Every day we push, pull, lift, throw, squeeze and stop things. In doing all this, we are applying a force. A force is a push or a pull. It is a fundamental idea in physics, because it explains why objects move, stop, speed up, slow down, change direction or change shape. A force cannot be seen, but its effects can always be observed and measured.
When you kick a football, it starts moving; when you catch a ball, it stops; when you squeeze a toothpaste tube, it changes shape. All these are effects of force. Force is a crucial concept not just in physics but in everyday life — from walking and running to driving a car and flying a kite. In this chapter we explore what force is, its effects, how it is measured, and the closely related idea of pressure.
2. Effects of Force
A force can have several observable effects on an object:
A force can move a stationary object: A ball at rest starts moving when kicked.
A force can stop a moving object: A moving cricket ball is stopped when caught by the wicketkeeper.
A force can change the speed of a moving object: A fast-moving bicycle can be made to go faster or slower by applying force.
A force can change the direction of a moving object: A football can be made to turn left or right by kicking it from the side.
A force can change the shape of an object: Squeezing a lump of clay or pressing a balloon changes its shape.
Often, a single force may cause more than one of these effects at the same time.
3. Contact Forces and Non-Contact Forces
Forces can be grouped according to whether they need physical contact between the objects:
Contact forces: Forces that act only when two objects touch each other. Examples:
Muscular force: The force applied by the muscles of our body, such as pushing a table or lifting a bucket.
Friction: The force that opposes the motion of one object over another when the surfaces are in contact.
Non-contact forces: Forces that act even when the objects are not in contact, at a distance. Examples:
Magnetic force: A magnet attracts or repels another magnet or iron objects without touching them.
Electrostatic force: A comb charged by rubbing attracts small bits of paper without touching them.
Gravitational force: The force by which the Earth attracts all objects towards its centre. It acts at a distance, and objects fall to the ground because of it.
4. How Forces Are Described and Measured
A force is described by its magnitude (size or strength) and its direction. The combined effect of two forces acting on an object depends on both their magnitudes and directions. When two forces act in the same direction, their effects add up; when they act in opposite directions, the larger force wins, and the object moves in the direction of the larger force.
Force is measured using a spring balance or a force gauge, and its SI unit is the newton (N). One newton is approximately the force of gravity on a mass of about 100 grams (the weight of an apple). Forces are also represented in diagrams by arrows: the length of the arrow shows the magnitude and the arrowhead shows the direction.
5. Pressure and the Effect of Area
Pressure is the force acting perpendicularly on a unit area of a surface. When you press a thumbtack, the force you apply is small, but because the point of the pin has a very small area, the pressure on the pin is very large, and it easily goes into the wall. Mathematically:
Pressure = Force / Area
The same force produces more pressure when applied over a smaller area, and less pressure when spread over a larger area. This is why:
The sharp edge of a knife cuts easily (small area, high pressure).
Nails and pins have sharp points (small area, high pressure).
A camel has broad feet so it does not sink into sand (large area, low pressure).
Tractor wheels are broad and wide (large area, low pressure), so tractors do not sink into muddy fields.
School bags have broad straps so that the force is spread over a larger area, causing less pressure on the shoulders.
The SI unit of pressure is the pascal (Pa), which is equal to one newton per square metre (1 Pa = 1 N/m²).
6. Pressure in Liquids
Liquids also exert pressure. The pressure exerted by a liquid has some important properties:
Pressure is exerted on the walls of the container in all directions.
The pressure at a point in a liquid increases with depth — deeper water exerts more pressure.
Liquids exert equal pressure at the same depth in all directions.
This is why dams are made thicker at the bottom: the water pressure is greatest at the bottom, so the dam wall must be stronger there. A swimmer feels more pressure on the ears when diving deeper into a swimming pool.
7. Atmospheric Pressure
The air around us has weight and presses on everything with a force. The pressure exerted by the envelope of air around the Earth is called atmospheric pressure. Although we do not feel it, the atmosphere presses on us with a force of about 100,000 newtons on our whole body, because air pressure pushes equally in all directions.
Atmospheric pressure has several effects:
A rubber sucker sticks to a surface because the air inside is pushed out, and atmospheric pressure holds it in place.
The atmospheric pressure decreases as we go higher into the atmosphere, which is why it is harder to breathe at high altitudes like mountains.
Liquids like ink in a dropper are drawn up because atmospheric pressure pushes the liquid into the low-pressure region.
Atmospheric pressure is measured using a barometer.
Quick Revision Tables
Table 1: Types of Force
Type
Contact?
Examples
Muscular force
Contact
Pushing a table, lifting weights
Friction
Contact
Stopping a moving object, walking
Magnetic force
Non-contact
Attracting iron nails with a magnet
Electrostatic force
Non-contact
Attracting paper bits with a charged comb
Gravitational force
Non-contact
Objects falling to the ground
Table 2: Effects of Force
Effect
Example
Move a stationary object
Kick a football
Stop a moving object
Catch a cricket ball
Change speed
Press accelerator or brakes
Change direction
Turn a bicycle by handlebar
Change shape
Squeeze clay or a balloon
Table 3: Pressure in Different Situations
Situation
Area
Effect
Sharp knife blade
Small
High pressure, easy cutting
Camel's broad feet
Large
Low pressure, no sinking in sand
Wide tractor wheels
Large
Low pressure, no sinking in mud
Broad bag straps
Large
Less pressure on shoulders
Dam built thicker at bottom
Smaller at bottom
Withstands high water pressure
Mind Map
graph TD
A["Force and Pressure"] --> B["Force: push or pull"]
B --> B1["Effects: move, stop, change speed/direction/shape"]
B --> B2["Contact: muscular, friction"]
B --> B3["Non-contact: magnetic, electrostatic, gravitational"]
A --> C["Measuring Force"]
C --> C1["Spring balance"]
C --> C2["Unit: newton (N)"]
A --> D["Pressure = Force / Area"]
D --> D1["Small area -> high pressure"]
D --> D2["Large area -> low pressure"]
D --> D3["Unit: pascal (Pa)"]
A --> E["Pressure in Liquids"]
E --> E1["Increases with depth"]
E --> E2["Thicker dam at bottom"]
A --> F["Atmospheric Pressure"]
F --> F1["Air presses all around"]
F --> F2["Measured by barometer"]
Important Diagrams (SVG)
Diagram 1: Effects of Force
Diagram 2: Pressure Depends on Area
Common Mistakes
Saying force needs contact: Forces like gravitational, magnetic and electrostatic act at a distance without any contact.
Confusing mass and force: Mass is the amount of matter; force is a push or pull measured in newtons.
Using "weight" for force of gravity: Gravitational force pulls objects to the Earth; weight is the measure of that force, but the force itself is called gravity.
Thinking pressure increases when area increases: Pressure = force/area, so pressure increases when the area decreases for the same force.
Believing air has no weight: The atmosphere has weight and exerts pressure on everything — about 100,000 N on our bodies.
Forgetting pressure in liquids increases with depth: This is why dams are thicker at the bottom and ears feel pressure when diving deep.
Calling the unit of force "pascal": The unit of force is the newton (N); the pascal (Pa) is the unit of pressure.
Exam Tips
Define force and pressure precisely at the start of any answer.
Remember Pressure = Force / Area and quote the pascal = N/m² relation.
List the effects of force with real examples — move, stop, change speed, change direction, change shape.
Classify forces into contact and non-contact with at least two examples of each.
For the camel/tractor question, explain: broad feet/wheels mean large area, low pressure, so they don't sink.
Mention the barometer for atmospheric pressure and spring balance for force.
Conclusion
Force is the push or pull that changes or tries to change the state of motion or shape of an object, and its effects can be observed in our everyday actions — moving, stopping, speeding up, turning or deforming things. Forces are divided into contact forces like muscular force and friction, and non-contact forces like magnetic, electrostatic and gravitational forces, each acting in its own remarkable way at a distance. Measuring force in newtons and understanding that pressure is force spread over an area explains a host of practical designs, from sharp knives and thin pins to camels' broad feet and thick-bottomed dams. Whether it is the pressure of deep water or the weight of the atmosphere pressing on us, the ideas of force and pressure connect the smallest push to the grandest movements of nature.