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1.3.3Forces and Their Effects

Newton’s Third Law

5 min read 5 parts Lesson 14 of 32 Unit quiz

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Section 1 of 5The Law and Its Tests

1.3.3 Newton's Third Law

A third-law pair must pass three tests, all of which must be satisfied together.

Statement
  • When two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction.
Different objects
  • The two forces in a third-law pair act on two different objects, which is the first test to apply.
Same type
  • Both forces in the pair are the same type, for example both gravitational or both normal contact forces.
Equal and opposite
  • The forces are always equal in size and opposite in direction, whatever the masses of the two objects.

Section 2 of 5Examples of Third-Law Pairs

Each interaction produces two forces, one acting on each object involved.

1
A walker's foot pushes backwards on the ground, and the ground pushes forwards on the foot, which moves the walker along.
2
A rocket pushes exhaust gases downwards, and the gases push the rocket upwards with an equal force.
3
A book pulls the Earth upwards with the same gravitational force that the Earth pulls the book downwards.
4
A swimmer pushes water backwards with the hands, and the water pushes the swimmer forwards with an equal force.

Section 3 of 5Third Law or Not?

Third Law or Not?
Book on a table, weight and normal force
  • Both forces act on the book, so they form a pair of balanced forces, not a third-law pair.
Book and Earth
  • The weight of the book pulls the Earth, and the Earth's pull on the book is its weight, so these form a third-law pair.
Book and table
  • The book pushes down on the table and the table pushes up on the book, so these normal contact forces are a third-law pair.
Equal forces on one object
  • Equal and opposite forces acting on the same object balance, and they are not a third-law pair.

Section 4 of 5Worked Example

Question

  • A box rests on a horizontal floor.
  • The floor exerts an upward normal contact force on the box.
  • State the third-law partner of this force, and say where it acts.

  1. The third-law partner must be the same type of force, so it is a normal contact force.
  2. It must act on a different object, so it acts on the floor rather than the box.
  3. It must be equal in size and opposite in direction, so the box pushes downwards on the floor with the same magnitude.

Answer The partner is the downward normal contact force of the box acting on the floor.

Section 5 of 5Exam Checklist & Tips

1.3.3 – Key Points for the Exam

  • Newton's third law When two objects interact, they exert equal and opposite forces on each other.
  • Third-law pair Two forces of the same type, equal and opposite, acting on two different objects.
  • Balanced forces Two equal and opposite forces acting on the same object, giving zero resultant force.
  • Normal contact force The support force a surface exerts perpendicular to its surface on an object in contact with it.

Common Mistakes

  • Calling the weight and normal contact force on a resting object a third-law pair;
    • both act on the same object, so they are balanced forces.
  • Thinking the larger object exerts a larger force; the two forces in a pair are always equal in size.
  • Assuming third-law forces cancel out; they act on different objects, so they cannot cancel each other.
  • Forgetting that the two forces in a third-law pair must be the same type.
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