Comprehensive theory, key formulas, diagrams, and memory aids for Force and Pressure.
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.
A force can have several observable effects on an object:
Often, a single force may cause more than one of these effects at the same time.
Forces can be grouped according to whether they need physical contact between the objects:
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.
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 SI unit of pressure is the pascal (Pa), which is equal to one newton per square metre (1 Pa = 1 N/m²).
Liquids also exert pressure. The pressure exerted by a liquid has some important properties:
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.
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:
Atmospheric pressure is measured using a barometer.
| 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 |
| 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 |
| 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 |
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"]
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.