Momentum + Friction force = me depressed

[dabouncerx24]

Hey guys, I need help on some physics problems again.

A 15g bullet strikes and becomes embedded in a 1.10kg block of wood placed on a horizontal surface just in front of the gun. If the coefficient of kinetic friction between the block and the surface is 0.25, and the impact drives the block a distance of 9.5m before it comes to rest, what was the muzzle speed of the bullet.

A gun is fired vertically into a 1.4kg block of wood at rest directly above it. If bullet has mass of 21g and speed of 310m/s, how high will the block rise into the air before the bullet becomes embedded in it?

Please give me hints on how to solve these two problems.

 

[Pyrrhus]

Use conservation of momentum, then work the initial speed for the block.

 

[dabouncerx24]

Sorry, some people (like myself) aren't well equipped with physics skills and know right off the bat what " Use conservation of momentum, then work the final speed for the block." means totally.

Cyclo you've helped me out many times man, can u explain just a bit more. It is a new topic that was taught just yesterday in class. Thanks.

I have no idea how the friction force fits in there...

 

[Pyrrhus]

Well you should use your knowledge that change of momentum will be 0, thus it will be conserved.

$$m_{bullet}v_{bullet_{o}} + m_{box}v_{box_{o}}= (m_{bullet} + m_{box})v_{both}$$

The box was at rest

$$m_{bullet}v_{bullet_{o}} = (m_{bullet} + m_{box})v_{both}$$

Now you need to know the speed of the both, use the rest of the information, Newton's 2nd law and kinematic.

 

[dabouncerx24]

thx a lot man :)