Kinetic Energy — A Sprinter at Top Speed (Worked Example)

Given

• m = 70 kg

• v = 8.0 m/s

Find

Kinetic energy KE.

Principle

The kinetic energy of a moving object is one-half its mass times the square of its speed.

Formula

Solution

1. Write the formula: $KE = \tfrac{1}{2}mv^2$

2. Substitute: $KE = 0.5 \times 70 \times 8.0^2$

3. Square the speed: $8.0^2 = 64$, so $KE = 0.5 \times 70 \times 64$

4. Calculate: $KE = 2240\ \mathrm{J}$

Unit check

kg × (m/s)² = kg·m²/s² = J. Units match energy.

What this means physically

2240 J is the energy the sprinter's muscles invested to reach top speed — about enough to lift the same 70 kg body roughly 3.3 m straight up (mgh).