Force and Pressure
Force and Pressure
Understand pushes and pulls, the different types of forces, and how pressure works — with a full practice worksheet and a quick revision.
Every time you open a door, kick a ball, squeeze toothpaste, or lift your school bag, you are using a force. Forces are all around us, quietly making things move, stop, speed up, slow down, turn, or change shape.
In this chapter we will find out what a force really is, what it can do, and the different kinds of forces that act on objects. We will also learn about pressure — why a sharp knife cuts easily, why a camel walks comfortably on sand, and how the air around us presses on everything, including you.
🎯 What You Will Learn
- What a force is and how it arises from interaction
- The many effects a force can produce
- Contact forces and non-contact forces
- The meaning of pressure and the pressure formula
- Pressure exerted by liquids, gases and the atmosphere
1. What is a Force?
In everyday language, a push or a pull on an object is called a force. When you push a stalled car you apply a force; when you pull a drawer open you apply a force too.
📚 Key Idea
A force is a push or a pull on an object that arises due to the interaction between two objects. At least two objects must interact for a force to come into play.
When two forces act in the same direction, they add up and the total (net) force becomes larger. When two forces act in opposite directions, the net force is the difference between them and acts in the direction of the larger force. If two equal forces act in opposite directions, they cancel out and the net force is zero.
Example: In a game of tug-of-war, both teams pull the rope in opposite directions. The team that applies the greater force wins, and the rope moves in that direction.
2. What Can a Force Do?
A force can produce several different effects on an object. Remember these four carefully — they are asked often.
▶️ Set an object into motion
A force can make a resting object start moving — like kicking a football that was lying still.
⚡ Change the speed of a moving object
A force can make a moving object faster or slower — pedalling harder speeds up a cycle; applying brakes slows it down.
↻ Change the direction of motion
A force can turn a moving object — a batsman changes the direction of a ball by hitting it.
🪨 Change the shape of an object
A force can squeeze, stretch or bend things — pressing dough, stretching a rubber band, or crushing a can.
State of motion: The state of motion of an object is described by its speed and the direction of its motion. An object at rest has zero speed. A force can change this state of motion.
3. Types of Forces
Forces are grouped into two families: contact forces (which act only when objects touch) and non-contact forces (which act even from a distance).
Note: Gravity acts on all objects, big or small. Every object in the universe attracts every other object — this pull is the gravitational force.
4. Pressure
The effect of a force depends not only on how hard you push, but also on the area over which the force acts. This is where pressure comes in.
Pressure is the force acting on a unit area of a surface.
Pressure = Force ÷ Area
For the same force, a smaller area gives more pressure, and a larger area gives less pressure. This single idea explains many everyday designs.
Remember: A blunt knife has a larger cutting area, so it exerts less pressure and cuts poorly. A sharp knife concentrates the same force on a tiny area, giving high pressure and a clean cut.
5. Pressure Exerted by Liquids and Gases
Solids press downward on the surface they rest on. But liquids and gases behave differently — they press in every direction.
💧 Liquids
A liquid exerts pressure on the bottom and the walls of its container, and in all directions. The pressure also increases with depth — which is why the walls of a dam are built much thicker at the bottom.
🌟 Gases
A gas fills its container and exerts pressure on all the walls. That is why an inflated balloon stays taut and a well-pumped tyre feels firm.
6. Atmospheric Pressure
The Earth is wrapped in a thick blanket of air called the atmosphere, stretching many kilometres up. This air has weight, so it presses down on everything below it.
📚 Definition
The pressure exerted by the air around us is called atmospheric pressure. It acts on us from every direction.
Atmospheric pressure is surprisingly large. So why aren’t we crushed? Because the pressure of the blood and fluids inside our bodies balances the atmospheric pressure pushing in from outside.
🧠 See It in Action
- Rubber sucker: Pressing it out squeezes the air out; atmospheric pressure then holds it firmly to the wall.
- Drinking straw: Sucking lowers the pressure inside; atmospheric pressure pushes the drink up the straw.
- Crushing can: Remove the air from a sealed tin and atmospheric pressure crushes it inward.
Key Terms to Remember
📝 Practice Worksheet
Attempt every question first, then check the answer given below it.
Part A · Multiple Choice Questions
1. A force is best described as a:
(a) push only (b) pull only (c) push or a pull (d) turn only
2. Which of these is a non-contact force?
(a) Muscular force (b) Friction (c) Gravitational force (d) Push of a hand
3. For the same force, pressure is greatest when the area is:
(a) largest (b) smallest (c) doubled (d) it makes no difference
4. The pressure exerted by a liquid increases as we go:
(a) shallower (b) deeper (c) sideways only (d) it stays the same
5. We are not crushed by atmospheric pressure because:
(a) it is very small (b) our body pressure balances it (c) air has no weight (d) we are too strong
Part B · Fill in the Blanks
1. A force arises due to the __________ between two objects.
2. The force of __________ always opposes the motion of one surface over another.
3. Pressure is equal to force divided by __________.
4. The envelope of air surrounding the Earth is called the __________.
5. Camels have __________ feet so they do not sink into the sand.
Part C · True or False
1. A force can change the shape of an object.
2. Magnetic force is a contact force.
3. Liquids exert pressure in all directions.
4. A sharp knife exerts less pressure than a blunt one.
Part D · Match the Following
Part E · Very Short Answer
1. Name two contact forces.
2. Write the formula for pressure.
3. Which force pulls all objects towards the Earth?
Part F · Short Answer
1. Why are the walls of a dam made thicker at the bottom?
2. Give any three effects a force can produce.
3. Why is it easier to cut vegetables with a sharp knife than a blunt one?
Part G · Long Answer
1. Explain the difference between contact and non-contact forces with two examples each.
2. What is atmospheric pressure? Describe one activity that demonstrates it.
Part H · Think Higher (HOTS)
1. Two friends push a box from opposite sides with forces of 40 units and 25 units. In which direction does the box move, and what is the net force?
2. Why does a heavy truck have many wide tyres instead of a few narrow ones?
3. When you suck a drink through a straw, what actually pushes the liquid up?
⚡ Quick Revision
- Force is a push or a pull that arises from the interaction of two objects.
- Forces in the same direction add up; forces in opposite directions give a net force equal to their difference.
- A force can set an object in motion, change its speed, change its direction, or change its shape.
- Contact forces: muscular force and friction. Non-contact forces: magnetic, electrostatic and gravitational.
- Friction always opposes motion; gravity pulls everything towards the Earth.
- Pressure = Force ÷ Area. Smaller area means greater pressure (sharp knives, nails); larger area means less pressure (wide straps, camel feet).
- Liquids and gases exert pressure in all directions; liquid pressure increases with depth.
- Atmospheric pressure is the pressure of the air around us; our internal body pressure balances it, so we are not crushed.
