Newton's Third Law (Edexcel IGCSE Physics)

What is Newton's Third Law?

• 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

Newton's Third Law in Collisions

• When one object exerts a force on another object, the second object will exert an equal force on the first object in the opposite direction
• When two objects collide, both objects will react, generally causing one object to speed up (gain momentum) and the other object to slow down (lose momentum)

Newton's third law can be applied to collisions

• Consider the collision between two trolleys, A and B:
  • When trolley A exerts a force on trolley B, trolley B will exert an equal force on trolley A in the opposite direction

• In this case:

F_B–A = –F_A–B

• While the forces are equal in magnitude and opposite in direction, the accelerations of the objects are not necessarily equal in magnitude
• From Newton's second law, acceleration depends upon both force and mass, this means:
  • For objects of equal mass, they will have equal accelerations
  • For objects of unequal mass, they will have unequal accelerations