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2.3Force and Translational Dynamics

Newton’s Third Law — AP PHYSICS 1 Revision Notes & Topical Questions

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Lesson summary

2.3 Newton's Third Law — AP PHYSICS 1 notes: State Newton's third law, identify third-law pairs and distinguish them from balanced forces.Newton's third law — if object A exerts a force on object B, then B exerts a force on A equal in magnitude and opposite in directionInteractio…

  • Interaction
  • Weight and normal force on the book
  • Real third-law pairs
  • Gravity pair
  • Explanation of the book at rest
  • Three-part label

Part 1 of 6Statement

Newton's third law
  • Newton's third law if object A exerts a force on object B, then B exerts a force on A equal in magnitude and opposite in direction
Interaction
  • Interaction forces arise in pairs when two objects interact

Part 2 of 6Criteria for a third-law pair

1
The forces are the same type.
2
They have the same magnitude.
3
They act in opposite directions.
4
They act on different objects.

Part 3 of 6Example: walking

Example: walking
Pair
  • Pair the foot pushes backward on the ground and the ground pushes forward on the foot
Type
  • Type both are normal contact forces
Objects
  • Objects the forces act on the ground and on the foot respectively

Part 4 of 6Common error: book on a table

1Weight and normal force on the book
  • Weight and normal force on the book equal and opposite but on the same object and of different types, so not a third-law pair
2Real third-law pairs
  • Real third-law pairs the book pushes on the table and the table pushes on the book (normal forces)
3Gravity pair
  • Gravity pair the Earth pulls on the book and the book pulls on the Earth
4Explanation of the book at rest
  • Newton's first law, since net force is zero

Part 5 of 6Naming pairs

Naming pairs
Three-part label
  • Three-part label object A pushes/pulls on object B, and object B pushes/pulls on object A
Weight
  • Weight the gravitational pull of Earth on the object, whose pair is the pull of the object on Earth

Part 6 of 6More from the specification

More from the specification
Incline normal force on a slope
  • Incline normal force on a slope resolving object weight reveals the perpendicular contact reaction force exerted by the surface of an inclined slope.
Inertia
  • Resistance of an object to changes in its motion.
Parallel
  • Components on separate branches, each with the full e.m.f.
External force
  • Force from the surroundings.
Reference frame
  • Coordinate system used by an observer.
Inclined plane
  • Flat surface tilted at an angle.
Driving force
  • Force applied by an engine or motor to produce motion.
Velocity
  • Rate of change of displacement.
Acceleration
  • Rate of change of velocity.
Constant
  • Not changing.
Resultant
  • Vector sum of two or more vectors.

Key terms

TermMeaning
Third-law pairTwo forces of equal magnitude, opposite direction, same type, on different objects.
Normal contact forcePerpendicular contact force from a surface.
InteractionMutual action between two objects.
Newton's third lawForces between two objects are equal and opposite.

Avoid these mistakes

Avoid these mistakes
Identifying weight and the normal force as a third-law pair.
Thinking the equal forces cancel each other — they act on different objects.
Mixing force types in a pair.

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