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Force A force is that which can cause acceleration Vector quantity Newtons (N)
Newton's Laws of Motion:
Law 1: A body will remain at rest or continue moving at constant velocity unless acted upon by an unbalancing external force Application: Seat belts are used to stop passengers in cars from continuing at constant velocity when the car decelerates rapidly
Law 2: The rate of change of momentum is proportional to the applied force and takes place in the direction of the force Application: Heavier objects require more force to move the same distance as lighter objects
Law 3: For every action there is an equal and opposite reaction Action and reaction do not happen on the same body Application: When a rocket takes off, the rocket's action is to push down with the force of its engines, and the reaction is that the ground pushes the rocket upwards with an equal force
F = ma A special case of Newton's 2nd law Relies on the fact that the standard unit of force, the Newton, is defined as 1 N = 1 kg m/s²
Derivation Based on Newton's second law: F ∝ (change in momentum) / time taken F ∝ (final momentum  initial momentum)/ time F ∝ (mv  mu)/ t F ∝ m((vu)/t) Substitute (vu)/t for 'a' from the first equation of linear motion: F ∝ ma Multiply by a constant (to go from "proportional to" to "equal to"): F = kma The unit Newton is defined in such a way as to make k = 1: F = ma
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