Newton's Third-Law Interaction Pairs
Key idea: Identify Newton's third-law pairs by reversing agent and target across two interacting bodies.
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The core idea
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Learning objectives
- Distinguish contact forces from non-contact forces
- State that mass measures the amount of matter in a body
- Describe a gravitational field as a region where a mass experiences gravitational force
- Define gravitational field strength as gravitational force per unit mass
- Apply weight = mass × gravitational field strength
- Distinguish mass from weight
- Describe the effect of balanced and unbalanced forces on a body
- Describe ways a force may change motion
- Identify action–reaction pairs on interacting bodies
- Draw free-body diagrams for force systems in at most two dimensions
- Solve three-force static equilibrium graphically
- Apply resultant force = mass × acceleration
- Relate mass to resistance to change in motion
- Explain the effects of friction on motion
- Describe falling with and without air resistance, including terminal velocity
- Describe a moment as a force's turning effect in everyday examples
- Apply moment = force × perpendicular distance from the pivot
- State the principle of moments for a body in equilibrium
- apply the principle of moments to new situations or to solve related problems
- show an understanding that the weight of a body may be taken as acting at a single point known as its centre of gravity
- Explain qualitatively how centre-of-gravity position affects stability
Retrieve: name a force precisely
A hand pushes a trolley. Complete the name force of ___ on ___. The contact force is the force of the hand on the trolley. Precise names expose which body exerts the force and which body experiences it.
One interaction, two bodies
If body A exerts a force on body B, body B simultaneously exerts an equal-magnitude, opposite-direction force on body A. The two forces:
- arise from the same interaction;
- are the same force type;
- act on different bodies;
- are found by reversing agent and target: A-on-B ↔ B-on-A.
Because the forces act on different bodies, they cannot cancel in the resultant for either single body.
Modelled example: reverse agent and target
Modelled example 1
A swimmer pushes water
Problem
Study the worked solution
Name the given force
Method
Write swimmer-on-water.Reason
The swimmer is the agent and the water is the target.Working
The force acts backward on the water.Reverse the names
Method
The partner is water-on-swimmer.Reason
It is the reverse direction of the same contact interaction.Working
It acts forward on the swimmer with equal magnitude.
Guided practice: the table and box
Guided practice 2
Find the partner to the normal force
Problem
Choose by body ownership
Hints
Hint 1: keep the interaction
Hint 2: reverse the names
View solution step by step
Separate same-body balance from the pair
Method
Choose the box’s downward contact force on the table.Reason
It has the same two bodies and force type, with agent and target reversed.Working
Weight and normal can balance on the box, but both act on the box and are not a third-law pair.
Two common confusions
- Weight and normal are a third-law pair. They can be equal and opposite, but both act on the box and arise from different interactions. The gravitational partner to Earth-on-box is box-on-Earth.
- Third-law forces cancel. They do not act on one selected body, so they cannot be added in one body’s resultant.
Contact transfer: a bouncing ball
Challenge 3
Ball and ground during a bounce
Problem
Name both bodies and then isolate the ball
Hints
Hint 1: name the interaction
Hint 2: isolate the ball
View solution step by step
Identify the contact pair
Method
Pair ground-on-ball upward with ball-on-ground downward.Reason
They reverse agent and target in the same contact interaction.Working
The two forces act on different bodies.Return to one body for acceleration
Method
On the ball, compare ground-on-ball with weight.Reason
These are the forces that contribute to the ball’s resultant.Working
During the bounce, ground-on-ball can exceed weight, producing an upward resultant.
Non-contact transfer: gravity
A falling ball and Earth exert gravitational forces on each other. Earth-on-ball is downward on the ball. Ball-on-Earth is toward the ball on Earth. These forces are equal and opposite despite the very different accelerations of the two bodies.
Independent evidence
Two skaters push off from each other. Without being told which law to use:
- name the two contact forces as agent-on-target;
- state whether they cancel;
- explain why the lighter skater has the greater acceleration although the forces are equal.
Check your reasoning
The pair is A-on-B and B-on-A, equal and opposite on different skaters, so it does not cancel on either skater. Each skater receives one pair member; for the same force magnitude, a = F/m gives the lighter skater the greater acceleration.
For mixed practice, always decide first whether the question selects one body for a resultant or two bodies for one interaction.
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Course and syllabus information
- Course
- SEC G3 Physics
- Edition
- SEC G3 Physics 2027