Pupil sheet · Energy, work and power

Energy, work and power — return questions

Name ______________________ Date ____________

Use one to two weeks after the lessons. Keep the notes and the lesson closed.

1. state / describe / explain

A wind-up toy car has a spring inside. You turn its key to wind the spring, put the car down and it whirrs across the floor, then slows down and stops. (a) State the store in which the energy is kept when the spring is wound up. (b) Describe how energy is transferred to the car’s kinetic store as it starts to move. (c) Explain why the car slows down and stops. State whether any energy has been destroyed.

(a) Wound spring

(b) Starting to move

2. calculate

An electric pump lifts water up to a tank on a roof. Each second it is supplied with 800 J by electrical work. In a Sankey diagram for the pump drawn to scale, the arrow for the 800 J going in is 40 mm wide and the arrow for the wasted energy is 12 mm wide. (a) Calculate the energy wasted by the pump each second. (b) A better pump is also supplied with 800 J each second, but it wastes only 160 J. Calculate the width of its wasted arrow, drawn to the same scale.

(a) Energy wasted each second: ______________ unit ________

(b) Width of the better pump’s wasted arrow: ______________ unit ________

3. calculate

An electric winch pulls a boat 16 m up a slipway from the water. It does 4800 J of work on the boat in 30 s. (a) Calculate the force with which the winch pulls the boat along the slipway. (b) Calculate the power of the winch in doing this work.

(a) Force of the winch: ______________ unit ________

(b) Power: ______________ unit ________

4. calculate

Supplement. A 0.020 kg arrow leaves a bow at 60 m/s. (a) Calculate the kinetic energy of the arrow. (b) The arrow hits a straw target and stops after moving 0.090 m into the straw. Calculate the average force of the straw on the arrow. (c) Another 0.020 kg arrow is fired straight upwards at 60 m/s. Calculate the height it would reach if there were no air resistance.

(a) Kinetic energy of the arrow: ______________ unit ________

(b) Average force of the straw: ______________ unit ________