Newton's Three Laws of Motion
Newton's First Law
- Newton's First Law states:
A body will remain at rest or move with constant velocity unless acted on by a resultant force
- If the forces on a body are balanced (the resultant force is 0), the body must be either:
- At rest
- Moving at a constant velocity
- Since force is a vector, it is easier to split the forces into horizontal and vertical forces
- If the forces are balanced:
- The forces to the left = the forces to the right
- The forces up = the forces down
- The resultant force is the single force obtained by combining all the forces on the body
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
- Newton's Second Law states:
The resultant force is equal to the rate of change in momentum. The change in momentum is in the same direction as the resultant force
- This can also be written as:
- This relationship means that objects will accelerate if there is a resultant force acting upon them
- This is derived from the definition of momentum as follows:
- An unbalanced force on a body means it experiences a resultant force
- If the resultant force is along the direction of motion, it will speed up (accelerate) or slow down (decelerate) the body
- If the resultant force is at an angle, it will change the direction of the body
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.
Examiner Tip
The direction you consider positive is your choice, as long as the signs of the numbers (positive or negative) are consistent throughout the question.It is a general rule to consider the direction the object is initially travelling in as positive. Therefore all vectors in the direction of motion will be positive and opposing vectors, such as drag forces, will be negative.
Newton's Third Law
- Newton's Third Law states:
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
- This means that every force has a paired equal and opposite force
- Newton’s Third Law force pairs must act on two 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 there 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
