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

A physics textbook is at rest on a dining room table. Eugene draws a free body force diagram for the book and labels the forces acting on it.

Eugene says the diagram is an example of Newton's third law of motion. William disagrees with Eugene and says the diagram is an example of Newton's first law of motion. By referring to the free-body force diagram, state and explain who is correct. 

Answer:

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

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.