In ANCAP's 2018 crash test of a Ford Ranger (1,871 kg), the seatbelt exerted a contact force of 3,400 N over 0.06 s during a 56 km/h frontal impact, a real-world demonstration of Newton's Third Law: the car decelerated but the unrestrained passenger would continue forward at 56 km/h unless a force acted.
Printable worksheets that build from foundations to exam-style questions.
A skateboarder pushes backwards against a wall and rolls forwards. The wall did not move. Where did the skateboarder's motion come from?
Which of the following is an example of a field-mediated force?
The misconceptions above claim a moving object must have a net force in its direction of motion, and that Third Law pairs cancel out. Here you can test both against the physics.
Try this: set 10 kg and μ = 0.30, then hunt for the applied force that makes the velocity trace go flat — the crate keeps moving with zero net force. Then set μ to 0, get it moving and press Release force. Finally press Show the pair to see where the reaction force actually acts.
Use the simulation. Set mass 10 kg and μ = 0.30, get the crate moving, then find the applied force that holds the velocity constant (the trace goes flat). What is it, and why is it not zero?
Use the simulation. Set μ = 0, push the crate up to speed, then press Release force so the applied force is zero. True or false: the crate slows down and stops.
True or false: Newton's Third Law action-reaction pairs can cancel each other out, resulting in no net force.
Core Content
Kick a soccer ball and it rolls across the field. Without the kick, it would have stayed still. The ball did not decide to move, something acted on it. That something was your foot pressing against the ball for a fraction of a second, in a specific direction. Remove the foot (and the ground), and the ball keeps moving. The moment of contact, the push, is a force.
Forces are vector quantities they have both magnitude (size) and direction. The SI unit of force is the Newton (N). One Newton is the force required to accelerate a 1 kg object at 1 m/s².
Forces cause objects to accelerate, decelerate, change direction, or deform. A force always requires a source, something must be exerting it on something else.
Require physical contact between objects. Examples: normal force, friction, tension, applied force. Acts only when objects touch.
Act across a distance, no contact required. Examples: gravity, magnetic force, electrostatic force. Acts through empty space via a field.
A force is a vector quantity (magnitude + direction) measured in Newtons (N); forces are either contact forces (normal, friction, tension, require physical contact) or field-mediated forces (gravity, electrostatic, magnetic, act through space via a field).
Pause, copy the highlighted definition into your book before moving on.
A book rests on a table. Which of the following correctly identifies the Newton's Third Law pair for the weight force on the book?
We just saw that forces are vector quantities, pushes or pulls with magnitude and direction. That raises a question: what happens to an object when no net force acts on it? This card answers it → Newton's First Law describes exactly that: constant-velocity motion unless acted on by an unbalanced force.
An object that is not being pushed stays put, and one that is moving keeps moving, unless something pushes back.
Formal statement: An object will remain at rest or continue moving at constant velocity in a straight line unless acted upon by a net external force.
The property that causes this behaviour is called inertia the tendency of an object to resist changes to its state of motion. Inertia is directly proportional to mass.
Object at rest. Net force = 0. Example: a book sitting on a table.
Object moving at constant velocity. Net force = 0. Example: a car at constant speed on a straight road.
Newton's First Law: an object remains at rest or at constant velocity unless a net external force acts on it; inertia is the resistance to changes in motion and is directly proportional to mass.
Pause, copy the highlighted law and definition into your book before moving on.
Complete the sentence: An object in dynamic equilibrium is ______ at a ______ velocity with a net force of ______.
We just saw that inertia keeps objects moving unless a net force acts. That raises a question: when a force acts on an object, does that object push back? This card answers it → Newton's Third Law says yes, every force is met by an equal and opposite force on the other object.
Every force has a twin, equal in size, opposite in direction, and acting on the other object.
Formal statement: For every action force exerted by object A on object B, there is an equal and opposite reaction force exerted by object B on object A.
The two forces in a Newton's Third Law pair are always: equal in magnitude, opposite in direction, the same type of force, and acting on different objects.
Newton's Third Law pairs act on different objects. When finding the net force on one object, you can only add forces acting on that same object. Forces on different objects never cancel each other.
| Scenario | Action Force | Reaction Force | Genuine Pair? |
|---|---|---|---|
| Skateboarder pushes wall | Skateboarder pushes wall backward (contact) | Wall pushes skateboarder forward (contact) | Yes, same type, different objects |
| Earth pulls ball down | Earth pulls ball down (gravitational) | Ball pulls Earth up (gravitational) | Yes, same type, different objects |
| Book on table | Weight pulls book down (gravitational) | Normal force pushes book up (contact) | No, different types, not a Third Law pair |
Scenario: A 5 kg textbook rests on a horizontal table. Identify all forces, draw the FBD, and identify the Newton's Third Law pairs.
Newton's Third Law: for every action force on object B by object A, there is an equal-magnitude, opposite-direction reaction force on object A by object B; the pair act on different objects (same force type) so they never cancel when finding the net force on one object.
Pause, copy the highlighted law into your book before moving on.
Which of these is NOT a genuine Newton's Third Law pair?
Activities
Draw, label and classify forces for three scenarios using the Vector Protocol.
For each scenario: Is the object in equilibrium? How do you know?
A person stands in a lift moving upward at constant speed. Which statement correctly describes the forces acting on the person?
A fresh five-question set drawn from this lesson's bank, feedback shown immediately.
Pick your answer, then rate your confidence.
UnderstandBand 2(3 marks) 1. Describe the difference between a contact force and a field-mediated force. Give one example of each and explain how each force is produced.
AnalyseBand 5(3 marks) 2. A horse pulls a cart forward. Using Newton's Third Law, explain why the cart also pulls back on the horse with an equal force, yet the system still accelerates forward.
EvaluateBand 6(4 marks) 3. Draw and annotate a free body diagram for a skateboarder in the moment they push off a wall. Apply the Vector Protocol. Identify all forces acting on the skateboarder only, classify each as contact or field-mediated, and identify the Newton's Third Law pair for each force.
MC answers and full explanations are shown inline as you complete each question.
Q1 (3 marks): A contact force requires physical contact between two objects to act, for example, the normal force between a book and a table. A field-mediated force acts across a distance without requiring contact, for example, the gravitational force between Earth and the Moon, produced because both objects have mass and create gravitational fields.
Q2 (3 marks): By Newton's Third Law, the cart exerts an equal and opposite force on the horse. However, these two forces act on different objects and cannot be combined. The net force on the cart is determined only by forces acting on the cart. If the horse exerts a forward force on the cart greater than any friction, the net force on the cart is forward and it accelerates.
Q3 (4 marks): Step 1: Positive direction = forward (away from wall) and upward. Step 2: Forces on skateboarder only: (a) Reaction force from wall, horizontal, forward, contact. (b) Weight, vertical, downward, gravitational. (c) Normal force from ground, vertical, upward, contact. Step 3: F_net (horizontal) = F_wall reaction = ma, skateboarder accelerates forward. Newton's Third Law pairs: (a) Skateboarder pushes wall backward / Wall pushes skateboarder forward, both contact. (b) Earth pulls skateboarder down / Skateboarder pulls Earth up, both gravitational. (c) Ground pushes skateboarder up / Skateboarder pushes ground down, both contact.
A full module quiz covering every lesson in this module, not just this one. Set aside a decent block of time and treat it like a real assessment.
Start the module quiz →In ANCAP's 2018 crash test of a Ford Ranger, the seatbelt exerted 3,400 N over 0.06 s, this is Newton's Third Law and the work-energy theorem in action. The seatbelt (contact force) acted on the occupant; without it, the occupant would obey Newton's First Law and continue forward at 56 km/h as the car decelerated.
The skateboarder's motion came from Newton's Third Law: the wall exerted an equal and opposite reaction force on the skateboarder, which was the only unbalanced horizontal force on the skateboarder, causing them to accelerate forward while the wall (attached to Earth) effectively didn't move.