Work done and energy transfer

Figure 5 shows a truck on a horizontal road.

A force of 1200 N pulls the truck along the road for a distance of 8.0 m.

Calculate the work done by the 1200 N force.

Use the equation

work done = force distance moved in the direction of the force

State the unit of work done.

work done =_........

unit_........

At 8.0 m the force is removed and the truck slows down until it stops.

Describe the energy transfers as the truck slows down.

A box has a mass of 90 kg.

Which of these is the weight of the box?

Figure 6 shows a truck lifting a different box.

A student calculates the change in gravitational potential energy, ΔGPE, for the box at different heights.

Figure 7 shows the results of the student’s calculations.

The student has made one incorrect calculation.

The truck lifts the box from the ground to a height of 2.0 m.

This takes a time of 5.0 s.

Using data from the graph in Figure 7, calculate the power needed to lift the box.

Use the equation

power =_........ W

Figure 8 shows an athlete training with a push sled.

The athlete pushes the sled with a force of 645 N.

Calculate the distance the sled moves when the force of 645 N does 7440 J of work on the sled.

Give your answer to an appropriate number of significant figures.

distance moved =_........ m

Figure 9 shows an electric motor lifting a set of masses.

Describe an experiment, using the apparatus in Figure 9, to determine the gravitational potential energy gained by the masses as they are lifted.

Your description should include any measuring devices to be used.

You may add to the diagram in Figure 9 if it helps your answer.

In one experiment, the change in gravitational potential energy of the masses was 5.8 J.

The total mass lifted was 320 g.

Calculate the vertical height the masses travelled through.

Use g = 10 N/kg.

height =_........ m

The efficiency of the motor was 59%.

State one reason why the motor was not 100% efficient.

A student investigates how current varies with potential difference across a filament lamp.

The student uses a power supply, a variable resistor, the filament lamp and two meters.

Part of the circuit diagram is shown in Figure 10.

Complete the circuit diagram needed for this investigation.

Figure 10

Another student repeats the investigation in part (a) using a data logger.

The data logger records observations using sensors instead of meters. The sensors are connected to a computer to collect and display the observations.

The data logger collects 555 pairs of data in 2 minutes.

The results are shown in Figure 11.

Suggest one advantage of using a data logger instead of meters in this investigation.

Describe how current varies with potential difference in the graph in Figure 11.

Use data from the graph in Figure 11 to show how the resistance changes with potential difference for the filament lamp.

Which of these equations is correct?

The unit of work is the joule.

Starting with the meaning of work, we may obtain an equivalent unit of work as Nm.

Using work F d

unit of work unit of force unit of distance Nm

The unit of potential difference is the volt.

Explain how, starting with the meaning of potential difference, we may obtain an equivalent unit of potential difference.

Figure 11 shows a pulley system that enables a person to lift a heavy barrel.

The person pulls down on the rope to make the barrel rise through 1.2 m.

The work done against gravity on the barrel is 1800 J.

Calculate the weight of the barrel.

Use the equation

work done = forcedistance moved in the direction of the force

weight of the barrel =_........ N

The efficiency of the system is 64%.

Calculate the total work done by the person.

Use the equation

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

Some energy is wasted due to friction.

Suggest another reason why some energy is wasted in using this pulley system.

*(b) A student has the equipment shown in Figure 12.

Devise an experiment to investigate how the efficiency of the pulley system varies with the weight of metal being lifted.

Your answer should include how you will use your measurements.

Three students carry out an investigation to determine their powers when running up stairs.

Figure 7 shows a diagram of the stairs with four distances, A, B, C and D, marked.

The students need to calculate the work done against gravity.

Which distance should be used in the calculation?

They take turns to run up the stairs and use a stopwatch to measure the time taken.

The students estimate their own weight.

Figure 8 shows a table of their results.

The table is not complete.

State the unit for work done.

unit for work done is_........

Use the data for student B to calculate his estimated weight.

weight =_........ N

Use the data for student C to calculate the time she takes.

time taken =_........ s

Use the data for all three students to calculate the average power of the students.

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

Identify a significant source of error in the investigation and state how this error can be reduced.

source of error

can be reduced by

This question is about energy changes.

Figure 8 shows a water slide. A person travels from the top to the bottom of the water slide.

The mass of the person, m = 72 kg. The change in vertical height, h = 7.0 m Gravitational field strength, g = 10 N / kg

Calculate the change in gravitational potential energy for the person.

Use the equation

change in gravitational potential energy = m × g × h

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

The person comes to rest after the end of the water slide.

Explain what happens to the energy as the person comes to rest after the end of the water slide.

Figure 9 shows a person pushing a box from the bottom of a slope to the top of the slope.

Explain which one of the three distances shown in Figure 9 should be used to calculate the work done against the force of friction between the box and the slope.

Calculate the kinetic energy of a tennis ball travelling at 28 m/s. The mass of the tennis ball = 58 g.

Use the equation

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

Two cyclists ride on a hilly road and go through points P, Q, R and S.

The diagram in Figure 7 shows how the vertical height of the road changes during the journey from P to S.

The greatest overall change in gravitational potential energy for each cyclist is between which two points on the journey?

The total weight of one cyclist and bicycle is 700 N.

Calculate the total amount of work done against gravity when the cyclist travels from point P to point Q in the journey.

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

The gravitational potential energy of the other cyclist changes by 11 250 J when travelling from point Q to point R.

Calculate the mass of this cyclist.

Gravitational field strength = 10 N / kg

Use the equation

∆ GPE = m × g × ∆ h

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

Explain why the total amount of work done by a cyclist between points Q and R is different from the change in gravitational potential energy of the cyclist between points Q and R.

The cyclists lubricated the chains and the wheel bearings of their bicycles before setting off.

Lubricating the chains and wheel bearings helps to

The kinetic energy of another cyclist is 2800 J.

The mass of the cyclist is 85 kg.

Calculate the velocity of this cyclist.

Use the equation

velocity =_........ m / s

Figure 4 shows a drone.

The drone has four spinning blades.

The upward force produced enables the drone to rise in the air.

The speed at which the blades spin is measured in turns per minute.

Figure 5 shows how the upward force produced by the four blades depends on the speed at which the blades spin.

Describe the relationship between upward force and speed shown by this graph.

A different drone has a mass of 4.5 kg.

This drone rises from the ground to a height of 20 m.

Calculate the change in gravitational potential energy when the drone rises through a height of 20 m.

The gravitational field strength g = 10 N/kg.

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

State the amount of useful work done by the blades as the drone rises through 20 m.

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

It takes 4 s for the drone to rise through 20 m.

Calculate the useful power developed by the blades in this time of 4 s.

useful power developed =_........ W

Which of these is the equation for work done?

A ball has a mass of 0.046 kg.

Calculate the change in gravitational potential energy when the ball is lifted through a vertical height of 2.05 m.

Use the equation

Δ GPE = m × g × Δ h

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

The ball is released.

Calculate the kinetic energy of the ball when the speed of the ball is 3.5 m/s.

kinetic energy of the ball =_........ J

The ball bounces several times.

Figure 7 shows how the height of the ball above the floor changes with time.

Use Figure 7 to estimate the maximum height that the ball reaches after the first bounce.

height after first bounce =_........ m

Explain why the ball does not bounce back to its starting height of 2.05 m.

A student plots a graph showing the height at the start and the maximum height reached after each bounce.

Figure 8 shows the student’s graph.

Describe how the maximum height reached changes with the bounce number in Figure 8.

Which of these is the equation for work done?

A ball has a mass of 0.046 kg.

Calculate the change in gravitational potential energy when the ball is lifted through a vertical height of 2.05 m.

Use the equation

Δ GPE = m × g × Δ h

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

The ball is released.

Calculate the kinetic energy of the ball when the speed of the ball is 3.5 m/s.

kinetic energy of the ball =_........ J

The ball bounces several times.

Figure 1 shows how the height of the ball above the floor changes with time.

Use Figure 1 to estimate the maximum height that the ball reaches after the first bounce.

height after first bounce =_........ m

Explain why the ball does not bounce back to its starting height of 2.05 m.

A cyclist is riding a bicycle at a steady velocity of 12 m/s.

The cyclist and bicycle have a total mass of 68 kg.

Calculate the kinetic energy of the cyclist and bicycle.

Use the equation

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

Describe the energy transfers that happen when the cyclist uses the brakes to stop.

The cyclist starts to cycle again.

The cyclist does 1600 J of useful work to travel 28 m.

Calculate the average force the cyclist exerts.

average force =_........ N

*(d) A class of students investigate the power output of each student in the class.

The class must decide whether they use a method using steps or a method using weights. The whole class must use the same method.

Plan what measurements the students should take and how these can be used to calculate and compare the power output of each student.

You may draw a diagram to help with your plan.

A cyclist is riding a bicycle at a steady velocity of 12 m/s.

The cyclist and bicycle have a total mass of 68 kg.

Calculate the kinetic energy of the cyclist and bicycle.

Use the equation

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

Describe the energy transfers that happen when the cyclist uses the brakes to stop.

The cyclist starts to cycle again.

The cyclist does 1600 J of useful work to travel 28 m.

Calculate the average force the cyclist exerts.

average force =_........ N

An athlete uses a training machine in a gym.

The display on the machine shows the time spent on the machine and the amount of energy transferred during a training session.

Figure 3 shows the displays for two different sessions by the same athlete.

Explain what the displays show about the average power of the athlete in each of these two sessions.