Newton's Third Law (Oxford AQA IGCSE Combined Science Double Award)

A physics textbook is at rest on a table. Student A draws a free body 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:

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

  • You will learn more about balanced forces in Resultant Forces

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

Free-body force diagrams always focus on multiple forces acting on one object, so these are used for Resultant Forces. 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.