Work and Power Problems and Questions

NOTE: WE ARE DOING THIS UNIT IN A DIFFERENT ORDER THAN THE WAY THE TEXT IS ORGANIZED

5-1 Work (Read 5-1, then answer the following)

  1. Why is no work done when an object is carried horizontally?
  2. Is the same amount of work done lifting a 15 kg rock on the earth and lifting a 15 kg rock on the moon the same distance? Explain.
  3. Under what circumstance is work a negative value?
  4. A car needs to be pushed 35 m across a parking lot. How much work is done when 825 N of force is applied to the car? 29 kJ
  5. How much work is done if the car is pushed straight back to its original position with the same amount of force?
  6. A flight attendant pulls her luggage down a 75 meter long corridor at an airport. How much work does she do if she pulls with a force of 25 N at a 38o angle (above the horizontal)? 1500 J
  7. How much work is done by a forklift to lift an 835 kg crate to a height of 2.7 meters at constant velocity? 22 kJ
  8. An airplane passenger carries a 215 N suitcase up a staircase. The displacement of the suitcase is 4.25 m vertically, and 4.61 m horizontally. How much work against gravity does the person do? 914 J
  9. How much work is done by the passenger in the previous problem in carrying the box down the stairs?-914 J
  10. Suppose a pendulum swings back and forth. Does the tension in the string do work on the bob? Does gravity do work on the bob?
  11. Work is not a vector quantity. Explain why it is not.
  12. Can the speed of an object change if the net work done on it is zero? Explain.

5-4 Power (Read 5-4, then answer the following)

  1. How much power is required to lift a 350 kg mass up 0.55 m in 0.35 seconds? 5400 W
  2. How much power would it take for a 650 N person to hike up a trail that gains 550 m in elevation if it takes 25 minutes? Only consider the work done against gravity. 240 W
  3. An electric motor develops 65 kW of power as it lifts a loaded elevator 17.5 m in 35 seconds. How much force does the motor exert? 130 kN
  4. An engine moves a boat through the water at a steady speed of 15 m/s. The engine and propeller must exert a constant force of 6500 N to balance the force that the water exerts against the hull. How much power does the engine develop? Hint: look at the units—what do you have to do with the values to get watts? 98 kW
  5. Based on the previous question, derive an equation for calculating power that includes force and velocity.
  6. The brightness of a light-bulb is not represented by its wattage (a 25 W fluorescent bulb is much brighter than a 25 W incandescent bulb). Based on the definition of power, what does the wattage represent?
  7. Suppose that the engine of a muscle car produces twice the power of a typical car. Compare the time it takes to accelerate both cars to the same speed (assume that each car has the same mass and experience the same amount of air resistance).