posted by cutie .
what force is necessary to stop a 20 g bullet moving at 200 m/s as it penetrates a wood at a distance of 4 m?
A 300 g piano is being lifted at a steady speed from the ground level straight upward to an apartment 15 m above the ground. The crane that is doing the lifting produces a steady power of 500 watts. How much time does it take to lift the piano?
Your question is badly worded. 4 m is not the distance from which the bullet was fired. It is the distance over with the bullet's velocity is decelerated to zero.
Otherwise the question cannot be answered.
I will ignore your second question, which should have been posted separately.
All of the kinetic energy is converted to work penetrating the wood. (That work becomes heat)
Force*Distance = (initial kinetic energy)
Solve for the force. Make sure bullet mass is in kg