Edexcel A Level Physics:復習筆記2.12 Newton's Third Law of Motion

Category: A-level課程, 教材筆記, 福利干貨 Date: 2022年9月30日下午3:32

Newton's Third Law of Motion

Newton’s?third law of motion?states:

Whenever two bodies interact, the forces they exert on each other are equal in size, act in opposite directions, and are of the same type

Newton’s third law explains the following important principles about forces:
- All forces arise in?pairs?– if object A exerts a force on object B, then object B exerts an?equal?and?opposite?force on object A
- Force pairs are of the?same type?– for example, if object A exerts a?gravitational force?on object B, then object B exerts an equal and opposite?gravitational force?on object A

Newton’s third law explains the forces that enable someone to walk
The image below shows an example of a pair of equal and opposite forces acting on two objects (the ground and a foot):

5.7.4-Newton-Third-law-in-action

Newton's Third Law: The foot pushes the ground backwards, and the ground pushes the foot forwards

One force is from the foot that pushes the ground backwards
The other is an?equal and opposite?force from the ground that pushes the foot forwards

Worked Example

A physics textbook is at rest on a dining room table.

Eugene draws a free body force diagram for the book and labels the forces acting on it. 5.7.4-WE-Newton-Third-law-Question-image ?Eugene says the diagram is an example of Newton's third law of motion. William disagrees with Eugene and says the diagram is an example of Newton's first law of motion.By referring to the free-body force diagram, state and explain who is correct.

Step 1: State Newton's first law of motion

- Objects will remain at rest, or move with a constant velocity unless acted on by a resultant force

Step 2: State Newton's third law of motion

- Whenever two bodies interact, the forces they exert on each other are equal and opposite

Step 3: Check if the diagram satisfies the two conditions for identifying Newton's third law

- In each case, Newton's third law identifies pairs of equal and opposite forces, of the same type, acting on two different objects
- The diagram only involves?one object
- Furthermore, the forces acting on the object are different types of force - one is a?contact?force?(from the table) and the other is a?gravitational force?on the book (from the Earth) - its?weight
- The image below shows how to apply Newton's third law correctly in this case, considering the pairs of forces acting:

5.7.4-WE-Newton-Third-law-Answer-image

Step 4: Conclude which person is correct

- In this case,?William is correct
- The free-body force diagram in the question is an example of?Newton's first law
- The book is?at rest?because the two forces acting on it are?balanced?- i.e. there is no?resultant force

Exam Tip

Remember that pairs of equal and opposite forces in Newton's third law act on?two different objects.?It's a really common mistake to confuse Newton's third law with Newton's first law. That also means it's a really common exam question, which often uses a version of thee example below.

Applying this check will help you distinguish between them.

Newton's first law involves forces acting on a?single?object.

These differences are shown in Scenario 1 (Newton's first law) vs. Scenario 2 (Newton's third law) 3.1.1.5-Worked-example-Newtons-third-law-pairs