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⚡ Newton's Laws of Motion

Spec 6.5.4.2.1–6.5.4.2.3 📗 Foundation
📖 In-Depth Theory

Newton's First Law

NEWTON'S FIRST LAW:
An object will remain at rest or continue moving at CONSTANT VELOCITY unless acted upon by a RESULTANT FORCE.
In other words:
No resultant force → no change in motion.
A resultant force is needed to START, STOP or CHANGE THE DIRECTION of motion.
EXAMPLES:
A book on a table: weight balanced by normal force → resultant = 0 → stays still.
A puck on frictionless ice: no horizontal forces → moves in straight line forever.
A car at constant speed: driving force = friction + air resistance → resultant = 0 → constant velocity.
INERTIA:
The tendency of an object to resist changes in its motion.
Heavier (more massive) objects have more inertia — harder to accelerate or stop.

Newton's Second Law

NEWTON'S SECOND LAW:
The acceleration of an object is proportional to the resultant force and inversely proportional to the mass.
EQUATION:
F = m × a
F = resultant force (newtons, N)
m = mass (kilograms, kg)
a = acceleration (m/s²)
Rearranging:
a = F ÷ m
m = F ÷ a
EXAMPLES:
1000 kg car, resultant force 3000 N:
a = 3000 ÷ 1000 = 3 m/s²
Greater force → greater acceleration (for same mass).
Greater mass → smaller acceleration (for same force).

Newton's Third Law

NEWTON'S THIRD LAW:
Whenever object A exerts a force on object B, object B exerts an equal and opposite force on object A.
Also called: 'action and reaction are equal and opposite'.
Key features of Third Law pairs:
EQUAL in magnitude.
OPPOSITE in direction.
ACT ON DIFFERENT OBJECTS — they cannot cancel each other.
SAME TYPE of force.
EXAMPLES:
Book on table: book pushes down on table (gravity transfers) → table pushes up on book (normal force). Equal magnitude, opposite direction.
Rocket: rocket pushes exhaust gas backward → exhaust pushes rocket forward.
Swimmer pushes water backward → water pushes swimmer forward.
Earth pulls you down (gravity) → you pull Earth up (equally) but Earth barely moves (enormous mass).
IMPORTANT: Newton's Third Law pairs must be the same TYPE of force between the same two objects.
⚠️ Common Mistake

Newton's Third Law pairs act on DIFFERENT objects — they cannot cancel. When a horse pulls a cart (action), the cart pulls back on the horse (reaction). These don't cancel because they act on different things. Newton's First Law is about ONE object — don't confuse the two.

📐 Variables
FResultant force (F) is measured in newtons (N)
mMass (m) is measured in kilograms (kg)
aAcceleration (a) is measured in m/s² (m/s²)
📐 Key Equations
F = m × a
📌 Key Note

N1: constant velocity (or stationary) if resultant = 0. N2: F = ma. N3: equal and opposite pairs on different objects. N2: bigger F → bigger a; bigger m → smaller a. N3 pairs: same type, same magnitude, opposite direction, different objects.

🎯 Matching Activity — Newton's Laws

Match each scenario to the Newton's Law it illustrates. — drag the symbols on the right to match the component names on the left.

Newton's First Law
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Newton's Second Law
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Newton's Third Law
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Newton's Second Law
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A swimmer pushes water backward — water pushes swimmer forward
Car at constant speed — resultant force is zero, velocity unchanged
Pushing a heavier trolley requires more force for the same acceleration
F = 1200 N, m = 600 kg → a = 1200/600 = 2 m/s²
⚽ FIFA Worked Examples
Newton's Second Law

A 900 kg car has a resultant forward force of 2700 N. Calculate its acceleration.

F

F = m × a, so a = F ÷ m

I

F = 2700 N, m = 900 kg

F

a = 2700 ÷ 900

A

a = 3 m/s²

🎯 Test Yourself
Question 1 of 2
1. A 1500 kg car accelerates at 2 m/s². What is the resultant force?
2. A book sits on a table. The book pushes down on the table with force 10 N (its weight). What is the Newton's Third Law reaction to this?
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