Force

1. Definition of Force

Force is a push or a pull that changes or tries to change the state of motion or shape of an object.

It can:

Start or stop motion

Change speed

Change direction

Change shape (stretch, compress, bend)

2. Unit of Force

The SI unit of force is Newton (N).

1 Newton = force required to accelerate a mass of 1 kg by 1 m/s².

3. Effects of Force

Force can:

Move a stationary object

Stop a moving object

Change the speed of an object

Change the direction of an object

Change the shape of an object

4. Types of Force

a) Balanced Force

When equal forces act on an object in opposite directions.

No change in motion.

Example: Book resting on a table.

b) Unbalanced Force

When unequal forces act on an object.

Causes change in motion.

Example: A football starts moving when kicked.

5. Contact and Non-contact Forces

Contact Forces

Applied force

Frictional force

Normal reaction force

Tension force

Non-contact Forces

Gravitational force

Magnetic force

Electrostatic force

6. Newton’s Laws of Motion (Brief SEE-Level)

1st Law – Law of Inertia

An object remains at rest or in uniform motion unless an external force acts on it.

2nd Law – Law of Acceleration

Force = mass × acceleration
F = m × a

3rd Law – Action and Reaction

Every action has an equal and opposite reaction.

7. Friction

Friction is a force that opposes motion between two surfaces.

Depends on:

Nature of surfaces

Normal force

Types of Friction:

Static

Sliding

Rolling

Fluid friction

Advantages: walking, writing, stopping vehicles
Disadvantages: wear and tear, loss of energy

8. Weight as a Force

Weight is the force of gravity acting on a body.

Formula:
Weight (W) = mass (m) × gravity (g)

Measured in Newtons (N).

g ≈ 9.8 m/s² on Earth.

9. Numerical Example

Example:
Find the force when a mass of 4 kg accelerates at 3 m/s².

Solution:
F = m × a = 4 × 3 = 12 N