Unit 4: (c) Work and power

What is work done in physics?

Work done is the energy transferred when a force moves an object through a distance.

What is the equation for work done?

W = F × d

What does W represent in the work done equation?

Work done measured in joules (J).

What does F represent in the work done equation?

Force measured in newtons (N).

What does d represent in the work done equation?

Distance moved in the direction of the force measured in metres (m).

What is the unit of work done?

Joule (J).

When is work done equal to zero?

When there is no movement in the direction of the force.

Why must the distance be in the direction of the force for work to be done?

Only the component of the force acting along the direction of movement transfers energy.

A person pushes a wall but it does not move. Is work done?

No, because the distance moved is zero.

A force of 50 N moves an object 4 m. Calculate the work done.

W = F × d = 50 × 4 = 200 J.

An object has 300 J of work done on it. What does this mean?

300 J of energy has been transferred to the object.

What is the relationship between work done and energy transferred?

Work done is equal to the energy transferred.

Why is work done measured in joules?

Because doing work transfers energy.

A force acts on an object but no energy is transferred. Explain.

The object has not moved in the direction of the force, so no work is done.

What happens to work done when force increases?

More energy is transferred because more force is used.

What happens to work done when distance increases?

More energy is transferred because the force acts over a greater distance.

What is gravitational potential energy (GPE)?

The energy stored in an object because of its position in a gravitational field.

What is the equation for gravitational potential energy?

GPE = m × g × h

What does m represent in the GPE equation?

Mass measured in kilograms (kg).

What does g represent in the GPE equation?

Gravitational field strength measured in newtons per kilogram (N/kg).

What does h represent in the GPE equation?

Height raised measured in metres (m).

What is the unit of gravitational potential energy?

Joule (J).

What value of gravitational field strength is usually used on Earth?

9.8 N/kg (often approximated as 10 N/kg in some questions).

Calculate the GPE of a 5 kg object lifted 10 m.

5 × 9.8 × 10 = 490 J.

A 20 kg object gains 2000 J of GPE. How could this happen?

It could be lifted higher, have a larger mass, or be in a stronger gravitational field.

Why does lifting an object increase its GPE store?

Work is done against gravity, transferring energy to the object's gravitational potential energy store.

What happens to GPE when an object falls?

The gravitational potential energy store decreases.

What happens to energy when an object is lifted?

Chemical energy store of the person decreases and gravitational potential energy store of the object increases.

What is kinetic energy (KE)?

The energy stored in a moving object.

What is the equation for kinetic energy?

KE = ½ × m × v²

What does m represent in the kinetic energy equation?

Mass measured in kilograms (kg).

What does v represent in the kinetic energy equation?

Speed measured in metres per second (m/s).

What is the unit of kinetic energy?

Joule (J).

Calculate the kinetic energy of a 4 kg object moving at 5 m/s.

KE = ½ × 4 × 5² = 50 J.

Why is speed squared in the kinetic energy equation?

Because kinetic energy increases much faster when speed increases.

If speed doubles, what happens to kinetic energy?

Kinetic energy increases by a factor of four.

If mass doubles, what happens to kinetic energy?

Kinetic energy doubles.

Why does a faster car have much more kinetic energy than a slower car?

Kinetic energy depends on the square of speed.

Why are high-speed vehicles harder to stop?

They have a much larger kinetic energy store, requiring more energy transfer to stop them.

How does conservation of energy link GPE and KE?

Energy can transfer between gravitational potential energy and kinetic energy stores without being created or destroyed.

Describe energy changes when an object falls.

GPE decreases and KE increases as energy is transferred from the gravitational potential energy store to the kinetic energy store.

Describe energy changes when an object is thrown upwards.

KE decreases and GPE increases as energy is transferred to the gravitational potential energy store.

Why does a falling object not gain all its GPE as KE?

Some energy is transferred to thermal energy stores due to air resistance.

What happens to energy during a roller coaster ride?

GPE is transferred to KE when going downhill and KE is transferred back to GPE when climbing.

How is work done related to changes in energy stores?

Work done transfers energy between stores.

A machine lifts an object. What energy transfer occurs?

Work is done, transferring energy from the machine's energy source to the object's gravitational potential energy store.

What is power?

Power is the rate of energy transfer or the rate of doing work.

What is the equation for power?

P = W ÷ t

What does P represent in the power equation?

Power measured in watts (W).

What does W represent in the power equation?

Work done or energy transferred measured in joules (J).

What does t represent in the power equation?

Time taken measured in seconds (s).

What is the unit of power?

Watt (W).

What does 1 watt mean?

1 watt means 1 joule of energy is transferred every second.

Calculate the power of a machine transferring 600 J in 30 seconds.

P = 600 ÷ 30 = 20 W.

A machine has a higher power rating. What does this mean?

It transfers energy or does work more quickly.

What is the difference between energy and power?

Energy is the amount transferred; power is how quickly it is transferred.

Two machines do the same work. One takes less time. Which has greater power?

The machine that takes less time has greater power.

Why are powerful motors useful?

They can transfer energy and do work more quickly.

How can power be increased?

Increase the amount of work done or reduce the time taken.

A 1000 W motor runs for 60 seconds. How much energy does it transfer?

Energy transferred = power × time = 1000 × 60 = 60000 J.

How is the equation P = W ÷ t rearranged to find work done?

W = P × t

How is the equation P = W ÷ t rearranged to find time?

t = W ÷ P

A 500 W device operates for 20 seconds. Calculate energy transferred.

W = 500 × 20 = 10000 J.

Why is power measured in watts rather than joules?

Power measures the rate of transfer, while joules measure the amount of energy.

Explain the energy changes when a ball is dropped.

The gravitational potential energy store decreases and kinetic energy store increases; some energy is transferred to thermal and sound energy.

Explain why stopping a moving object requires work.

A force acts over a distance to remove kinetic energy from the object.

Explain why braking a car produces heat.

Work is done against friction, transferring kinetic energy into thermal energy.

A heavier object and a lighter object fall from the same height. Which has more GPE?

The heavier object because GPE is proportional to mass.

Two objects have the same mass but one is twice as high. Compare their GPE.

The higher object has twice the GPE because height is directly proportional to GPE.

Two objects have the same mass but one moves twice as fast. Compare their KE.

The faster object has four times the kinetic energy because speed is squared.

Why is energy transferred when lifting an object?

A force acts over a distance against gravity, so work is done.

Why is climbing stairs an example of work done?

The person's muscles exert a force and move their body upwards.