Newton’s laws

Welcome to Newton’s Laws!

In this chapter, we are going to explore how objects move and why they stop. Whether you are kicking a football, riding a bike, or watching a rocket blast off, you are seeing Newton’s Laws of Motion in action. Don't worry if some of this seems tricky at first—once you see the patterns, Physics becomes a lot like solving a puzzle!

1. How Objects Interact

Before we look at the laws, we need to understand how things "push" or "pull" on each other. Forces happen when two objects interact. These can be contact forces (where they touch) or non-contact forces (where they don't).

Types of Interactions:

• Contact Forces: Examples include friction (the "rubbing" force) and the normal contact force (the force that stops you from falling through the floor).
• Non-contact Forces: These act over a distance. Examples include gravity, magnetism, and electrostatics (static electricity).

Free Body Diagrams (Drawing Forces)

In Physics, we use vectors to represent forces. A vector is just an arrow. The length of the arrow shows how big the force is, and the direction shows where it is pointing. A free body diagram is a simple sketch of an object with these arrows drawn on it. This helps us see which way an object will move.

Quick Review: Remember that a vector has both size and direction, while a scalar only has size. Force and velocity are vectors; speed and mass are scalars!

Key Takeaway: Forces always come in pairs and are represented by arrows called vectors.

2. Newton’s First Law: The Law of "Laziness"

Newton’s First Law tells us that objects are basically "lazy"—they want to keep doing exactly what they are already doing!

The Rule: An object will remain stationary (still) or keep moving at a constant velocity unless an unbalanced force acts on it.

What are Balanced Forces?

If the forces on an object are balanced (the same size in opposite directions), the resultant force is zero. This means:
• If the object was still, it stays still.
• If the object was moving, it keeps moving at the exact same speed and in a straight line.

Example: A car driving at a steady 70 mph on a straight motorway has balanced forces. The engine's push equals the air resistance's pull.

Higher Tier: Circular Motion

If an object is moving in a circle (like a moon orbiting a planet), its direction is always changing. Since velocity is a vector (speed + direction), a change in direction means the velocity is changing. This means there must be a resultant force acting on it, even if the speed stays the same!

Did you know? This "laziness" of objects is called inertia. It’s why you lurch forward when a car brakes suddenly—your body wants to keep moving at the same speed!

Key Takeaway: No resultant force = No change in motion.

3. Newton’s Second Law: Force and Acceleration

This law explains what happens when forces are not balanced. When there is a resultant force, the object will accelerate (speed up, slow down, or change direction).

The Famous Equation:

\( Force (N) = mass (kg) \times acceleration (m/s^2) \)
Or: \( F = ma \)

Think of it this way:
1. If you push something harder (more Force), it accelerates more.
2. If the object is heavier (more Mass), it is harder to accelerate.

Analogy: Imagine pushing an empty shopping trolley versus a trolley full of bricks. You need much more force to get the heavy trolley moving at the same speed!

Higher Tier: Inertial Mass

Inertial mass is a measure of how difficult it is to change the velocity of an object. It is defined as the ratio of force over acceleration: \( m = \frac{F}{a} \).

Key Takeaway: Force equals mass times acceleration. Resultant forces cause objects to speed up or slow down.

4. Newton’s Third Law: Action and Reaction

You might have heard this one before: "For every action, there is an equal and opposite reaction."

The Rule: Whenever two objects interact, the forces they exert on each other are equal in size and opposite in direction.

Example: When you sit on a chair, you push down on the chair (Action). The chair pushes back up on you with the exact same force (Reaction). If it didn't, you'd fall through it!

Common Mistake to Avoid!

Students often confuse Newton’s Third Law with "balanced forces."
• Balanced forces (Newton's 1st Law) act on the same object (e.g., gravity pulling you down and the chair pushing you up).
• Newton’s Third Law pairs act on different objects (e.g., You push the chair, the chair pushes you).

Key Takeaway: Interactions always involve two forces that are equal, opposite, and act on different objects.

5. Terminal Velocity (Falling Objects)

When an object falls through a fluid (like air), it eventually reaches a maximum speed called terminal velocity.

Step-by-Step: A Skydiver's Journey

1. The Jump: The skydiver starts to fall. The only major force is weight (gravity). They have a huge resultant force downwards, so they accelerate fast.
2. Speeding Up: As they get faster, air resistance (friction) increases. The resultant force gets smaller, so they accelerate more slowly.
3. Terminal Velocity: Eventually, air resistance grows until it is equal to their weight. The forces are now balanced. The resultant force is zero. They stop accelerating and stay at a steady speed.

Key Takeaway: Terminal velocity happens when weight and air resistance are balanced.

6. Momentum (Higher Tier Only)

Momentum is a property of moving objects. It depends on how much "stuff" is moving and how fast it’s going.

The Formula:

\( momentum (kg\ m/s) = mass (kg) \times velocity (m/s) \)
Or: \( p = mv \)

Conservation of Momentum

In a collision (like two bumper cars hitting each other), the total momentum before the crash is the same as the total momentum after the crash, as long as no external forces act on them. This is called the Law of Conservation of Momentum.

Key Takeaway: Momentum stays the same before and after a collision.

7. Work, Power, and Energy

When a force moves an object, energy is transferred. We call this doing work.

Calculating Work Done:

\( Work\ done (J) = force (N) \times distance (m) \)
(The distance must be in the same direction as the force!)

Calculating Power:

Power is just the speed at which you do work. It’s how many Joules of energy are transferred every second.
\( Power (W) = \frac{Work\ done (J)}{time (s)} \)

Quick Tip: 1 Joule per second = 1 Watt. So, a 60W lightbulb transfers 60 Joules of energy every second!

Key Takeaway: Work is energy transfer; Power is how fast that transfer happens.

You've reached the end of the Newton's Laws notes! Take a deep breath—you're doing great. Try practicing some \( F = ma \) calculations to see these laws in action!