Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

4.2.4Energy Resources and Energy Transfers

Work, GPE and KE — Edexcel International GCSE Science Double Award 4SD0 - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 11 of 12

📚 Edexcel International GCSE Science Double Award 4SD0 - Physics Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read

Part 1 of 7The big idea

Work done, gravitational potential energy and kinetic energy are three descriptions of the same thing: energy being moved or stored.

The big idea
The work-energy idea says that the energy transferred to an object by a resultant force equals the change in its kinetic energy.
Where there is no energy loss, the sum of GPE and KE stays constant.
Where there is friction or air resistance, some energy is dissipated, and this is the work done against those forces.

Part 2 of 7Falling and rising without losses

For motion with negligible air resistance, GPE lost = KE gained when falling and KE lost = GPE gained when rising.

Falling and rising without losses
A ball thrown vertically up at speed u reaches a maximum height h where all KE has changed to GPE: ½mu2 = mgh, so h = u2 ÷ 2g.
At the top its speed is zero but it is still accelerating downwards at g.

On a smooth frictionless curve like a skate park the speed at any point depends only on the vertical drop from the start: v = √(2gΔh), regardless of the shape of the path.

Part 3 of 7Including work against resistance

When there is friction, write an energy balance: initial energy = final energy + work done against friction (energy dissipated).

Including work against resistance
For a car braking to rest on a level road, the braking force F acts over the stopping distance d, and F d = ½mv2.
Hence stopping distance d = mv2 ÷ 2F.
Doubling the speed multiplies the braking distance by four for the same braking force and the same mass.

For a box sliding down a rough slope, GPE lost = KE gained + work done against friction. The friction work is Ff × distance along the slope, using the sloping distance (not the vertical height).

Part 4 of 7Strategy for multi-step problems

1. Identify the start and end points and their heights and speeds. 2. Write down the energy stores at each point. 3. Include any work done by an external force (add it to the starting energy) or against a resistive force (subtract it). 4. Solve for the unknown. This approach works for roller coasters, pendulums, ramps, vehicles and projectiles.

An important point

  • energy methods give speeds and heights but not times.
  • If the question needs time you might need other equations, but for "how fast" or "how high" the energy method is quickest.

Part 5 of 7Power and energy chains

Real systems have chainsa battery supplies electrical energy to a motor, the motor does work lifting a load (GPE), and some energy is wasted.

Power and energy chains
Combine this chapter with efficiency: efficiency = useful energy out ÷ energy in.
A motor lifting 20 kg through 5 m with efficiency 40% needs 20 × 9.8 × 5 ÷ 0.40 = 2450 J of electrical energy.
Always decide which quantity is useful (GPE) and which is the input (electrical).

Part 6 of 7Exam style reasoning

Examination questions in this area often give a diagram with heights marked at different points and ask for the speed at a lower point.

Exam style reasoning
Work out the vertical height difference between the two points, equate mgΔh with ½mv2, and cancel the mass.
If a speed is given at the bottom and the question asks how much energy was lost, find GPE lost and KE gained and subtract.
For stopping distances, remember that the same work-energy equation applies: the force multiplied by the distance equals the KE removed.

When explaining, use words such as "transferred", "dissipated" and "work done against friction".

Exam style reasoning
Avoid saying energy is "lost" or "used up".
Where two vehicles of equal mass travel at different speeds, remember to use the square relationship rather than simple proportion.
State units clearly and show each substitution on a new line to collect method marks even if the arithmetic goes wrong.

Part 7 of 7Worked examples

  1. A 0.10 kg ball is thrown vertically upwards at 14 m s−1. Ignoring air resistance, find the maximum height. (g = 9.8 N kg−1)

  2. A 900 kg car travelling at 20 m s−1 brakes to rest with a constant braking force of 6000 N. Find the stopping distance.

  3. A 40 kg child slides down a slide that drops 3.0 m vertically and reaches the bottom at 5.0 m s−1. How much energy is dissipated?

Key terms

TermMeaning
Work-energy ideaNet work done on an object equals its change in kinetic energy.
DissipationEnergy spread into thermal stores through friction and drag.
Stopping distanceDistance travelled while braking to rest.
Braking forceThe force applied to stop a vehicle.
Energy balanceAn equation that equates energy before and after, including losses.

Avoid these mistakes

1
Using the length of the slope for the GPE change.
2
Forgetting to include dissipated energy when the final speed is smaller than predicted.
3
Using KE = mv2 without the half.
4
Saying work done by friction increases the object's KE; it removes energy from the motion.
5
Thinking an object at the top of a throw has no forces on it; gravity still acts.

Last updated: