Topic 1.3: Work, Energy and Power Questions

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 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.

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.

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

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 what is meant by the work done by a force.

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

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

Calculate its kinetic energy.

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

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

Describe how a Sankey diagram represents the energy transfers in a machine, and explain why the widths of the output branches must add up to the width of the input arrow.

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 spring of spring constant 500 N m⁻¹ is compressed by 4.0 cm and then released, transferring all of its stored energy in 0.025 s.

A worker pushes a trolley 8.0 m along a corridor using a handle inclined at 60° to the horizontal floor.

The push along the handle has magnitude 15 N.

Calculate the work done by this push on the trolley.

A 1.5 kg melon is dropped from rest off a balcony 8.0 m above the ground.

Air resistance is negligible and g = 9.8 N kg⁻¹.

Calculate the gravitational PE it has at the top, and use conservation of energy to find its speed just before it hits the ground.

A 0.040 kg golf ball rolls off a shelf and falls from rest through a height of 2.5 m.

Air resistance is negligible and g = 9.8 N kg⁻¹.

Determine the speed of the ball just before it lands.

A pendulum bob of mass 0.25 kg is pulled to one side so that it is raised 0.20 m above its lowest point, then released from rest.

Air resistance and friction are negligible and g = 9.8 N kg⁻¹.

Determine the speed of the bob as it passes through the lowest point of its swing.

The force on a model rocket sled varies with distance and is plotted on a force–distance graph as a curve that bulges above a straight line.

The grid is marked so that each small square represents 2.0 N by 0.50 m.

By counting squares, a student estimates the region under the curve from 0 to 4.0 m contains about 45 small squares.

Estimate the work done on the sled over this distance.

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?

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.