Unit Analysis

d

If you don't see the animation, you need to download the Flash plug-in.

Knowing the composition of compound units helps you to check solutions of physics problems. You know that work and energy are measured in joules (J) and power is measure in watts (W). So, if you substitute the units into the equations the should reduce to the appropriate unit. Using Newton's second law we can check the force unit:

F = ma
F = kg × m/s²
F = N

Force is measured in newtons, therefore:

kg × m/s² = N

What about work, energy and power?

Work:

W = Fd
W = N × m
W = J
N × m = J

Gravitational Potential Energy:

E_p = mgh
E_p = kg × m/s² × m
E_p = N × m
E_p = J

Kinetic Energy:

E_k = ½ (mv²)
E_k = kg × (m/s)²
E_k = kg × (m/s²) × m
E_k = N × m
E_k = J

Power:

P = W ÷ t
P = Fd ÷ t
P = N × m ÷ s
P = J ÷ s
P = W
J/s = W