You are here
What is force?
- A force is a push or a pull on an object.
- Example — When we push or pull a door, we apply force.
What can a force do to an object?
- A force can make an object move from rest, change its speed, change its direction, or change its shape.
| What a force does | Example |
|---|---|
| Makes an object move from rest | A ball at rest starts moving when you kick it |
| Changes the speed of a moving object | If we push a rickshaw, its speed increases |
| Changes the direction of a moving object | A cricket bat hits a ball and the ball changes its direction |
| Changes the shape of an object | A lemon gets squeezed by the force applied by your fingers |
Can we measure force?
Real life example
- When we hold a 100 g namkeen packet in our hand, what happens?
- We apply a force of about 1 N on the packet, in the upward direction.
Important points
- We can measure a force.
- A force is expressed in newtons.
- We write it as 1 N, with a capital N.
- We also write the direction, here upward.
Why is direction always written along with a force?
- A force is never just a push. It is a push in some direction.
- The same force gives a different result when its direction is changed.
- A force can act in many different directions.
| Direction | Example |
|---|---|
| Downward | The Earth pulls a stone down towards itself |
| Upward | Water pushes a floating ball up |
| Forward | Your foot pushes the ball forward when you kick it |
| Backward | Friction pulls a rolling ball backward and slows it |
| Towards each other | Two unlike poles of magnets pull each other closer |
| Away from each other | Two like poles of magnets push each other apart |
- So the direction has to be stated every time, or the force is only half described.
- Notice that every force you learnt earlier was described with its direction.
What has to be stated for every force?
Magnitude
how strong the push or pull is
Unit
the newton
Direction
the line along which it acts
↓
A force of 5 N is not complete until you say 5 N towards the right
Everyday example: holding a 100 g namkeen packet is a force of about 1 N, upward
Which forces do we already know?
| Force | Direction in which it acts | Example |
|---|---|---|
| Force of friction | Opposite to the direction of motion of the object | A ball rolling on the ground slows down |
| Magnetic force | Like poles repel, unlike poles attract | Two like poles of a magnet push each other away |
| Electrostatic force | Like charges repel, unlike charges attract | Two unlike charges pull each other closer |
| Gravitational force | The Earth attracts objects towards itself | A stone falls down when you drop it |
| Buoyant force | Upward, on an object placed in a liquid | Water pushes up on a ball floating in it |
What is the unit of force?
- The SI unit of force is the newton, whose symbol is N.
📌 Note
- When we write newton, it is a small n.
- When we write N, it is a capital N.
What kind of quantity is force?
- It is a physical quantity because it can be measured.
-
For a force we must state three things.
- Magnitude — the size of the force, which expresses its strength
- Unit — the newton
- Direction — the line along which the force acts
-
The quantities of Chapter 4 worked the same way.
- Example: Position, displacement, velocity and acceleration each needed a direction too.
What do we mean by a physical quantity?
Physical
something in the world around us
Quantity
an amount that
can be measured
↓
Anything in science that can be measured, written as a number with a unit
📌 Note — What are physical quantities?
- A physical quantity is anything in science that can be measured.
- Every physical quantity is written as a number with a unit.
- General example: The length of a table is 2 m. The number is 2 and the unit is the metre.
- Force example: The force applied on a box is 5 N. The number is 5 and the unit is the newton.
- Some physical quantities also need a direction to be complete.
- Force is one of them. A force of 5 N is not complete until you say 5 N towards the right.
What happens if the force is changed?
- The effect of a force depends on its magnitude and its direction.
- If the magnitude changes, the effect of the force changes.
- If the direction changes, the effect of the force changes.
- If both change, the effect changes as well.
GIF
Watch: the effect of a force depends on its magnitude and its direction
![6.1 The Concept of Force - How Forces Affect Motion - [Teachoo] - Concepts](https://cdn.teachoo.com/ea96c065-5bde-45dc-a714-10edcb49c078/animation-for-what-happens-if-the-force-is-changed-.-a-short-looping-gif-three-or-four-beats-drawn-in-the-flat-classroom-style-of-the-cha.gif)
📝 Important points about force
| Point | Detail |
|---|---|
| What it does | Moves an object from rest, changes speed, changes direction, changes shape |
| What must be stated | Magnitude, unit and direction |
| SI unit | newton |
| Symbol | N |
| Meaning of magnitude | The strength of the force |
💡 Worth remembering
The unit is written as newton with a small n, but its symbol is a capital N.
✅ Quick self-check
-
Name two things a force can do to a moving object.
View Answer
Change its speed, change its direction. -
What is the SI unit of force?
View Answer
The newton. -
In which direction does the force of friction act on a moving object?
View Answer
Opposite to the direction of its motion.
Key terms and units
| Term | Meaning | Unit |
|---|---|---|
| Force | A push or a pull on an object | newton (N) |
| Physical quantity | Anything in science that can be measured | - |
| Magnitude | The size of a quantity, here the strength of the force | newton (N) |
| Direction | The line along which the force acts | - |
| Buoyant force | The upward force applied by a liquid on an object placed in it | newton (N) |
