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
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).
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