Edutropica - Beginner Physics

The Foundation of Classical Mechanics

Sir Isaac Newton formulated three fundamental laws that describe how objects move. These laws are the foundation of classical mechanics and explain almost all motion we see in everyday life.

⚡ The Three Laws at a Glance:

• First Law: Objects resist changes to their motion (inertia)

• Second Law: Force = Mass × Acceleration (F = ma)

• Third Law: Every action has an equal and opposite reaction

1st Law

"An object stays at rest or in constant motion unless acted on by a force"

Also called the Law of Inertia

2nd Law

"F = m × a"

Force causes acceleration

3rd Law

"Every action has an equal and opposite reaction"

Forces always come in pairs

🎯 Which Law? Quick Practice

The Law of Inertia

📜 Newton's First Law States:

An object will remain at rest or continue to move at a constant velocity unless acted upon by a resultant force.

⚡ What This Means:

If resultant force = 0 N:
• Stationary objects stay stationary
• Moving objects keep moving at constant speed in a straight line

If resultant force ≠ 0 N:
• The object will accelerate (change velocity)

💡 What is Inertia?

Inertia is an object's resistance to changing its motion.

• More mass = more inertia = harder to start/stop
• A heavy truck has more inertia than a bicycle
• This is why heavy objects are harder to push.

Low Mass

Low inertia
Easy to start/stop

Medium Mass

Medium inertia
Moderate effort needed

High Mass

High inertia
Hard to start/stop

Example 1: Car at Constant Velocity

A car travels at a constant 30 m/s on a motorway.

Forces acting:
• Driving force forward: 2000 N
• Air resistance + friction backward: 2000 N

Resultant force: 2000 − 2000 = 0 N

Because resultant = 0, the car maintains constant velocity (Newton's 1st Law).

Example 2: Tablecloth Trick

When you pull a tablecloth quickly from under plates:

• The plates have inertia — they resist motion
• The quick pull doesn't give enough time for friction to accelerate them
• The plates stay (roughly) in place.

This demonstrates Newton's First Law in action.

🎯 First Law Practice

The Relationship Between Force, Mass, and Acceleration

📜 Newton's Second Law States:

The acceleration of an object is directly proportional to the resultant force and inversely proportional to its mass.

The Most Important Equation in Mechanics:

F = m × a

F = Resultant Force (N) | m = Mass (kg) | a = Acceleration (m/s²)

⚡ What This Means:

• Bigger force → Bigger acceleration (directly proportional)
• Bigger mass → Smaller acceleration (inversely proportional)

Rearranged forms:
• $a = \frac{F}{m}$ (find acceleration)
• $m = \frac{F}{a}$ (find mass)

Example 1: Finding Acceleration

A 1000 kg car has a resultant force of 4000 N. Find its acceleration.

Step 1: Write the formula
F = m × a

Step 2: Rearrange for a
a = F / m

Step 3: Substitute and calculate
a = 4000 / 1000
a = 4 m/s²

Example 2: Finding Force

A 70 kg sprinter accelerates at 5 m/s². What force do they exert?

F = m × a
F = 70 × 5
F = 350 N

Example 3: Finding Mass

An object accelerates at 8 m/s² when a force of 200 N is applied. What is its mass?

m = F / a
m = 200 / 8
m = 25 kg

🎯 F = ma Practice

Forces Always Come in Pairs

📜 Newton's Third Law States:

For every action, there is an equal and opposite reaction.

⚡ What This Means:

• Forces always come in pairs
• If object A pushes object B with force F...
• ...then object B pushes object A with force F in the opposite direction

Key point: The forces act on different objects.

Action Force
A pushes B

⟷

Reaction Force
B pushes A

🧱 Example: Pushing a Wall

Action:
You push wall with 50 N →

=

Reaction:
← Wall pushes you with 50 N

The wall doesn't move because it's fixed. You don't move because the floor provides friction.

🪑 Example: Sitting on a Chair

Action:
Your weight pushes down on chair

=

Reaction:
Chair pushes up on you (normal force)

🚀 Example: Rocket Launch

Action:
Rocket pushes exhaust gases down

=

Reaction:
Exhaust gases push rocket up

This is how rockets work in space — no air needed

🏊 Example: Swimming

Action:
Swimmer pushes water backward

=

Reaction:
Water pushes swimmer forward

💡 Common Misconception:

Wrong: "A book on a table — weight down and normal force up are action-reaction pairs"

Correct: These are not an action-reaction pair because they both act on the same object (the book).

The actual pairs are:
• Book pushes table down ⟷ Table pushes book up
• Earth pulls book down ⟷ Book pulls Earth up

🎯 Third Law Practice

Putting It All Together

⚡ Summary of the Three Laws:

1st Law: No resultant force → no acceleration (constant velocity or at rest)

2nd Law: Resultant force → acceleration (F = ma)

3rd Law: Forces come in pairs (action = reaction, opposite directions, different objects)

Example: Car Accelerating Then Cruising

A 1200 kg car starts from rest and accelerates.

Phase 1: Accelerating
• Driving force: 3600 N forward
• Resistive forces: 1200 N backward
• Resultant: 3600 − 1200 = 2400 N
• Using F = ma: a = 2400 / 1200 = 2 m/s²
• (Newton's 2nd Law)

Phase 2: Constant Velocity
• Driving force: 1200 N forward
• Resistive forces: 1200 N backward
• Resultant: 0 N
• The car maintains constant velocity
• (Newton's 1st Law)

Law	Key Concept	Formula/Rule	Example
1st	Inertia	No force = no change in motion	Seatbelts, objects sliding when car brakes
2nd	F = ma	Force causes acceleration	Kicking a ball, car engine power
3rd	Action-Reaction	Equal and opposite pairs	Walking, rockets, swimming

Real Life Uses:
• Seatbelts: Stop you continuing forward (1st Law) when car stops suddenly
• Airbags: Increase time of deceleration, reducing force (2nd Law)
• Rockets: Push exhaust down, exhaust pushes rocket up (3rd Law)
• Walking: Push ground backward, ground pushes you forward (3rd Law)
• Sports: Heavier athletes need more force to accelerate (2nd Law)

🎯 Final Challenge: Identify the Law

2.3 Newton's Laws

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