Question

1. A reckless professor doing a demonstration swings a ball attached to the end of a 3-meter string at constant speed in a horizontal circle with a circumference of 3 m. What is the work done on the ball by the 10 N tension force in the string during one revolution of the ball?

A 60 J

B 30 J

C 20 J

D 1 0 J

E 0 J

2.

A player throws a softball (weight 1.8 N) to a teammate on the other side of the field. The softball leaves the thrower's hand with an initial kinetic energy of 90. J and its minimum kinetic energy during the flight is 40. J. What is the maximum height reached by the softball?

A 72 m

B 50. m

C 28 m

D 22 m

E 11 m

3. A stunt person in a movie jumps from the roof of a tall building, but no injury occurs because she lands on a large air-filled bag. Which one of the following best describes why she can accomplish this stunt safely?

A The bag provides the necessary force to stop the person.

B The bag reduces the impulse to the person.

C The bag increases the amount of time the force acts on the person and reduces the change in momentum.

D The bag decreases the amount of time during which the momentum is changing and reduces the average force on the person.

E The bag increases the amount of time during which the momentum is changing and reduces the average force on the person.

Answer 1

1. Tension in the string provides the centripetal force. It always acts towards the center. Hence there is no displacement in the direction of Tension. Thus the work done is 0 joules.

2. By conservation of energy

Mgh = change in kinetic energy

Mgh = 90 - 40 = 50 J

Given Mg = 1.8 N

h = (50/1.8) = 28 m

3. F = change in momentum / time taken for change

Hence on increasing the time taken for the change in momentum , The force on the body decreases.

Hence the correct answer is option E