A bullet of mass 10gm is fired from a rifle with velocity of 800 m/s. After passing through a wall of mud of 180 cm thick the velocity drops to 100m/s.The average force resisted by wall?

I'm assuming you have to ignore gravity here?

Find the work done on the bullet by the wall taking the change in kinetic energy of the bullet :)

Once you get that, use the work done equation (W = Fd) to get the force.

One last thing, be careful with the units.

Unk's aproach is probably the best for this problem, but just in case you haven't sudied about work and eneregy yet - another approach is to use one of the equations of motion to find the acceleration of the bullet:



and then once you have a value for a use to find the force. You'll get the same answer, but without using work & energy principles.