A man weighing 180 lbf pushes a block weighing 100 lbf along a horizontal plane. The dynamic coefficient of friction between the block and plane is 0.2. Assuming that the block is moving at constant speed, calculate the work required to move the block a distance of 100 ft considering (a) the man and (b) the block as the system. Express your answers in both lbf·ft and Btu.

a)If a team pulls with a constant force and moves the plane 3.7 m in 6.1 s (an excellent time), what fraction of the team's work goes to kinetic energy?  b)If a team pulls with a constant force and moves the plane 3.7 m in 6.1 s (an excellent time), what fraction of the team's work goes to thermal energy?  Express your answer numerically.

A 7.80 g bullet is initially moving at 660 m/s just before it penetrates a block of wood to a depth of 4.40 cm.   (a) What is the magnitude of the average frictional force (in N) that is exerted on the bullet while it is moving through the block of wood? Use work and energy considerations to obtain your answer. _______________N   (b) Assuming the frictional force is constant, how much time (in s) elapses between the moment the bullet enters the block of wood and the moment it stops moving? _______________s

A 7.80-g bullet moving at 530 m/s penetrates a tree trunk to a depth of 4.50 cm.  (a) Use work and energy considerations to find the average frictional force that stops the bullet. (b) Assuming the frictional force is constant, determine how much time elapses between the moment the bullet enters the tree and the moment it stops moving.