// physics › Energy
Work out the kinetic energy of a moving body from its mass and speed, or solve for mass or speed.
KE = ½ m v²
Kinetic energy is just the energy something has because it is moving. A rolling football has it. A running dog has it. A parked car has none, because it is not moving. The moment anything starts to move, it gets kinetic energy.
Two things: how heavy it is, and how fast it goes. A truck has more than a bicycle going the same speed, because the truck is heavier. And the same car going fast has far more than when it goes slow. Speed matters the most — we will see why next.
Because in the formula the speed is squared (multiplied by itself). If the speed goes from 10 to 20, you do not just double the energy — you square the doubling, so it becomes 2 × 2 = 4 times bigger. That is exactly why a car crash at high speed is so much worse than at low speed: a little more speed is a lot more energy.
Sure. Take a 2 kg ball thrown at 10 m/s. The formula is KE = half × mass × speed × speed. Step 1: speed × speed = 10 × 10 = 100. Step 2: multiply by the mass: 100 × 2 = 200. Step 3: take half: 200 ÷ 2 = 100. So the ball has 100 joules of kinetic energy. A joule (J) is simply the unit we measure energy in.
Everywhere things move! A cricket ball flying to the boundary, water rushing down a slide, a skateboard rolling downhill, wind spinning a turbine. When a moving thing hits something and makes it move too, it is handing over its kinetic energy.
They are best friends. The work you do pushing something is exactly the kinetic energy it gains. Push a trolley and it speeds up — the effort you put in (work) becomes the trolley's energy of motion (kinetic energy). This idea is called the work–energy theorem.