What this chapter is about
This chapter helps you understand what forces are and how they affect objects around us. A force is simply a push or a pull that can change the motion or shape of an object. Every time you kick a football, lift a bag, or squeeze a lemon, you are applying a force.
You will learn about different types of forces such as muscular force, gravitational force, friction, and contact and non-contact forces. The chapter also explains how forces can make objects move faster, slow them down, change direction, or change their shape. Understanding how pressure depends on force and area is another important idea you will explore.
By the end of this chapter, you should be able to identify forces acting on objects, explain their effects, and calculate pressure in simple situations. This knowledge connects to many real-life situations, from understanding why sharp knives cut better to why heavy trucks have wide tyres.
Key ideas
- A force is a push or pull acting on an object. It has both magnitude (how strong) and direction (which way).
- Forces can be contact forces (requiring touch, like friction or muscular force) or non-contact forces (acting at a distance, like gravitational force or magnetic force).
- The effect of a force depends on its magnitude, direction, and the point where it is applied.
- A force can change an object's speed, direction of motion, or shape.
- Friction is a force that opposes motion between two surfaces in contact. It can be useful (helping us walk) or undesirable (causing wear in machines).
- Gravitational force is the force of attraction between any two objects. Earth pulls all objects towards its centre, which is why things fall down.
- Pressure is the force acting per unit area. The same force produces more pressure when applied on a smaller area.
- When two or more forces act on an object, the net force (combined effect) determines the object's motion.
Formulas and facts to remember
- Pressure = Force ÷ Area, or P = F/A. Pressure is measured in pascals (Pa) or newtons per square metre (N/m²).
- Weight is the gravitational force on an object: Weight = mass × acceleration due to gravity (approximately 10 m/s² on Earth).
- Friction always acts opposite to the direction of motion or attempted motion.
- Static friction prevents an object from starting to move; kinetic friction acts on a moving object.
- Gravity acts on all objects with mass, pulling them towards Earth's centre.
- A force of 1 newton is roughly the force needed to hold a 100-gram apple.
- Liquids and gases also exert pressure on surfaces they touch.
Worked examples
Example 1: Calculating pressure
Ravi stands on the ground. His weight is 500 newtons and the total area of his shoes in contact with the ground is 0.02 square metres. What pressure does he exert on the ground?
Step 1: Write down the formula. Pressure = Force ÷ Area
Step 2: Put in the values. Pressure = 500 N ÷ 0.02 m²
Step 3: Calculate. Pressure = 25,000 N/m² or 25,000 Pa
So Ravi exerts a pressure of 25,000 pascals on the ground.
Example 2: Why do sharp blades cut better?
A vegetable cutter applies a force of 20 newtons with a sharp knife. The blade edge has an area of 0.0001 m². What is the pressure?
Step 1: Use P = F/A Pressure = 20 N ÷ 0.0001 m²
Step 2: Calculate. Pressure = 200,000 Pa
Compare this with a blunt knife having edge area 0.001 m²: Pressure = 20 N ÷ 0.001 m² = 20,000 Pa
The sharp knife creates 10 times more pressure with the same force, so it cuts more easily.
Example 3: Net force on a box
Two boys push a box. One pushes with 30 newtons towards the east, and the other pushes with 20 newtons towards the west. What is the net force?
Step 1: Forces act in opposite directions, so subtract. Net force = 30 N − 20 N = 10 N towards the east.
The box will move eastward.
Common mistakes
- Thinking weight and mass are the same thing → Mass is the amount of matter (in kilograms); weight is the gravitational force (in newtons).
- Believing friction is always harmful → Friction helps us walk, write, and grip objects; it is essential in many activities.
- Confusing force and pressure → Force is the total push or pull; pressure is force spread over an area.
- Thinking non-contact forces need some invisible connection → Gravitational and magnetic forces act through empty space without any material link.
- Assuming heavier objects always exert more pressure → Pressure also depends on area; a light heel can exert more pressure than a heavy flat shoe.
Quick revision
- Force is a push or pull; measured in newtons.
- Contact forces need touch; non-contact forces (gravity, magnetism) act at a distance.
- Friction opposes motion and depends on the surfaces involved.
- Pressure = Force ÷ Area; smaller area means greater pressure for the same force.
- Net force decides whether an object speeds up, slows down, or changes direction.