Energy and Momentum Review Thrill-a-Minute Worksheet

1. What is the definition of energy?

2. In what circumstance(s) does a force acting on an object do zero work?

3. What are the two broad categories of energy and how are they defined?

4. Give an example of the following:  (a) a transfer of energy, (b) a transformation of energy.

5. Just like beauty, the gravitational potential energy of an object is in the eyes of the beholder.  What the heck am I talking about?

6. What does it mean to say energy is conserved?

7. A wind blowing at 25.0° exerts a force of 675 N on the sail of a boat which in turn is propelled 1.55 km, 45.0°.  (Consider directions as viewed from overhead.) Find the work done by the wind.

8. A 50.0 kg kid swings on a playground swing.  His height above ground varies from 0.500 m at the lowest point to 2.00 m at the highest point.  Using the ground as a reference, find:  (a) PE and KE at the highest point, (b) PE and KE at the lowest point, (c) the kid's maximum speed during the swing, (d) the height at which the kid has speed 3.00 m/s.

9. A 2500 kg car is running on flat ground at 35 m/s.  Just as it reaches the bottom of a hill with slope 25° the engine cuts out.  (a) Find the kinetic energy of the car initially.  (b) How far up the hill will the car go, measured along the road?  Neglect friction.

10. An electric light bulb is rated at 150 W.  (a) How much electric energy does it use in 55 minutes?  (b) In how much time would this bulb use as much electric energy as there is chemical energy in a 150 Calorie cookie (628,500 J)?

11. A weight lifter raises a 150 kg barbell from the floor to a height of 2.2 m in 0.80 sec.  What is his power output?

12. A 4.00 kg box is pushed 2.00 m across a level surface by an applied force of 32.0 N, 0°.  The box then slides an additional 3.00 m before coming to a rest.  (a) Determine the amount of friction.  (b) Determine the box’s maximum speed over the 5.00 m distance traveled.

13. A 1230 kg car has an engine with power output 155 h.p. (1 h.p. = 746 W).  (a) What is the minimum amount of time the car could possibly accelerate from 0 to 60.0 mph (26.8 m/s)?  (assuming friction is negligible)  (b) In reality of course there is friction.  Repeat the above question assuming there is a steady 1.20 kN of friction.  (This is MUCH harder to figure!)

14. A 2500 kg truck tops a hill with elevation 15.0 m doing 30.0 m/s.  The driver applies the brakes and brings the truck to a stop just as it reaches the bottom of the hill, 200 m farther down the road.  (a) What amount of work is done by friction in the process?  (b) Assume this work represents heat being generated in the brake discs and pads.  How much water could this heat raise from 0.0 °C to 100.0 °C?  (It takes 4.19 J to raise 1.00 gram of water 1.00 Celsius degrees.)  (c) Determine the average frictional force on the car during the slow down.

15. The Raccoon Mountain Pumped Storage Plant uses electricity to run the four most powerful reversible pump-turbines in the world to pump water from the Tennessee River to a reservoir 1160 feet (354 m) upward.  Energy is stored in this manner until it is needed to meet peak demands for electricity, at which point water flows back to the river below turning the same turbines which in this mode produce electricity at 1530 MW.  The upper reservoir contains 38,000 acre·feet of water (46,870,000 m³).  (a) How much energy can be stored?  (b) What is the longest possible amount of time the plant can generate electricity before having to pump water back to the reservoir?  (Note:  density of water = 1.00 g/cm³)

16. Consider the “bumper cars” ride that you often see at the fair or an amusement park.  In this ride you have multiple bumper cars riding around in an enclosed “arena”.  The cars are powered by electricity that comes through an electrode that touches an electric grid on the ceiling.  Let all of the cars and the people riding in the cars be a “system”.  (a) What are the external forces acting on this system?  (b) What are the internal forces for this system?  (c) Would the total momentum of all the bumper cars remain constant as the people are driving them about?  Explain this by referring to the forces listed previously, stating which forces, if any, would affect the total and which forces, if any, would not.

17. (Honors physics only)  A ball with momentum 7.00 kg m/s, 0.0° collides with a wall and bounces off in the opposite direction.  During the collision the ball exerts an average force of 120 N, 0.0° on the wall for a duration of 0.110 seconds.  Ignore the effect of gravity.  (a) Determine the impulse of the ball on the wall.  (b) Determine the momentum of the ball after the collision. 

18. (Honors physics only)  The propeller on the fan cart has a diameter of 15 cm and it produces a “wind” with speed 4.0 m/s.  Given the density of air is 1.29 kg/m³, estimate the thrust produced by the spinning propeller.

19. A kid with mass 70.0 kg stands at rest on a frozen lake holding a 0.50 kg snowball.  If the kid throws the snowball with velocity 15 m/s, 180° what will be his resulting velocity?  Ignore friction.

20. If the Moon (mass 7.35 × 10²² kg) were to somehow stop moving forward in its orbit it would fall to the Earth (mass 5.974 × 10²⁴ kg) and impact the surface with an impact speed of 11.0 km/s.  Supposing the Moon would plow into the Earth and become a part of the Earth, by how much would this change the Earth’s velocity?

21. A rubber ball of mass 105 grams is thrown at a wooden block that is on the frozen surface of an ice rink (ignore friction).  The ball hits the block with velocity 12 m/s, 180.0° and rebounds with velocity 5.0 m/s, 0.0°.  (a) Determine the change in momentum of the wooden block due to the impact.  (b) Determine the mass and final speed of the block assuming that the collision is perfectly elastic and assuming that the block was initially at rest.

Answers

1. The ability to do work.

2. When the force is perpendicular to the displacement or there is no motion.

3. Potential, Kinetic -- definitions?

4. a. a cue ball hitting the eight ball (transfer of KE)

    b. a pendulum swinging (transformation from PE to KE to PE, etc.)

5. Potential energy is always measured relative to an arbitrary point of reference and therefore
    could be anything you want it to be.

6. Energy is never created or destroyed, only transferred or transformed.

7. 983 kJ

8. a. PE = 980 J, KE = 0

    b. PE = 245 J, KE = 735 J

    c. 5.42 m/s

    d. 1.54 m

9. a. 1.53 MJ

    b. 148 m

10. a. 495 kJ

      b. 1.2 hrs

11. 4.0 kW

12. a. 12.8 N

      b. 4.38 m/s

13. a. 3.8 s

      b. 4.4 s

14. a. -1.49 MJ

      b. 3.56 kg (about 1 gallon!)

      c. 7.46 kN

15. a. 1.63 x 10¹⁴ J  (45,000,000 kW-hrs)

      b. 29.5 hrs.

16. a.

      b.

      c.

17. a. 13.2 Ns, 0.0°

      b. 6.2 kg m/s, 180.0°

18. 0.36 N

19. 0.11 m/s, 0.0°

20. 134 m/s (not 135 m/s)

21. a. 1.8 kg m/s, 180.0°

      b. 255 g, 7.0 m/s