Which statement defines force?

Question 4

Which statement defines force?

A When a force acts on a body that is free to move, the force is the product of the mass of the body and its acceleration.

B When a force acts on a body that is free to move, the force is the rate of change of

momentum of the body.

C When a force acts on a body that is free to move, the force is the work done by the force divided by the distance moved by the body.

D When a force acts on a lever and causes a moment, the force is the moment divided by the perpendicular distance of the force from the pivot.

Reference: Past Exam Paper – March 2016 Paper 12 Q9

Solution:

Answer: B. 

Force is DEFINED from Newton’s second law of motion. This tells us that the force is proportional to the rate of change of momentum of a body.

F = Δp / Δt = Δ(mv) / Δt

The formula ‘F = ma’ is only a consequence of the second law if we assume that the mass is constant.

F = m(Δv/ Δt) = ma

Choices C and D only deal equations that contain ‘force’. These do not defined force.