15. A person applies a rightward force to a box on a rough surface. The box slides to the right at constant velocity. Earth a. On the figure above draw and correctly label all the forces acting on the box as it slides across this rough surface. Or if you prefer, draw a force diagram in the space next to the figure, remembering to include F. b. If the force the person applies to the box is +20 N, what is the magnitude and direction of the frictional force of the surface on the box? c. If the force of the person on the box is doubled and the mass of the box is m = 10 kg, what is the acceleration of the box (frictional force stays the same)? (2 m/s) Two-Dimensional Motion and Relative Motion 16. A cannonball is fired horizontally off a cliff as depicted below. 352 0 a) Describe the motion of the cannonball in the horizontal and vertical directions. Use complete sentences. b) If the cliff is 40 m tall, and the cannonball is launched horizontally with a velocity of v=20 m/s, calculate the time the ball spends in the air. 17. You are fixing a broken rotary lawn mower. The circular blade on the mower has a period of 0.02 s. What is the magnitude of the force needed to hold the outer portion of the blade to the inner portion of the blade? The outer part is 0.21 m from the center of the blade, and the mass of the outer portion is 0.007 kg. (145 N) 18. A flea is riding on the body of a dog a few centimeters behind the dog's neck. The dog sees a squirrel and starts to accelerate towards it. Describe how the squirrel sees the motion of the dog and how the flea sees the motion of the dog (well, actually only the dog's neck). What is the reference frame of each observer? Answer below.