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
- The pair of forces exerted by the interacting objects are known as force pairs
- Newton's third law explains the forces that enable someone to walk
- The foot exerts a push force on the ground
- The ground exerts a push force force on the foot
- The forces are equal in magnitude and opposite in direction
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
- Vector diagrams can be used to represent Newton's third law
- Use the following three rules to help you identify a third law pair:
- The two forces in a third law pair act on different objects
- The two forces in a third law pair always are equal in size but act in opposite directions
- The two forces are always the same type: weight, normal contact force, etc.
Worked example
A physics textbook is at rest on a table. Student A draws a force diagram for the book and labels the forces acting on it as weight and normal contact force.
Student A says the diagram is an example of Newton's third law of motion. Student B disagrees with Student A. By referring to the vector diagram, state and explain who is correct.
Answer:
Answer:
Step 1: Identify the forces and objects involved
- The gravitational pull of the Earth acts downwards on the book (weight) and the push force of the table acts upwards on the book (normal contact force)
Step 2: State Newton's third law of motion
- Whenever two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction
Step 3: Check if the diagram satisfies the two conditions for identifying Newton's third law
- Newton's third law identifies pairs of equal and opposite forces, of the same type, acting on two different objects
- In this example:
- Both forces are acting on the book
- The forces acting on the book are different forces- normal contact force and weight
- The image below shows how to apply Newton's third law correctly in this case, considering the pairs of forces acting:
- The third law pairs in this scenario would be:
- The gravitational pull of the Earth on the book (weight) and the gravitational pull of the book on the Earth (weight)
- Both forces are the same type (weight)
- Both forces are equal and opposite
- The arrows in the vector diagram of the book on the table are equal and opposite, which is where lots of students get confused
- This is because the forces are balanced
Step 4: Conclude who is correct
- In this case, Student B is correct
- The vector diagram in the question is an example of Newton's first law
- In the vector diagram of the book on the table, both forces are acting on one object and the forces are not the same type
Examiner Tip
Third law pairs will always be different objects exerting the same type of force on each other. Once you have identified a set of third law pairs, then you know that the forces will be equal in magnitude and opposite in direction.
