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Mechanics - 2 objects collide travelling in the same direction

Can someone help me solving these types of problems?

You have 2 objects(A and B) at different masses travelling at 2 different speeds. They are travelling in the same direction. They then collide and move as a single object. F1 is the force B exerts on A, and F2 is the force A exerts on B.

Generally speaking, how would you find the values of F1 and F2?

Am I right in thinking that F1 and F2 are the same due to Newtons 3rd Law?
You need to think about the conservation of momentum laws which involve both velocity and mass.

So then they collide, which object will pass on its momentum to the other? It's a simple sum.

So now you have the momentum of both objects and their combined direction so rearrange the momentum formula to get the velocity of both.
Original post by ThatPerson
Can someone help me solving these types of problems?

You have 2 objects(A and B) at different masses travelling at 2 different speeds. They are travelling in the same direction. They then collide and move as a single object. F1 is the force B exerts on A, and F2 is the force A exerts on B.

Generally speaking, how would you find the values of F1 and F2?

Am I right in thinking that F1 and F2 are the same due to Newtons 3rd Law?


Why do you need to find the forces?
Generally speaking you don't know what these are unless you know the impact time dt of the collision. Then Fdt = change in momentum.
As this time is unknown you can't find the forces.
In these problems you are only interested in change in momentum, initial and final velocities, and kinetic energy. You solve them using mass, momentum and velocity. Not force.
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ThatPerson
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Original post by Stonebridge
Why do you need to find the forces?
Generally speaking you don't know what these are unless you know the impact time dt of the collision. Then Fdt = change in momentum.
As this time is unknown you can't find the forces.
In these problems you are only interested in change in momentum, initial and final velocities, and kinetic energy. You solve them using mass, momentum and velocity. Not force.


I actually think that I've misread the question.

A hint/assumption given in the question is:

"F1 and F2 are not the magnitudes of the forces, remember, but the actual forces themselves."

So, what does this mean?
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joostan
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Original post by ThatPerson
I actually think that I've misread the question.

A hint/assumption given in the question is:

"F1 and F2 are not the magnitudes of the forces, remember, but the actual forces themselves."

So, what does this mean?


Forces are vectors. They have both magnitude and direction.
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ThatPerson
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Original post by joostan
Forces are vectors. They have both magnitude and direction.


Oh.... So it is 1 and -1, hence the answer for my specific question is -1. :/ oops
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joostan
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Original post by ThatPerson
Oh.... So it is 1 and -1, hence the answer for my specific question is -1. :/ oops


Lol it happens all the time :tongue:

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