What next?
Kinetic energy links mass and speed to work done.
How to use this calculator
- Enter the object’s mass in kilograms.
- Enter its velocity in metres per second.
- Read the kinetic energy in joules.
What your result means
Kinetic energy is the energy of motion — what a moving object would give up if brought to rest. Because velocity is squared, doubling speed quadruples the energy, which is why high speeds are so dangerous.
Why this one is different
Alongside the energy in four units, the speed you typed is turned into the height it would take to reach it in free fall — the same quantity expressed as a drop, which is far easier to picture than a number of joules. Because velocity is squared, a negative entry returns the energy of its positive twin, and the page says so rather than looking broken.
Double the speed, quadruple the energy
Kinetic energy rises with the square of velocity. A car at 60 mph carries four times the energy it had at 30 mph — not twice. That squared relationship is the single biggest reason stopping distances and crash forces climb so steeply with speed.
How it works
Kinetic energy equals one half of the mass times the velocity squared. Mass adds energy in direct proportion, but velocity is squared, so it dominates the result.
Formula
Worked example
A 1,200 kg car travelling at 20 m/s (72 km/h):
E = ½ × 1,200 × 20²
E = 0.5 × 1,200 × 400
E = 240,000 J (240 kJ)
At 40 m/s the same car carries 960 kJ — four times the energy, because kinetic energy scales with the square of speed. This is why stopping distances grow so sharply.
Velocity is squared, and that changes everything
Kinetic energy is ½mv², so doubling mass doubles the energy while doubling speed quadruples it. This is the single most useful thing the formula tells you. A car at 60 mph carries four times the energy it does at 30 mph, which is why stopping distances grow so much faster than speed and why impact severity rises so steeply. In design terms it means that limiting velocity is almost always more effective than reducing mass when the goal is to control energy.
Where the energy goes
Kinetic energy is never destroyed, only converted — into heat in brakes, deformation in a crash structure, sound, or potential energy as something climbs. That conversion is what braking distance, crumple zones and flywheel storage all exploit. Two caveats on this calculator: it uses the classical formula, which is accurate below roughly 10% of the speed of light and diverges above it, and it treats the body as translating rather than spinning. A rolling wheel or a rotating flywheel also carries rotational kinetic energy, which needs the moment of inertia rather than the mass.
Frequently asked questions
What is kinetic energy measured in?＋
Joules (J) — the SI unit of energy. One joule is a mass of two kilograms moving at one metre per second.
Does direction matter?＋
No. Kinetic energy is a scalar, so only the speed (magnitude of velocity) matters, not the direction of travel.
Why does stopping distance grow so fast with speed?＋
Brakes work by turning kinetic energy into heat at a roughly constant rate, and kinetic energy rises with the square of speed. Double your speed from 30 to 60 mph and there is four times as much energy to dissipate, so the braking distance is around four times longer — before you add reaction time.
How does kinetic energy relate to braking distance?＋
Braking must dissipate all of it, and energy rises with the square of speed. Doubling speed quadruples the energy, and therefore roughly quadruples the braking distance once reaction time is set aside.
What is the difference between kinetic energy and momentum?＋
Momentum is mass times velocity and is conserved in every collision. Kinetic energy is half mass times velocity squared and is only conserved in a perfectly elastic one. That difference is why crumple zones work.
Does rotation count as kinetic energy?＋
Yes, and it can be significant. A rolling wheel stores energy in both its forward motion and its spin, which is why rotating mass matters more than static mass in vehicle design.
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Assumptions & limitations
Engineering formulas are exact; the situations they model are not. Read your result with these limits in mind:
- Covers translational kinetic energy only. A rotating wheel or flywheel also stores rotational energy, which this does not include.
- Energy scales with velocity squared, so a small speed error produces a large energy error — double-check the units of your speed input.
- Assumes classical mechanics; the formula diverges from reality at relativistic speeds.
- Says nothing about how the energy is dissipated in a collision, which depends on crumple distance, materials and geometry.