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4.2.3Energy Resources and Energy Transfers

Kinetic Energy

5 min read 5 parts Lesson 11 of 15 1 quiz

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Section 1 of 5Kinetic Energy Basics

4.2.3 Kinetic Energy

Any moving object stores energy in its kinetic store, and the amount depends on two quantities.

Kinetic energy
  • The energy an object has because it is moving, held in its kinetic store.
Speed
  • Kinetic energy depends on the square of the speed, so doubling the speed makes the energy four times greater.
Mass
  • For a given speed, kinetic energy is directly proportional to the mass of the object.
Units
  • Kinetic energy is measured in joules, with mass in kilograms and speed in metres per second.

Section 2 of 5Effects of Speed and Mass

The equation explains many real effects in transport and road safety.

Effects of Speed and Mass
Braking distance
  • A faster car has more kinetic energy, so the brakes must do more work to bring it to rest.
Collisions
  • A faster vehicle carries more kinetic energy, which causes more damage when it crashes.
Energy transfer
  • When an object slows down, its kinetic store transfers energy to the thermal store.
Rearranging
  • If the kinetic energy and mass are known, the equation can be rearranged to find the speed.

Section 3 of 5Using the Equation

Careful handling of the square and the units avoids most errors.

1
Write down the equation Ek = ½ × m × v² with the correct symbols and units.
2
Square the speed first, since kinetic energy depends on v² and not on v.
3
Multiply the squared speed by the mass and then by one half to find the kinetic energy in joules.
4
Check that the mass is in kilograms and the speed is in metres per second before calculating.

Section 4 of 5Worked Example

Question

  • A car of mass 1200 kg travels at 27 m/s.
  • Calculate its kinetic energy.

  1. Write the equationEk = ½ × m × v².
  2. Substitute the valuesEk = ½ × 1200 kg × (27 m/s)².
  3. Ek = ½ × 1200 × 729 = 437 400 J.

Answer The kinetic energy of the car is 437 400 J, or about 4.4 × 10⁵ J to two significant figures.

Section 5 of 5Exam Checklist & Tips

4.2.3 – Key Points for the Exam

  • Kinetic energy The energy stored in a moving object, calculated from its mass and speed.
  • Kinetic store The energy store that holds the kinetic energy of a moving object.
  • Joule The SI unit of energy, equal to one newton metre.
  • Directly proportional Two quantities increase at the same rate, so doubling one doubles the other.
  • Speed The rate of change of distance with time, in metres per second.

Common Mistakes

  • Using v instead of v² in the equation, which gives an answer that is far too small.
  • Forgetting the factor of one half in the kinetic energy equation.
  • Saying kinetic energy doubles when speed doubles; it increases by a factor of four.
  • Using a speed in km/h without first converting it to metres per second.
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