Newton's Third Law (Cambridge O Level Physics)

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