Part 03 · Energy, work and power · Card 3 of 8
Kinetic energy
The energy of a moving object. Kinetic energy = 0.5 x mass x speed squared.
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Analogy
The harder a ball is thrown, the more it can knock over.
Example
Cyclist
m = 80 kg, v = 5 m/sKE = 0.5 x 80 x 5 x 5KE = 0.5 x 80 x 25KE = 1,000 J
In practice
A lorry at the same speed has far more kinetic energy than a bicycle, which is why it takes longer to stop.
Practice
Flashcards
- Give the kinetic energy formula.KE = 0.5 x mass x speed squared.
Interview questions
- Explain why a bouncing ball does not return to its original height.
- The ball has gravitational energy at the top
- Some energy becomes thermal and sound on impact
- Total energy is conserved but not all stays useful
- So the next bounce has less kinetic energy
Questions people ask
Why does doubling speed quadruple kinetic energy?
Speed is squared in the formula KE = 0.5 x m x v squared. Doubling v makes v squared four times as big, so the energy is four times as big. This is why a crash at 60 km/h is far more damaging than one at 30 km/h.
What are the units in the kinetic energy formula?
Use mass in kilograms and speed in metres per second. The answer comes out in joules. If a question gives a mass in grams or a speed in km/h, convert them first, or the final energy will be wrong by a large factor.