Newton's Third Law (WJEC GCSE Physics): Revision Note

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 reaction 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

  • But the forces are both acting on the book (one object)

  • Remember that Newton's third law describes two different objects interacting

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

Higher-tier students need to learn the definition of Newton's third law that is easiest to remember. Other students need to be able to apply the law to different situations. Pairs of equal and opposite forces in Newton's third law act on two different objects.