Kinetic energy · Year 9
In Year 9 you learn: kinetic energy = ½mv².
Anything that moves has energy in its kinetic store. The kinetic energy depends on the mass and, even more, on the speed: KE = ½mv². Doubling the mass doubles the kinetic energy, but doubling the speed makes it four times bigger. That is why a small rise in a car’s speed needs a much longer distance to stop.
Key facts
- Kinetic energy KE = ½mv².
Worked examples
Example 1
A 0.5 kg ball moves at 4 m/s. Find its kinetic energy.
Answer: 4 J
Example 2
A 60 kg runner moves at 5.0 m/s. Find her kinetic energy.
Answer: 750 J
Example 3
A 1,200 kg car travels at 15 m/s. Find its kinetic energy.
Answer: 135,000 J (135 kJ)
Example 4
A 1,000 kg car travels at 20 m/s. What is its kinetic energy in kJ?
Answer: 200 kJ