CORE Physics (9223) | Forces and their interactions

📚 CORE Physics (9223) Study Notes: Forces and Their Interactions 📚

Hello Future Physicist!

Welcome to the exciting world of forces! Don't worry if this chapter seems tricky at first—physics is all about understanding how things move, and forces are the invisible pushes and pulls that make everything happen, from lifting your phone to orbiting the Earth.
These notes will break down forces into simple, manageable pieces, ensuring you understand the core concepts needed for your exam!

Key Goal: To understand what forces are, how they interact, and the difference between mass and weight.

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1. Defining Force: The Basics

What Exactly is a Force?

In physics, a force is simply a push or a pull that acts on an object.

When a force acts on an object, it can cause four main things to happen:

1. Change its speed (start moving, speed up, or slow down).
2. Change its direction of movement.
3. Change its shape (e.g., squeezing a rubber ball).
4. Keep it stationary (balanced forces).

Units and Measurement

The unit we use to measure force is the Newton, symbol \(N\), named after the famous scientist Sir Isaac Newton.

Important Concept: Force is a Vector Quantity
A force is a vector quantity. This means that to fully describe a force, you need two things:

• The magnitude (size or strength)
• The direction in which it acts

Analogy: Saying "I pushed the box with 10 N of force" isn't enough. You must say, "I pushed the box with 10 N of force to the East."

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2. Contact Forces vs. Non-Contact Forces

Forces can be grouped into two main categories, based on whether the objects must touch to interact.

2.1. Contact Forces

These forces require the two interacting objects to be physically touching.

• Friction: The force that opposes motion when two surfaces slide past each other (e.g., rubbing your hands together).
• Air Resistance (or Drag): Friction from air molecules opposing the movement of an object through the air (e.g., slowing down a parachute).
• Tension: The force transmitted through a rope, string, or cable when it is pulled tight.
• Normal Contact Force: The force exerted by a surface perpendicular to an object resting on it. (If you stand on the floor, the floor pushes up on you—that's the normal force!)

2.2. Non-Contact Forces (Field Forces)

These forces act on objects even when they are separated by a distance. They are transmitted through a field.

There are only three non-contact forces you need to know:

1. Gravitational Force
2. Magnetic Force
3. Electrostatic Force

Did you know? These non-contact forces are responsible for everything from keeping the Earth orbiting the Sun to static cling when you take clothes out of the dryer!

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3. Detailed Look at Non-Contact Forces

3.1. Gravitational Force

Gravity is the force of attraction between any two masses. It is the force that pulls things down towards the centre of the Earth.

• It is always an attractive force (it pulls, never pushes).
• The strength of gravity decreases the further apart the objects are.

Connection to Weight: Gravitational force is the reason why objects have weight (see section 4).

3.2. Magnetic and Electrostatic Forces

These forces share a similar rule, often described as the "opposites attract" rule:

Magnetic Force: Acts between magnets or magnetic materials.

• Like poles repel (North pushes North away).
• Opposite poles attract (North pulls South).

Electrostatic Force: Acts between charged objects.

• Like charges repel (Positive pushes Positive away).
• Opposite charges attract (Positive pulls Negative).

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4. Mass vs. Weight: The Essential Difference

This is one of the most important, and often confusing, topics. We use these terms interchangeably in everyday life, but in physics, they mean very different things!

4.1. Mass (The 'Stuff' in You)

Mass is the amount of matter (stuff) contained in an object.

• Unit: Kilograms (kg).
• Nature: It is a scalar quantity (size only, no direction).
• Key Fact: Your mass never changes, no matter where you are in the universe (Earth, Moon, or deep space).

4.2. Weight (The Force of Gravity)

Weight is the force of gravity acting on an object's mass.

• Unit: Newtons (\(N\)). (Remember, weight is a force!)
• Nature: It is a vector quantity (it always acts downwards, towards the centre of the planet).
• Key Fact: Your weight changes depending on the gravitational field strength where you are.

The Relationship: Calculating Weight

Weight is calculated using the following formula:

$$\text{Weight} = \text{Mass} \times \text{Gravitational Field Strength}$$

In symbols:

$$\mathbf{W = m \times g}$$

• \(W\) = Weight (measured in Newtons, \(N\))
• \(m\) = Mass (measured in kilograms, \(kg\))
• \(g\) = Gravitational field strength (measured in \(N/kg\))

On Earth, the gravitational field strength (\(g\)) is approximately \(10 \, N/kg\) (you should use this value unless the exam states otherwise).

Step-by-Step Example Calculation:

If an astronaut has a mass of 80 kg on Earth:

Step 1: Identify known values.
\(m = 80 \, kg\)
\(g = 10 \, N/kg\) (on Earth)

Step 2: Apply the formula.
\(W = m \times g\)
\(W = 80 \, kg \times 10 \, N/kg\)
\(W = 800 \, N\)

The astronaut weighs 800 Newtons on Earth.

Did you know? On the Moon, \(g\) is only about \(1.6 \, N/kg\). If that 80 kg astronaut landed on the Moon, their mass would still be 80 kg, but their weight would drop to only \(80 \times 1.6 = 128 \, N\)!

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5. Force Interactions: Newton's Third Law (Action-Reaction Pairs)

When studying "interactions," we focus on how forces always come in pairs. This is summarized by Newton's Third Law of Motion (simplified for core physics):

When two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction.

This means forces always occur in pairs, often called Action-Reaction Pairs.

Key Rules for Interaction Pairs

1. The two forces are equal in size (magnitude).

2. The two forces are opposite in direction.

3. Crucially: The two forces act on different objects.

Example: Pushing a Wall

Imagine you push a wall with a force of 50 N:

• Action Force: You push the wall (50 N, forward).
• Reaction Force: The wall pushes back on you (50 N, backward).

If the forces acted on the same object, nothing would ever move! Since the action force is on the wall and the reaction force is on *you*, it explains why your hand might hurt if you push too hard!

Example: Earth and Apple (Non-Contact Interaction)

When an apple falls from a tree, gravity pulls it down:

• Action: Earth pulls the Apple down.
• Reaction: The Apple pulls the Earth up.

Wait, does the apple really pull the Earth? Yes! The force is equal and opposite. However, because the Earth’s mass is absolutely enormous, the tiny upward force from the apple causes no noticeable acceleration on the Earth. The effect is entirely seen in the tiny mass of the apple.