CIE A Level Physics復(fù)習(xí)筆記3.1.3 Newton's Laws of Motion

Category: A-level課程, 教材筆記, 福利干貨 Date: 2022年8月16日上午11:19

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? WE-Newtons-first-law-question-image

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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.

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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.

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Exam 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.

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