3.5MechanicsCore

Newton’s Laws of Motion

Three short statements, written over 300 years ago, still form the backbone of every mechanics calculation in this course. From why a rocket needs to expel gas to move, to why your body lurches forward when a car brakes suddenly, Newton’s three laws quietly explain almost everything about how forces produce motion.

40 min Video by Physics Online Module 3.5 Newton’s laws of motion and momentum

Newton’s Laws Explained — A Level PhysicsWatch the full walkthrough before the notes below.

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What you'll be able to do

State Newton’s first law and explain it in terms of inertia
State and use Newton’s second law, $F = ma$
State Newton’s third law and correctly identify force pairs
Apply Newton’s laws to solve problems involving connected objects

Newton’s first law

Newton’s first law states that an object remains at rest, or continues moving at constant velocity in a straight line, unless acted on by a resultant (net) external force. This property of "resisting a change in motion" is called $inertia$ , and it depends entirely on an object’s mass — a more massive object is harder to start moving, and harder to stop once moving.

Tip — The first law is really a statement about EQUILIBRIUM: zero resultant force means zero acceleration, whether the object happens to be stationary or cruising at constant velocity.

Newton’s second law

Newton’s second law states that the resultant force on an object equals the rate of change of its momentum — for constant mass, this simplifies to the familiar $F = ma$ : resultant force equals mass times acceleration.

F = ma

Newton’s second law (constant mass).

1Use

a = \frac{F}{m}

a = \frac{45}{15}

Answer

a = 3.0

m s⁻²

Tip — For connected objects, it’s often quickest to treat the whole system together first (to find acceleration), then isolate just ONE object to find an internal force like tension.

Newton’s third law

Newton’s third law states that if object A exerts a force on object B, then object B exerts an equal and opposite force on object A. These two forces form a $Newton’s third law pair$ : they are always the same type of force, act on two DIFFERENT objects, and act simultaneously — never on the same object, and never cancelling each other out.

1The third-law pair to "Earth pulls book down" must be the same TYPE of force (gravitational), acting on the OTHER object involved.

2That pair force is: the book pulls the Earth upward, with an equal gravitational force.

3Note this is NOT the same as the normal contact force from the table — that is a completely different (contact) force, and its own third-law pair is "book pushes down on table".

AnswerThird-law pair to the book’s weight: the book’s gravitational pull on the Earth (equal and opposite, acting on the Earth).

Tip — The most common exam mistake here: pairing weight (gravity, downward, on the book) with the normal contact force (a completely different TYPE of force, upward, also on the book). A genuine third-law pair is always the SAME type of force, acting on the OTHER object.

Equation recap

F = ma

Newton’s second law.

Common mistakes to avoid

Pairing an object’s weight with the normal contact force acting on it as a "Newton’s third law pair".

A genuine third-law pair must be the SAME type of force, acting on the OTHER object — weight and normal force are different force types acting on the same object, so they are not a third-law pair (they simply happen to balance in equilibrium).

Applying F=ma to just one block in a connected system, using the total applied force.

Only include the forces actually acting on the chosen object — a connected block only feels the tension (or other internal contact forces) linking it to the rest of the system, not the full external applied force.

Thinking a stationary object has no forces acting on it at all.

The first law describes zero RESULTANT force, not zero forces — a stationary book still has weight and a normal contact force acting on it; they simply balance to zero.

Key takeaways

Newton’s first law: zero resultant force means zero acceleration (object is in equilibrium).
Newton’s second law: F = ma (resultant force = mass × acceleration).
Newton’s third law: force pairs are equal, opposite, the same TYPE of force, and act on two DIFFERENT objects.
For connected objects, find system acceleration first (treating them as one mass), then isolate a single object to find internal forces like tension.

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Newton’s Laws of Motion

40 min Video by Physics Online Module 3.5 Newton’s laws of motion and momentum