# Collision is perfectly elastic?

Trying to figure this problem...
A bullet moves with a speed 5560 cm/s, strikes an 8.45 kg block resting on the table, and bounces straight back with a speed of 1260 cm/s. Find the speed of the block immediately after collision.

P before = P after
I'm unsure of how to solve without knowing the mass of the bullet.

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Tom Mattson
Staff Emeritus
Gold Member
Does the problem tell you that the collision is perfectly elastic? If so, then you will get a second independent equation from conservation of kinetic energy.

No it doesn't, it just has a diagram. With the bullet being fired into the block from a distance, on a horizontal plane.

Tom Mattson
Staff Emeritus
Gold Member
If that is all that is given, then it seems you will have to assume kinetic energy conservation, because that is the only way you are going to generate a second equation.

I feel like I have just been staring at this problem and getting know where....

So, would I need to find the distance or acceleration first and then use the work energy theorem??

Think of what role the (elastic) change in velocities for the particular masses has to do with the initial and final momenta and energies.

I'm just getting more confused.
Ek1 + Ek2 =0 to determine if inelastic or elastic right? I'm unsure of where to go, I'm not getting how to approach the question. We've dealt with inelastic problems, determining whether it is or not in the beginning and using the above equation. However, if never approached a question that way. Help?!?!

Doc Al
Mentor
Do as Tom says: assume that energy is conserved.

Now write down the equations for 1) conservation of momentum & 2) conservation of energy. (Hint: call the mass of the bullet "m" and the final speed of the block "V"; those are your unknowns.)

Thank you, this really helped!!