newton's laws of motion |
law I |
Unless a resultant force acts on a body, its velocity will not change. i.e. if F = 0, _{}v = 0 This gives us an intuitive meaning of force: a resultant force is that agent which changes the velocity (and momentum) of a body. Law I is a special case of law II. |
law II |
The rate of change of momentum of a body is proportional
to the resultant force that acts on it. i.e. F d(mv)/dt or F = k d(mv)/dt Hence F = km dv/dt + kv dm/dt = km dv/dt (since dm/dt = 0 in most problems in classical mechanics) = kma We then choose k=1, and in so doing we also define our unit for force. F[N] = m[kg]a[ms^{-2}] 1 newton (N) is that force which accelerates a mass of 1 kg at 1 ms^{-2}. F = ma is one form of Newton's second law. |
law III |
If body A exerts a force F on body
B, then body B exerts a force F on body
A of the same size and along the same line, but in the opposite
direction. i.e. F_{AB} = - F_{BA} Law III refers to a pair of forces which must always act on two different bodies. These two forces have the same size at every instant of time. |