Kinetic energy formula
Kinetic energy is the energy an object has because of its motion. This formula relates the mass and speed of an object to the energy stored in that motion. Because speed is squared, doubling the speed quadruples the energy — which is why speed matters so much in impacts.
What each symbol means
| Symbol | Meaning | Units |
|---|---|---|
| KE | Kinetic energy | joules (J) |
| m | Mass of the object | kilograms (kg) |
| v | Speed of the object | meters per second (m/s) |
Rearranged forms
Worked example
A 1,200 kg car travels at 20 m/s. Find its kinetic energy.
- Start with KE = ½ m v².
- Substitute m = 1200 kg and v = 20 m/s.
- Square the speed: 20² = 400.
- Multiply: KE = 0.5 × 1200 × 400.
Use SI units — kilograms for mass and meters per second for speed — to get energy in joules. If speed is in km/h, convert to m/s first by dividing by 3.6. It is easy to drop the one-half or the square; keep both.
Want the answer without the arithmetic?
Enter mass and speed and the Kinetic Energy Calculator returns the energy instantly, and can solve for mass or speed too.
How the kinetic energy formula works
The formula comes from the work needed to accelerate an object from rest up to its speed. Because speed is squared, energy rises sharply as things move faster: a car at 40 m/s carries four times the kinetic energy it has at 20 m/s, not twice. That square relationship is why stopping distances and crash forces grow so quickly with speed.
Where it is used
Kinetic energy appears across physics and engineering — vehicle safety and braking, ballistics, flywheels and energy storage, wind and turbine power, and any collision or impact analysis. Anywhere a mass is moving, this formula measures the energy that motion represents.
FAQ
What is the kinetic energy formula?
KE = ½ m v², where m is mass in kilograms and v is speed in meters per second, giving energy in joules.
Why is velocity squared in the formula?
Because the work to accelerate an object grows with the square of its speed. Doubling speed quadruples kinetic energy, a key reason high speed is so dangerous in crashes.
What units does kinetic energy use?
Joules (J) in SI units, when mass is in kilograms and speed in meters per second. One joule equals one kg·m²/s².