Normal force act up wards so the middle box will be acting on the top box and the bottom box will acting up on the middle box. So the middle box acts upwards on the top box with the same force as the weight of the top box.
Just remember that for any force there is an equal and opposite reaction.
You need to use simple algebra to isolate r.
start with multiplying both sides by the rights side denominator. then you need to multiply out Earth and divide everything by r2. By doing this you should be able to get a single r2 term. then just isolate r.
The question was how long does the block slide on the surface after the bullet leaves it. I noticed though that it said "after the bullet leaves the block". This means that the bullet is no longer acting on the block meaning you can use the equation of impulse to give you.
ƩFblockΔt=mblockΔv...
Homework Statement
a 200-g projectile is fired with a velocity of 900m/s towards the center of the 15-kg wooden block, which rests on a rough surface. If the projectile penetrates and emerges from the block with a velocity of 300m/s, determine the velocity of the block just after the...