Work Energy Power And Efficiency
A child pulls a sledge along level ground using a rope inclined at 40° above the horizontal. The tension in the rope is 60 N and the sledge moves 15 m. Calculate the work done by the tension on the sledge.
State why the centripetal force in uniform circular motion does no work on the orbiting object.
A waiter walks 12 m along a horizontal corridor while holding a tray of mass 2.0 kg at a constant height. Calculate the work done by the waiter’s upward force on the tray.
A car of mass 1500 kg travels along a straight road at 20 m s⁻¹. Calculate its kinetic energy.
The speed of a moving body is tripled while its mass remains constant. By what factor does its kinetic energy change?
A golf ball of mass 0.045 kg has a kinetic energy of 90 J at the moment of impact. Calculate its speed.
A skier of mass 50 kg descends a straight slope of length 30 m inclined at 25° to the horizontal. Calculate the loss in gravitational potential energy. (g = 9.81 N kg⁻¹)
A stone is released from rest and falls freely through a vertical height of 5.0 m. Air resistance is negligible. Calculate its speed just before impact. (g = 9.81 N kg⁻¹)
A box slides across a rough horizontal floor and gradually slows to rest. State what happens to its kinetic energy.
A crane raises a load of mass 500 kg through a vertical height of 12 m at constant speed in 8.0 s. Calculate the useful output power of the crane. (g = 9.81 N kg⁻¹)
A car travels at a constant velocity of 25 m s⁻¹ along a level road against a total resistive force of 800 N. Calculate the useful power output of the engine.
An electric heater transfers 2.5 kW h of energy during one operating cycle. Calculate this energy in joules.
An electric pump uses 1200 W of electrical power and does useful work against gravity at a rate of 800 W. Calculate the efficiency of the pump as a percentage.
Which of the following values is impossible for the efficiency of a real device?
A ball of mass 0.20 kg is thrown vertically upwards from ground level with an initial speed of 15 m s⁻¹. Because of air resistance, it reaches a maximum height of only 9.0 m. Calculate the energy transferred to the thermal store of the air and ball during the ascent. (g = 9.81 N kg⁻¹)