Newton's Third Law (AQA GCSE Physics: Combined Science)

• 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):

The foot pushes the ground backwards, and the ground pushes the foot forwards. Newton's third law explains the forces that enable people to walk

• 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 who 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