Derivation of P = Fv
- Moving power is defined by the equation:
- This equation is only relevant where a constant force moves a body at constant velocity. Power is required in order to produce an acceleration
- The force must be applied in the same direction as the velocity
Derivation
- The derivation for the equation P = F x v is shown below:
Derivation of P = F × v
Worked example
A lorry moves up a road that is inclined at 14.5° to the horizontal.
The lorry has mass 3500 kg and is travelling at a constant speed of 9.4 m s-1. The force due to air resistance is negligible.Calculate the useful power from the engine to move the lorry up the road.
Examiner Tip
The force represented in exam questions will often be a drag force. Whilst this is in the opposite direction to its velocity, remember the force needed to calculate the power is equal to (or above) this drag force to overcome it therefore you equate it to that value.
