Class 8 | exploring forces | Science

 

What is a Force?

A force is a push or pull acting on an object.

Examples

• Pushing a door → force
• Pulling a drawer → force
• Kicking a football → force
• Pulling a bucket with a rope → force

SI Unit of Force

The SI unit of force is newton (N).

1 newton is the force that produces an acceleration of 1 m/s² in a mass of 1 kg.

Effects of Force

A force can produce several changes in an object:

A. Change the state of motion

  A stationary object can start moving.

  Example: Kicking a stationary football.

B. Change the speed

Force can increase or decrease the speed of a moving object.

Example: Applying brakes decreases the speed of a bicycle.

C. Change the direction of motion

A force can change the direction in which an object is moving.

Example: Hitting a moving cricket ball with a bat.

D. Change the shape or size

Force can deform an object.

Example: Pressing a sponge changes its shape.

E. Stop a moving object

Example: Catching a moving ball brings it to rest.

Important: The effect of force depends on its magnitude and direction.

Forces as Interaction

A force is an interaction between two or more objects.

This means an object cannot exert a force without interacting with another object.

Examples

A person pushes a box → person and box interact.

Earth attracts an apple → Earth and apple interact.

A magnet attracts an iron nail → magnet and nail interact.

Important Point

Forces always occur in pairs of interacting objects.

Types of Forces

Forces can broadly be classified into:

Contact forces

Non-contact forces

Contact Forces

A contact force acts only when two objects are in physical contact with each other.

Two important contact forces are:

A. Muscular Force

The force produced by the muscles of humans or animals is called muscular force.

Examples:

Lifting a bag

Pushing a table

Pulling a cart

A horse pulling a cart

B. Frictional Force

Friction is a force that opposes the relative motion or attempted motion between two surfaces in contact.

Examples:

Friction between our feet and the ground helps us walk.

Friction between brake pads and a wheel helps stop a bicycle.

Friction between a pencil and paper allows us to write.

Advantages of friction

• Helps us walk.

• Helps vehicles move.

• Helps us write.

• Helps brakes work.

• Disadvantages of friction

• Causes wear and tear.

• Produces unwanted heat.

• Reduces efficiency of machines.

Disadvantages of friction

• Causes wear and tear.

• Produces unwanted heat.

• Reduces efficiency of machines.

Non-Contact Forces

A non-contact force acts between objects without physical contact.

Important non-contact forces are:

A. Magnetic Force

The force exerted by a magnet on another magnet or magnetic material is called magnetic force.

Example: A magnet attracts an iron nail.

Magnetic force can be:

Attractive

Repulsive

Like magnetic poles repel each other, while unlike poles attract each other.

Electrostatic Force

The force exerted by an electrically charged object on another charged or uncharged object is called electrostatic force.

Example: Rub a plastic comb on dry hair and bring it near tiny pieces of paper. The paper may be attracted to the comb.

Electrostatic force can be:

Attractive between unlike charges

Repulsive between like charges

Gravitational Force

The attractive force between any two objects having mass is called gravitational force.

The Earth attracts objects towards its centre due to gravity.

Examples:

An apple falls towards Earth.

A ball thrown upward comes back down.

We remain on the Earth's surface.

F=G m1 m2 /r²

Weight and Its Measurement

What is Weight?

Weight is the gravitational force acting on an object.

Weight depends on:

• Mass of the object.

• Gravitational acceleration at that place.

Formula

Weight = mass × acceleration due to gravity

$$W=mg$$

where:

• W = weight
• m = mass
• g = acceleration due to gravity

Near Earth's surface:

g ≈ 9.8 m/s²

SI Unit of Weight

Since weight is a force, its SI unit is newton (N).

Spring Balance

A spring balance is an instrument used to measure force, including the weight of an object.

Working

When an object is attached to a spring balance, its weight stretches the spring. The amount of stretching is related to the force acting on the spring.

The scale is generally marked in newtons (N).

Floating and Sinking

When an object is placed in a liquid, the liquid exerts an upward force on the object.

This upward force is called buoyant force or upthrust.

What happens to an object?

If buoyant force > weight:
→ Object moves upward/floats.

If buoyant force < weight:
→ Object sinks.

If buoyant force = weight:
→ Object can remain suspended or in equilibrium.

Archimedes' Principle

Archimedes' Principle states that when an object is wholly or partially immersed in a fluid, it experiences an upward buoyant force equal to the weight of the fluid displaced by the object.

Example

When a stone is placed in water, it displaces some water. The water exerts an upward force on the stone.

This principle helps explain:

• Why objects float or sink
• How ships float
• How hydrometers work
• Measurement of volume/density in some applications

Why Does a Huge Ship Float?

A steel ship is much heavier than a small steel ball, yet a ship can float.

The reason is not simply that the ship is light.

A ship is designed with a hollow shape that allows it to displace a large amount of water. This produces sufficient buoyant force to balance its weight.