Kinetic Energy Calculator
Motion energy in joules and foot pounds from mass and speed.
What it does
Kinetic energy is the energy of motion: half the mass times the speed squared. It sets braking distances, crash damage, and sports impacts, always growing with the square of velocity rather than linearly.
The tool above takes mass and speed in metric or imperial. It returns joules first with foot pounds beside them, plus the speed converted for a quick check of the inputs.
The default of 1000 kg at 20 m/s (72 km/h) holds 200,000 J, about 147,512 ft-lb. The imperial mirror of 2205 lb at 44.7 mph lands within rounding at 199,688 J, showing the conversion path holds.
The half m v squared form follows NASA motion references with the same derivation taught in Khan Academy physics units. Both treat it as the slow speed limit; fractions of light speed need the relativistic form instead.
How to use
- Pick metric or imperial to match your numbers.
- Enter the moving mass in kg or lb.
- Enter the speed in m/s or mph.
- Read joules first for the SI answer.
- Compare foot pounds and the speed check before using the number.
Formula
- 1/2
- Half from integrating force over distance.
- mass
- Moving kilograms or converted pounds.
- speed^2
- Squared velocity, the reason speed dominates.
Joules convert to foot pounds at 0.7375621493 each. Zero speed returns zero energy at any mass, and the formula assumes speeds far below light.
Worked example
Example: 1000 kg at 20 m/s
Metric defaults: 1000 kg, 20 m/s.
- Square speed: 20^2 = 400.
- Half mass: 1000 / 2 = 500.
- Energy: 500 x 400 = 200,000 J.
- Foot pounds: 200,000 x 0.7375621493 = 147,512 ft-lb.
- Speed check: 20 x 3.6 = 72 km/h. Tool shows 200,000 J, 147,512 ft-lb, 72 km/h.
Reference table
| Speed | Energy | Reads as |
|---|---|---|
| 36 km/h (10 m/s) | 50,000 J | Quarter of the example |
| 72 km/h (20 m/s) | 200,000 J | Example here |
| 144 km/h (40 m/s) | 800,000 J | Four times the example |
Limits
- Classical speeds only; relativistic corrections ignored.
- Point mass assumed; rotation needs its own term.
- No friction, drag, or deformation in the number.
- Inputs cap at sane ranges to keep results physical.
Common mistakes
- Forgetting to square the speed.
- Mixing mph with kilos without converting.
- Dropping the half and doubling the answer.
- Reading joules as watts (energy vs power).
- Assuming double speed means double energy.
Try it yourself
Quick checks. Answers sit tucked under each question.
2 kg at 3 m/s?
0.5 x 2 x 9 = 9 J.
Double 20 m/s to 40 at 1000 kg?
800,000 J, four times 200,000.
1500 kg at 10 m/s in ft-lb?
75,000 J x 0.7375621493 = 55,317 ft-lb.
Questions
What is the kinetic energy formula?
Half mass times speed squared. 1000 kg at 20 m/s is 200,000 J.
Why does speed matter more than mass?
Speed is squared while mass is linear, so doubling speed quadruples energy.
Joules or foot pounds?
Same energy in SI vs imperial dress: multiply joules by 0.7375621493 for ft-lb.
Can energy be negative?
No. Mass is positive and squares remove direction, so motion energy stays zero or above.
What about spinning objects?
Add rotational half I omega squared beside this translational term.
When does this formula fail?
Near light speed, where relativity replaces the half m v squared form.