1. What Is Force?
Force is a push or a pull that can change the state of motion of an object. You use force every time you open a door, kick a ball, or lift a bag.
A force can make an object start moving, stop moving, change speed, or change direction.
1.1. Force as an Interaction
Force always comes from an interaction between two objects. For example, when you push a table, your hand and the table interact through force.
1.2. Effects of Force
- It can start motion.
- It can stop motion.
- It can speed up or slow down an object.
- It can change direction.
- It can change the shape of objects.
2. Force and Change in Motion
If an object is at rest, it needs a force to start moving. If an object is moving, it needs a force to change its speed or direction.
2.1. Force Changes Velocity
Velocity changes when a force acts on an object. This connection is explained by Newton’s second law:
\( F = ma \)
This means the greater the force, the greater the acceleration produced.
2.2. Force Can Change Direction
When you swing a ball tied to a string, the string applies force and changes the direction of motion continuously. This keeps the ball moving in a circle.
3. Different Types of Forces
Forces can be broadly classified into two types: contact forces and non-contact forces.
3.1. Contact Forces
Contact forces act only when two objects touch. Examples include:
- Muscular force (pushing a table)
- Friction (opposes motion between surfaces)
- Normal reaction force
3.2. Non-Contact Forces
These forces act without physical contact. Examples include:
- Gravitational force
- Magnetic force
- Electrostatic force
4. Force Is a Vector Quantity
Force has both magnitude and direction, which means it is a vector quantity.
If you push a box to the right, the direction matters. Pushing to the left gives a completely different effect.
4.1. Representing Force
We often draw force using an arrow. The length of the arrow shows the strength of the force, and the arrowhead shows the direction.
5. Units of Force
The SI unit of force is the Newton (N).
One Newton is the force required to give a mass of 1 kilogram an acceleration of 1 m/s²:
\( 1 \, \text{N} = 1 \, \text{kg·m/s}^2 \)
5.1. Everyday Sense of Size of Newton
Roughly, the force needed to lift an apple is about 1 Newton.