1. A sprinter with a mass of 65 kg reaches a speed of 10 m/s during a race. Find the sprinter's linear momentum.

3. In Section 2.4, we computed the force needed to accelerate a 1,000-kg car from 0 to 27 m/s in 10 s. Compute the force using the alternate form of Newton's second law. The change in momentum is the car's momentum when traveling 27 m/s minus its momentum when going 0 m/s.

7. A pitcher throws a 0.5-kg ball of clay at a 6-kg block of wood. The clay sticks to the wood on impact, and their joint velocity afterward is 3 m/s. What was the original speed of the clay?

9. A 50-kg boy on roller skates moves with a speed of 5 m/s. He runs into a 40-kg girl on skates. Assuming they cling together after the collision, what is their speed?

11. A loaded gun is dropped on a frozen lake. The gun fires, with the bullet going horizontally in one direction and the gun sliding on the ice in the other direction. The bullet's mass is 0.02 kg, and its speed is 300 m/s. If the gun's mass is 1.2 kg, what is its speed?

13. A motorist runs out of gas on a level road 200 m from a gas station. The driver pushes the 1,200-kg car to the gas station. If a 150-N force is required to keep the car moving, how much work does the driver do?

15. A weight lifter raises a 100-kg barbell to a height of 2.2 m. What is the barbell's potential energy?

17. A personal watercraft and rider have a combined mass of 400 kg. What is their kinetic energy when they are going 15 m/s?

19. The kinetic energy of a motorcycle and rider is 60,000 J. If their total mass is 300 kg, what is their speed?

23. A 25-kg child uses a pogo stick to bounce up and down. The spring constant, k, of the toy equals 8750 N/m. (a) By how much would the spring be compressed by the child if she simply balanced herself vertically on the pedals of the stick? (b) How much energy is stored in the spring under this circumstance?

27. At NASA's Zero Gravity Research Facility in Cleveland, Ohio, experimental payloads fall freely from rest in an evacuated vertical shaft through a distance of 132 m. (a) If a particular payload has a mass of 45 kg, what is its potential energy relative to the bottom of the shaft? (b) How fast will the payload be traveling when it reaches the bottom of the shaft? Convert your answer to mph for a comparison to highway speeds.

29. A bicycle and rider going 10 m/s approach a hill. Their total mass is 80 kg.

(a) What is their kinetic energy? (b) If the rider coasts up the hill without pedaling, how high above its starting level will the bicycle be when it finally rolls to a stop?

31. The ceiling of an arena is 20 m above the floor. What is the minimum speed that a thrown ball would need to just reach the ceiling?

35. How long does it take a worker producing 200 W of power to do 10,000 J of work?

39. In the annual Empire State Building race, contestants run up 1,575 steps to a height of 1,050 ft. In 2003, Australian Paul Crake completed the race in a record time of 9 min and 33 s. Mr. Crake weighed 143 lb (65 kg). (a) How much work did Mr. Crake do in reaching the top of the building? (b) What was his average power output (in ft-lb/s and in hp)?

41. Two small 0.25-kg masses are attached to opposite ends of a very lightweight rigid rod 0.5 m long. The system is spinning in a horizontal plane around a vertical axis perpendicular to the rod located halfway between the masses. Each mass is moving in a circle of radius 0.25 m at a speed of 0.75 m/s. What is the total angular momentum of this system?