Work, Power & Efficiency Practice Questions

Which of these statements about the watt is correct?

Put a cross ( ☒ ) in the box next to your answer.

☐ A one watt is equal to one joule per metre

☐ B one watt is equal to one newton per second

☐ C one watt is equal to one joule per second

☐ D one watt is equal to one joule per newton

The worker pushes the crate with a horizontal force of $85\ \mathrm{N}$.

The crate moves $12\ \mathrm{m}$ in the direction of the force.

Calculate the work done on the crate by the worker.

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

The bucket is raised through a vertical height of $4.5\ \mathrm{m}$.

Calculate the change in the gravitational potential energy store of the bucket of water.

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

It takes the window cleaner $15\ \mathrm{s}$ to raise the bucket through this height.

Calculate the useful power output of the window cleaner.

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

Calculate the efficiency of the food mixer.

Give your answer as a percentage.

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

State what happens to the other $1800\ \mathrm{J}$ of energy supplied to the mixer.

The kettle and its surroundings can be treated as a closed system.

State what this tells you about the total energy of this system during the transfer.

Calculate the energy transferred to the surroundings, shown by the ? on the diagram.

energy transferred to surroundings = ............................................... kJ

Calculate the efficiency of the kettle.

Give your answer as a percentage.

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

The machine raises the hammer through a vertical height of $2.4\ \mathrm{m}$.

Calculate the useful work done by the machine in raising the hammer.

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

The hammer is released and falls.

Assume that all of the energy in the gravitational potential energy store of the hammer is transferred to its kinetic energy store as it falls.

Calculate the speed of the hammer just before it hits the post.

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

The motor of a lift (elevator) in an office block does $1.44\ \mathrm{MJ}$ of useful work in raising passengers during a period of $4.0$ minutes.

Calculate the useful power output of the motor.

Give your answer in kW.

useful power output = ............................................... kW

Describe how the student can measure the work done by the pulling force on the block.

State the relationship between the work done by the pulling force and the energy transferred.

Complete the table by stating, for each situation, which one of these three ways the energy of the system is changed.

Situation	How the energy of the system is changed
A cyclist pedals to push a bicycle up a slope
A kettle element is connected to the mains supply
A saucepan of soup is placed on a lit gas ring

A mechanical process becomes wasteful when it causes a rise in temperature.

Explain why.

A manufacturer claims that a stair lift has a useful power output of $150\ \mathrm{W}$.

The stair lift raises a person of mass $75\ \mathrm{kg}$ through a vertical height of $3.2\ \mathrm{m}$ in a time of $20\ \mathrm{s}$.

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

Evaluate the manufacturer's claim.

You must support your answer with a calculation.

A ski lift carries skiers from the bottom of a slope to the top.

The motor of the lift is supplied with electrical energy at a rate of $45\ \mathrm{kW}$.

The lift has an efficiency of $0.65$ (65%).

The skiers are raised through a vertical height of $320\ \mathrm{m}$.

Calculate the total mass of skiers that can be raised through this height in one minute.

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

Give your answer to 3 significant figures.

mass = ............................................... kg

A cyclist stops pedalling at the top of a hill and freewheels (coasts) down to the bottom, speeding up as she goes.

At the bottom of the hill she applies the brakes and the bicycle comes to rest. The brake blocks become hot.

Explain the energy changes that take place from the top of the hill until the bicycle is at rest, in terms of energy stores and the work done by forces.

Calculate the distance the car travels while braking.

Assume that all of the energy in the kinetic energy store of the car is transferred by the work done by the braking force.

distance = ............................................... m

The same car brakes from a speed of $50\ \mathrm{m/s}$ with the same braking force.

Deduce the distance the car now travels while braking.

You do not need to do a full calculation.

distance = ............................................... m