• Newton’s?third law of motion?states:

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

• 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):

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

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:

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