Work, Power & Efficiency Practice Questions

Question 1

3 marks

(a)

Which of these is equal to one watt?

A $1\ \text{J}$

B $1\ \text{J/s}$

C $1\ \text{J s}$

D $1\ \text{N/s}$

/ 1

(b)

A winch does $7200\ \text{J}$ of work lifting a load.

The winch takes $40\ \text{s}$ to lift the load.

Calculate the average power of the winch.

average power = ............................................... W

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Question 2

4 marks

A gardener is tidying a garden.

(a)

The gardener pushes a wheelbarrow along a level path.

The gardener pushes with a horizontal force of $85\ \text{N}$ .

The wheelbarrow moves $24\ \text{m}$ in the direction of the force.

Calculate the work done by the gardener.

work done = ............................................... J

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(b)

State the energy transferred by the gardener in pushing the wheelbarrow along the path.

energy transferred = ............................................... J

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(c)

The gardener then switches on an electric leaf blower.

State the way in which the energy of the leaf blower is changed when it is switched on.

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Question 3

3 marks

A builder lifts a bag of cement of mass $12\ \text{kg}$ from the ground onto a platform $1.8\ \text{m}$ above the ground.

gravitational field strength $g = 10\ \text{N/kg}$

(a)

Name the energy store of the bag of cement that is filled as the bag is raised.

/ 1

(b)

Calculate the change in gravitational potential energy of the bag of cement.

change in gravitational potential energy = ............................................... J

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Question 4

3 marks

Figure 1 is an energy transfer diagram for an electric food mixer. The diagram is drawn for every $800\ \text{J}$ of energy transferred to the mixer.

Figure 1

(a)

Calculate the energy wasted by the mixer for every $800\ \text{J}$ supplied to it.

energy wasted = ............................................... J

/ 1

(b)

Calculate the efficiency of the food mixer.

Give your answer as a percentage.

efficiency = ............................................... %

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Question 0

5 marks

A ten-pin bowling ball is rolled along a lane.

(a)

The ball has a mass of $6.4\ \text{kg}$ and moves at a speed of $5.0\ \text{m/s}$ .

Calculate the kinetic energy of the ball.

kinetic energy = ............................................... J

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(b)

A second ball has a mass of $4.0\ \text{kg}$ .

This ball has the same kinetic energy as the ball in part (a).

Calculate the speed of the second ball.

speed = ............................................... m/s

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Question 0

6 marks

A student of mass $54\ \text{kg}$ runs up a flight of stairs in a school.

The top of the stairs is $4.2\ \text{m}$ vertically above the bottom.

gravitational field strength $g = 10\ \text{N/kg}$

(a)

Calculate the work done by the student against gravity in running up the stairs.

work done = ............................................... J

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(b)

The student takes $6.0\ \text{s}$ to run up the stairs.

Calculate the useful power output of the student.

useful power output = ............................................... W

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(c)

The student later walks up the same flight of stairs, taking $15\ \text{s}$ .

Explain the effect this has on the useful power output of the student.

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Question 0

5 marks

A factory tests three electric motors, A, B and C. The table shows the energy supplied to each motor and the useful energy transferred by each motor during one test.

Motor	Energy supplied (J)	Useful energy transferred (J)
A	5000	4000
B	8000	6000
C	12 000	9600

(a)

Calculate the efficiency of motor A.

Give your answer as a percentage.

efficiency = ............................................... %

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(b)

Deduce which motor wastes the greatest amount of energy during its test.

You must support your answer with a calculation.

/ 2

(c)

State what happens to the energy that is wasted by a motor.

/ 1

Question 0

7 marks

A car of mass $1200\ \text{kg}$ is travelling at $15\ \text{m/s}$ along a straight, level road.

The driver applies the brakes and the car comes to rest in a distance of $25\ \text{m}$ .

(a)

Describe the changes in the way energy is stored as the car slows down and stops.

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(b)

Calculate the kinetic energy of the car before the brakes are applied.

kinetic energy = ............................................... J

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(c)

All of this kinetic energy is transferred by the work done by the braking force.

Calculate the average braking force acting on the car.

average braking force = ............................................... N

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Question 0

4 marks

A student is investigating the work done in dragging a wooden block across a laboratory bench.

The student has a wooden block, a forcemeter (newtonmeter) and a metre rule.

(a)

Describe how the student should measure the work done in dragging the block across the bench.

/ 3

(b)

State how the energy transferred in dragging the block compares with the work done by the student.

/ 1

Question 0

7 marks

A child of mass $32\ \text{kg}$ sits on a swing. The swing is pulled back so that the child is raised $0.80\ \text{m}$ above the lowest point of the swing, and is then released from rest.

gravitational field strength $g = 10\ \text{N/kg}$

(a)

Calculate the gain in gravitational potential energy of the child when the swing is pulled back.

gain in gravitational potential energy = ............................................... J

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(b)

Assume that all of this energy is transferred to the kinetic energy store of the child.

Calculate the speed of the child at the lowest point of the swing.

speed = ............................................... m/s

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(c)

In practice, the measured speed of the child at the lowest point is less than the value calculated in part (b).

Explain why.

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Question 0

6 marks

A crane on a building site lifts a steel beam of mass $450\ \text{kg}$ through a vertical height of $18\ \text{m}$ .

The lift takes $30\ \text{s}$ .

The motor of the crane is supplied with energy at a rate of $4.5\ \text{kW}$ .

gravitational field strength $g = 10\ \text{N/kg}$

(a)

Calculate the useful power output of the crane.

useful power output = ............................................... W

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(b)

Calculate the efficiency of the crane.

Give your answer as a decimal.

efficiency = ...............................................

/ 2

(c)

Explain why the crane becomes warm while it is lifting the beam.

/ 1

Question 0

8 marks

A worker pushes a crate of mass $40\ \text{kg}$ up a ramp into the back of a van, as shown in the diagram.

The crate moves $5.0\ \text{m}$ along the ramp and rises a vertical height of $1.5\ \text{m}$ .

The worker pushes with a steady force of $160\ \text{N}$ directed along the ramp.

gravitational field strength $g = 10\ \text{N/kg}$

(a)

Calculate the work done by the worker in pushing the crate up the ramp.

work done = ............................................... J

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(b)

Calculate the gain in gravitational potential energy of the crate.

gain in gravitational potential energy = ............................................... J

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(c)

Explain why the work done by the worker is greater than the gain in gravitational potential energy of the crate.

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(d)

The worker then carries a box of weight $120\ \text{N}$ a distance of $8.0\ \text{m}$ along a level floor, holding it at a constant height.

State the work done against gravity in carrying the box and explain your answer.

/ 2

Key Concepts of Physics

Motion & Forces

Conservation of Energy

Waves

Light & the Electromagnetic Spectrum

Radioactivity

Astronomy

Energy – Forces Doing Work

Forces & their Effects

Electricity & Circuits

Static Electricity

Magnetism & the Motor Effect

Electromagnetic Induction

Particle Model

Forces & Matter

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