# Physics homework - need help

We were given a WebAssign for homework dealing with forces. I got most of the problems, but there were a few I didn't understand at all. If anyone could help me, it would be much appreciated.

1. The helicopter in the drawing is moving horizontally to the right at a constant velocity. The weight of the helicopter is W = 59500 N. The lift force L generated by the rotating blade makes an angle of 21.0° with respect to the vertical. What is the magnitude of the lift force? Determine the magnitude of the air resistance R that opposes the motion.

2. A 85 kg water skier is being pulled at a constant velocity. The horizontal pulling force is 370 N. Find the magnitude of the upward force exerted on the skier by the water.

3. When a 58 g tennis ball is served, it accelerates from rest to a constant speed of 48 m/s. The impact with the racket gives the ball a constant acceleration over a distance of 44 cm. What is the magnitude of the net force acting on the ball?

## Answers and Replies

jamesrc
Science Advisor
Gold Member
1. There is no net force on the helicopter since it does not move in the vertical direction and it moves with constant velocity in the horizontal direction. Write two equations (force balances in each relevant axis). You should find that Lsin(21) = R and Lcos(21) = W, which is enough information (combined with what is given) to solve for L and R.

2. Similar problem to the one above. The vertical component of the reaction force on the skis must balance the weight of the skier while the horizontal component must balance the pulling force. The two unknowns in this problem are the magnitude and direction of the reaction force.

3. Have you studied the work-energy theorem yet? If so, use that; if not, use Fnet = ma. Find a using the kinematic data given (hint: v2 = v2o + 2a&Delta;x)

Thank you so much! :-) We haven't studied that theorem yet...the topics we're working on in physics are Newton's Laws of Motion and different Force equations. (Friction, Tension, Gravity, etc.)