Physics problems, just want to double check my answers?

Question

1. A worker pulls a sled with a force of 80 N directed at an angle of 60 degrees above the horizontal over a level distance of 6 m. If a frictional force of 24 N acts on the sled in a direction opposite to that of the worker, what net work is done on the sled?

a. 240 J, b. 216 J, c. 144 J, d. 96 J


2. A model rocket sits on the launch pad until its fuel is ignited, blasting the rocket upward. During the short time of blast-off, as the ignited fuel goes down, the rocket goes up because:

a. momentum is conserved in this process
b. the fuel pushes on the ground
c. air friction pushes on the escaping fuel
d. the downward force of gravity is less than the downward momentum of the fuel

please explain why you believe the answers are what they are just in case yours are different than mine. thanks a lot guys!

3. A 10 g bullet with an initial speed of 100 m/s is fired horizontally into a 90 g wodden block initially at rest on a frictionless horizontal surface. The bullet passes completely through the block and emerges with a final speed of 20 m/s in the horizontal direction. How much mechanical energy is lost in this process?

a. 44.4 J
b. 50 J
c. 26.4 J
d. 20 J

Answer 1

The net horizontal force is 80N*cos(60) -24N or
80*(1/2) - 24 = 16N so the total work done is 16*6 = 96J

a. Momentum is conserved. MV = mv and, since the initial momentum was 0, the rocket must move upwards as the exhaust gas travels downwards.


Doug