Newton's Three Laws of Motion
- Newton’s First Law: A body will remain at rest or move with constant velocity unless acted on by a resultant force
Worked example
If there are no external forces acting on the car, other than friction, and it is moving at a constant velocity, what is the value of the frictional force F?
- Newton’s Second Law: A resultant force acting on a body will cause a change in momentum in the direction of the force. The rate of change in momentum is proportional to the magnitude of the force
- This can also be written as F = ma
Worked example
A girl is riding her skateboard down the road and increases her speed from 1 m s-1 to 4 m s-1 in 2.5 s.If the force driving her forward is 72 N, calculate the combined mass of the girl and the skateboard.
- Newton’s Third Law: If body A exerts a force on body B, then body B will exert a force on body A of equal magnitude but in the opposite direction
- Newton’s Third Law force pairs must act on different objects
- Newton’s Third Law force pairs must also be of the same type e.g. gravitational or frictional
Worked example
Using Newton's third law describe why when a balloon is untied, it travels in the opposite direction.
Examiner Tip
You may have heard Newton’s Third Law as: ‘For every action is an equal and opposite reaction’. However, try and avoid using this definition since it is unclear on what the forces are acting on and can be misleading.
Newton’s Third Law force pairs are only those that act on different objects
