Topic 1.3: Work, Energy and Power Questions

State what is meant by the kinetic energy of an object.

State what the area under a force–extension graph for a spring represents.

State the principle of conservation of energy, and identify the form into which most of the energy wasted by a real machine is transferred.

State what is meant by the work done by a force.

A box is dragged at a steady speed across a rough floor by a horizontal rope.

Identify the unit of work done, and explain why the weight of the box does no work as it is dragged.

A 0.40 kg hockey ball is struck and moves at 30 m s⁻¹.

Calculate its kinetic energy.

A coin falls from rest off the edge of a table.

Air resistance is negligible.

State the quantity that stays constant as the coin falls.

State the SI unit of power and express it in terms of the joule and the second.

Sketch a force–distance graph for a constant 10 N force acting over a distance of 4.0 m, and indicate on your sketch how the work done is found from the graph.

A 1.5 kg watering can is lifted from the ground onto a shelf 1.2 m high.

Taking g = 9.8 N kg⁻¹, calculate the gravitational potential energy it gains.

A spring gun stores 4.5 J of elastic potential energy when its spring (spring constant 900 N m⁻¹) is fully compressed.

Calculate the compression of the spring, in centimetres.

A 3.0 kg block slides along a rough horizontal floor.

A constant force of 14 N pushes it through 6.0 m, while a steady friction force of 5.0 N opposes its motion.

The block starts from rest.

A student of mass 65 kg climbs a flight of stairs of vertical height 3.2 m in 4.0 s.

Estimate the average useful power the student develops against gravity.

(Take g = 9.8 m s⁻².)

A loaded cart of mass 800 kg has a kinetic energy of 90 kJ.

During a test, a spring stores 200 J of elastic potential energy when it is compressed by 50 cm.

Estimate the spring constant of the spring.

A water pump is supplied with 1300 W of electrical power.

The useful power it delivers in raising water is 800 W.

Show that the efficiency of the pump is about 0.62.

A toy launcher uses a spring of spring constant 800 N m⁻¹. A child compresses the spring by 0.10 m and then releases it. The spring returns all of its stored elastic potential energy to a foam dart in a time of 0.020 s.

What is the average power delivered to the dart?

Two stones, one twice the mass of the other, are dropped from rest from the same height with negligible air resistance.

Explain why they reach the ground with the same speed.

A trolley of mass 8.0 kg is initially at rest on a level track.

A constant horizontal force of 30 N pushes it through 5.0 m while a steady friction force of 6.0 N opposes the motion.

Show that the trolley's speed at the end of the push is about 5.5 m s⁻¹.

A Sankey diagram for a small petrol generator shows 4000 J of chemical energy supplied by the fuel.

The useful electrical output arrow is 1000 J.