A 400 km road trip taking 4.5 hours
Average speed is 400 / 4.5 ≈ 88.9 km/h — the time actually driving, including stops, is what matters here, not just the driving portions.
Kinematics & dynamics calculator
Result
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How it works
Average speed only tells you distance divided by time — it says nothing about whether the motion was steady, since a trip with stops and bursts of speed can have the same average as a constant one.
Acceleration is the rate of change of velocity, not of speed alone: a car braking or turning is also accelerating. A positive value here means speeding up over the interval, a negative one means slowing down.
Newton’s second law, F = m × a, gives the net force needed to produce a given acceleration on a given mass — it is the force required, not necessarily the only force acting (friction and gravity may need to be added or subtracted separately).
Kinetic energy grows with the square of velocity, which is why doubling a vehicle’s speed roughly quadruples the energy that has to be shed to stop it.
Formulas used
Average speed
v = d / t
- v
- Average speed
- d
- Distance travelled
- t
- Time taken
Acceleration
a = (v_f − v_i) / t
- v_i
- Initial speed
- v_f
- Final speed
Newton's second law
F = m × a
- F
- Net force, in newtons (N)
- m
- Mass, in kilograms (kg)
Kinetic energy
KE = ½ × m × v²
- KE
- Kinetic energy, in joules (J)
Worked examples
A car braking from 100 km/h to a stop in 4 s
Acceleration is (0 − 100 km/h) / 4 s ≈ −6.94 m/s², about 0.71 g of deceleration.
Assumptions and limits
- Motion is treated as one-dimensional (along a single line).
- "Force" here means the net force producing the given acceleration, not a specific applied force.
- No air resistance, friction or relativistic effects are modelled.
Frequently asked questions
Is average speed the same as velocity?
No. Speed is a magnitude (how fast); velocity also carries a direction. Average speed over a round trip that returns to the start is positive, while average velocity would be zero because the net displacement is zero.
Why is kinetic energy shown in kilocalories and watt-hours as well as joules?
The joule is the SI unit, but kilocalories make sense when comparing to food energy and watt-hours when comparing to an electrical battery — the same physical quantity, expressed in units suited to different contexts.
Updated