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1.7.2Motion, Forces and Energy

Kinetic Energy — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 65 of 91 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

1.7.2 Kinetic Energy — IGCSE PHYSICS notes: Calculate kinetic energy using Ek = ½mv² and relate it to work done in speeding up or stopping an object.Kinetic energy = the energy an object has because it is moving.Any moving mass has it, such as a river, a cyclist or an air molecul…

  • What the symbols mean
  • Change in kinetic energy
  • Work done by the resultant force
  • Braking force F over braking distance d does work F × d, so F × d
  • Rearranged forms
  • Transfer pathways

Part 1 of 7Kinetic energy

Kinetic energy
Kinetic energy the energy an object has because it is moving.
Any moving mass has it, such as a river, a cyclist or an air molecule.
The faster and heavier the object, the more kinetic energy it has.
Equation
  • Ek = ½mv2.
    What the symbols mean: Ek = kinetic energy (J) · m = mass (kg) · v = speed (m s−1)
  • Ek is kinetic energy in joules (J).
  • m is mass in kilograms (kg).
  • v is speed in metres per second (m/s).

Part 2 of 7The effect of speed

1
Kinetic energy depends on the square of the speed.
2
Doubling the speed multiplies Ek by 4 and tripling the speed multiplies it by 9.
3
Doubling the mass only doubles Ek.
4
A car at 30 m/s has four times the kinetic energy of the same car at 15 m/s.
5
Kinetic energy is a scalar and never negative.

Part 3 of 7Change in kinetic energy

Speeding up transfers energy to the kinetic store and slowing down transfers energy away from it.
Change in kinetic energy
  • ΔEk = ½mv2 − ½mu2, where v is the final speed and u is the initial speed.
Ek is directly proportional to mass and to speed squared.
Doubling the kinetic energy increases the speed only by a factor of √2 (about 1.41).

Part 4 of 7Work and kinetic energy

Work and kinetic energy
Work done by the resultant force
Work done by the resultant force change in kinetic energy.
Stopping an object
  • Stopping an object its kinetic energy is transferred by friction to the thermal store of the brakes, road and air.
Braking force F over braking distance d does work F × d, so F × d
½mv2.
Braking distance d
½mv2 ÷ F.
Doubling the speed with the same braking force makes the braking distance four times longer.

Part 5 of 7Method for calculations

1
Convert the mass to kg and the speed to m/s, dividing km/h by 3.6.
2
Square the speed first.
3
Multiply by the mass and halve it.
4
Give the answer in J, or kJ if large.
Rearranged forms
  • Rearranged forms v = √(2Ek ÷ m) and m = 2Ek ÷ v2.
Write the substitution carefully to avoid squaring the whole product or forgetting to square.

Part 6 of 7Kinetic energy compared with momentum

Kinetic energy compared with momentum
Kinetic energy is a scalar; momentum (mv) is a vector.
Momentum is conserved in all collisions, while kinetic energy is conserved only in elastic collisions.
Kinetic energy is exchanged with gravitational potential energy in falling objects and pendulums.

Part 7 of 7More from the specification

1Kinetic energy and mechanical power output
  • Kinetic energy and mechanical power output multiplying accelerating force by instantaneous velocity determines mechanical power, providing a means of energy evaluation.
2Efficiency
  • Useful output divided by total input.
3Surroundings
  • Everything outside the system.
4Wasted energy
  • Energy transferred to stores that are not useful, usually thermal in the surroundings.
5Gravitational field
  • Region where mass experiences gravitational force.
6Transfer pathways
  • Transfer pathways electrical, light and sound are not stores of energy;
    • they are ways in which energy is transferred.

Key terms

TermMeaning
Kinetic energyEnergy of a moving object; Ek = ½mv².
SpeedDistance travelled per unit time.
Braking distanceDistance a vehicle moves while the brakes slow it to rest.
ScalarA quantity with size but no direction.
Work doneEnergy transferred when a force moves an object.
JouleUnit of energy.

Avoid these mistakes

1
Forgetting to square the speed, or squaring only part of the expression.
2
Using km/h instead of m/s — divide by 3.6 first.
3
Thinking doubling the speed doubles the kinetic energy — it quadruples it.
4
Giving kinetic energy a direction or a negative value.
5
Confusing kinetic energy with momentum.

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