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IGCSE Chapter 3: Forces and Energy

Core Physics Concepts Visual Dashboard

Forces, Speed, and Motion

1. The Nature of Forces

A force is a push, pull, or twist exerted on an object. Every force has two distinct properties: a size (measured in Newtons, N) and a direction.

2. Balanced Forces (Net Force = 0 N)

When forces acting on an object are equal in size but opposite in direction, they cancel each other out. This is called a balanced force.

  • If the object is stationary, it stays stationary.
  • CRITICAL: If the object is moving, it keeps moving at a constant speed in a straight line. (It does NOT stop!)

3. Unbalanced Forces (Net Force > 0 N)

When forces are not equal or not exactly opposite, a resultant (net) force is created. Unbalanced forces cause an object to accelerate, decelerate, or change direction.

4. Speed, Distance & Time

Speed = Distance ÷ Time

Distance-Time Graphs

Why do scientists use graphs?

Scientists use graphs to describe how two variables are related. Graphs are more useful than words for describing movement because:

  • It is easier to see trends and patterns.
  • You can read any value of distance or time during the journey from the graph.
  • Other values, such as speed, can be calculated from a graph (Gradient = Speed).
  • Information about the whole journey can be seen easily at a glance.

Turning Forces (Moments)

1. The Turning Effect

Forces don't just push or pull objects in straight lines; they can also cause objects to turn or rotate around a fixed point. This turning effect is called a Moment.

  • Pivot: The fixed point around which the object turns.
  • Lever: The object that is turning.

2. Calculating Moments

The size of a moment depends on two factors: the size of the force applied, and how far away the force is from the pivot.

Moment = Force (N) × Perpendicular Distance (m)

3. The Principle of Moments (Equilibrium)

For an object to be perfectly balanced (in equilibrium), it must not rotate. This means the forces trying to turn it clockwise must perfectly cancel out the forces trying to turn it anticlockwise.

Pressure in Solids

1. What is Pressure?

Pressure is a measure of how much force is applied over a specific area. A sharp object (small area) creates high pressure, while flat snowshoes (large area) create low pressure.

Pressure = Force ÷ Area (N/m² or Pa)

Pressure in Liquids & Gases

1. Pressure and Depth in Liquids

The pressure in a liquid increases with depth. Why?

  • As you go deeper, there is more liquid above your position. The weight of this liquid, caused by gravity, pushes down on the particles.
  • Because particles in a liquid move randomly in all directions, the pressure in the liquid is equal in all directions at a specific depth.

2. Effects of Atmospheric Pressure & Temperature

Crushing Can: If you pump air out of a sealed metal container, the pressure inside becomes close to zero. The pressure outside does not change. The massive unbalanced atmospheric pressure from the outside crushes the container!

Temperature: As the temperature of a gas increases, the speed (Kinetic Energy) of the particles increases. Their collisions exert more force. Therefore, increasing temperature increases the pressure of the gas.

Diffusion of Particles

1. What is Diffusion?

The movement of the particles of each gas is called diffusion.

Diffusion is the overall random movement of particles from an area of higher concentration to an area of lower concentration.

2. The Diffusion Process

  • At the start: Particles are at high concentration in their own spaces, and zero concentration elsewhere.
  • Tap Opened: Gases start to spread out (diffuse). At random, some particles pass through the space.
  • After some time: The gases have completely diffused. There is equal concentration of both gases on both sides. Individual particles are still moving randomly!

3. Speed of Diffusion

The speed of diffusion depends on two main factors:

  • Concentration Difference: The bigger the difference in concentrations, the faster the diffusion.
  • Temperature: The higher the temperature, the faster the diffusion. Higher temperature gives particles more Kinetic Energy (KE), making them move and spread out faster.