m2v2
left side stands for before collision
right side stands for after collision

edit: that's conservation of momentum by the way.

 

[WHOAguitarninja]

At the point of collision, don't the objects exert an equal but opposite in direction force on each other? And if force=rate of change in momentum, then what one object gains in momentum, the other will lose in momentum, but the rate of change is the same, right?

Thanks BTW.

Hint: Approach the problem from the standpoint of conservation of energy.

EDIT-Actually don't do that...it doesn't specifically say if the collision is elastic or not.

 

[Yaweee]

Whatever momentum is lost by the first glider is gained by the second glider. So it is a trick question. Momentum as a vector is always conserved.

You can't really discuss energy whether you know it is elastic/(perfectly)inelastic.

"Rate" of change (Force) is also irrelevant except as a nice mental proof, since you don't know the duration over which the collision takes place.

 

[Peronthious]

Since the question doesn't state that one object sticks to the other after impact or something to that effect, I'm going to assume that the two objects elastically bounce off one another after the impact occurs. Empirically, the 2kg object would move faster away from the 32kg one, but of course you need to demonstrate this mathematically. It looks like you should be able to assume that the coefficient of restitution is equal to 1, so that 1 = (v2'-v1')/(v1-v2). You can set that up with the conservation of momentum equation, m1*v1+m2*v2 = m1*v1'+m2*v2' and solve simultaneously to determine the respective final velocities.

 

[weekend_warrior]

I don't know much about physics but lets see here. A 7kg object going 32 m/s runs into a 2kg object. Seems like the heavier object, already in motion, is going to experience a much smaller momentum change then the light weight object.

 

[speculawyer]

Equal. (?)

 

[tokkun]

Whatever momentum is lost by the first glider is gained by the second glider. So it is a trick question.

This is the correct answer. The magnitude of the momentum change is the same for both gliders. The purpose of the question is for you to show that you recognize the concept of conservation of momentum.

 

[SoulPlaya]

This is the correct answer. The magnitude of the momentum change is the same for both gliders. The purpose of the question is for you to show that you recognize the concept of conservation of momentum.

So, the momentum in the beginning is the same at the end? While the two objects may experience momentum change, they will still add up to the same momentum as in the initial condition, and thus, the rate of change is the same? Thus, conservation of momentum.