Kinetic energy
Kinetic energy is the energy which makes objects move or vibrate.
Calculating kinetic energy
If we know the:
- mass of the object, in kilograms,
kg - speed of the object, in metres per second,
m/s
…then we can calculate the amount of kinetic energy it has!
The formula we use is:
If we put it into symbols, we get:
E_k = \frac{1}{2}mv^2
Where:
E_k is the kinetic energy, in joules,J m is the mass of the object, in kilograms,kg v is the speed of the object, in metres per second,m/s
Examples
A car has a mass of 1000 kg and is moving at a speed of 20 m/s. What is its kinetic energy?
m = 1000\,kg v = 20\,m/s E_k = \frac{1}{2}mv^2 E_k = \frac{1}{2} \times 1000\,kg \times (20\,m/s)^2 E_k = 200,000\,J
A cyclist has a kinetic energy of 1400 J and is moving at a speed of 7 m/s. What is the mass of the cyclist?
E_k = 1400\,J v = 7\,m/s E_k = \frac{1}{2}mv^2 m = \frac{2E_k}{v^2} m = \frac{2 \times 1400\,J}{(7\,m/s)^2} m = 57.14\,kg
A ball has a kinetic energy of 50 J and a mass of 0.5 kg. What is the speed of the ball?
E_k = 50\,J m = 0.5\,kg E_k = \frac{1}{2}mv^2 v = \sqrt{\frac{2E_k}{m}} v = \sqrt{\frac{2 \times 50\,J}{0.5\,kg}} v = 14.14\,m/s