Formulas
- Force (Newton’s second law): F = m·a, in newtons (N).
- Weight: P = m·g.
- Kinetic energy: Ek = ½·m·v², in joules (J).
- Potential energy: Ep = m·g·h.
Example
A 10 kg object with an acceleration of 2 m/s² feels a force of 20 N and weighs 98.1 N. At 5 m/s it has 125 J of kinetic energy; 3 m above the ground it has 294.3 J of potential energy.
Dropped from 3 m, it hits the ground at about 7.672 m/s (√(2·g·h)), without air resistance.
Frequently asked questions
Is mass the same as weight?
No. Mass (kg) is the amount of matter and is the same everywhere. Weight (N) is the force of gravity on it: on the Moon you weigh about 1/6 as much.
What is a joule?
The work done by a force of 1 N over 1 m. 1 kWh = 3.6 million joules.
Why is g 9.81?
It is the average acceleration of gravity at the Earth’s surface. You can change it, for example to 1.62 for the Moon.
See also
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