Force - Physics

1. Force as a Push or Pull

The movement of a body is often obtained as a result of a push or a pull. This push or pull is called force.

Force as a Push

When we move an object away from us, we apply a push.

Examples:

  1. Pushing a shopping trolley in a supermarket.

  2. Pushing a door to open it.

Force as a Pull

When we bring an object towards us, we apply a pull.

Examples:

  1. Pulling a bucket out of a well.

  2. Pulling a drawer to open it.

2. Definition of Force

Force is a push or a pull acting on an object that can change its state of rest, state of motion, shape, or size.

Force is required to move, stop, or change the direction of an object.

3. Force Applied by Stretching

A stretched object can exert force.

Examples:

  1. A stretched rubber band can move a paper ball.

  2. A stretched bow can shoot an arrow.

A force applied by stretching can change the size and shape of an object.

4. Effects of Force

Force can produce several effects on objects.

1. A Force Can Move a Body

A force can make a stationary object start moving.

Example:

Pushing a stationary football causes it to move.

2. A Force Can Stop a Moving Body

A force can stop a moving object.

Example:

Applying brakes stops a moving bicycle.

3. A Force Can Make a Moving Body Move Faster

A force can increase the speed of a moving object.

Example:

Pedalling harder makes a bicycle move faster.

4. A Force Can Slow Down a Moving Body

A force can reduce the speed of a moving object.

Example:

Brakes slow down a moving car.

5. A Force Can Change the Direction of Motion of a Moving Body

A force can alter the path of a moving object.

Example:

A bat changes the direction of a moving cricket ball.

6. A Force Can Change the Shape or Size of a Body

A force may deform an object.

Example:

Pressing a sponge changes its shape and size.

5. Kinds of Forces

Forces are mainly of two types:

  1. Contact Forces

  2. Non-Contact Forces

A. Contact Forces

A contact force acts only when two objects touch each other.

Examples:

  • Force of friction

  • Muscular force

A contact between objects is necessary for the force to act.

Muscular Force

Muscular force is the force applied by the muscles of humans or animals.

Examples:

  1. Lifting a school bag.

  2. Pulling a cart.

B. Non-Contact Forces

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

Examples:

  • Gravitational force

  • Magnetic force

Gravitational Force

Gravitational force is the force with which the Earth attracts objects towards its centre.

Examples:

  1. A fruit falling from a tree.

  2. A stone dropped from a height falling to the ground.

6. Force of Friction

Friction is the force that opposes the motion of one object over the surface of another object.

Friction always acts opposite to the direction of motion.

Example:

A ball rolling on the ground eventually stops because of friction.

7. Effects of Friction

1. Friction Opposes Motion

Friction acts against movement and tries to stop it.

Example:

A sliding book gradually comes to rest.

2. Friction Always Acts Opposite to the Direction of Motion

The direction of friction is always opposite to the movement of an object.

Example:

If a box moves forward, friction acts backward.

3. Friction Produces Heat

When two surfaces rub against each other, heat is generated.

Example:

Rubbing your hands together makes them warm.

4. Friction Causes Wear and Tear

Continuous rubbing damages surfaces over time.

Example:

The soles of shoes wear out after long use.

8. Factors Affecting the Force of Friction

1. Smoothness of the Surfaces

Rough surfaces produce more friction than smooth surfaces.

Example:

Dragging a box on a rough road is more difficult than on a smooth floor.

2. Nature of the Medium Through Which the Body Moves

The medium may be a solid, liquid, or gas.

Example:

A swimmer moves more slowly in water than a person walks in air because water offers greater resistance.

3. Weight of the Moving Body

The heavier the object, the greater the friction produced.

Example:

A loaded cart is harder to pull than an empty cart.

9. Kinds of Friction

1. Static Friction

The friction acting on an object at rest is called static friction.

Example:

Pushing a heavy cupboard before it starts moving.

2. Sliding Friction

The friction acting when one surface slides over another is called sliding friction.

Example:

A book sliding across a table.

3. Rolling Friction

The friction acting when an object rolls over a surface is called rolling friction.

Example:

A bicycle wheel moving on a road.

Rolling friction is less than sliding friction.

10. Disadvantages of Friction

1. Friction Opposes Motion

It makes movement difficult and reduces efficiency.

Example:

More effort is needed to push a heavy box.

2. Friction Causes Wear and Tear

Machine parts and shoes gradually wear out due to friction.

Example:

Tyres become worn after long use.

3. Friction Produces Heat

Energy is lost as heat due to friction.

Example:

Machine parts become hot while working.

11. Ways of Reducing Friction

1. By Making the Surfaces Smooth

Smooth surfaces produce less friction.

Example:

Polishing furniture makes it smoother.

2. By Using Lubricants

Lubricants such as oil and grease reduce friction between moving parts.

Example:

Lubricating bicycle chains.

3. By Using Ball Bearings

Ball bearings convert sliding friction into rolling friction.

Example:

Ball bearings in bicycle wheels.

4. By Streamlining

Streamlining reduces friction caused by air or water.

Example:

Aeroplanes, ships, and racing cars have streamlined shapes.

12. Advantages of Friction

Although friction has disadvantages, it is also very useful.

1. Helps Us Walk

Without friction, our feet would slip on the ground.

Example:

Walking becomes difficult on an icy surface.

2. Helps Vehicles Move

Tyres grip the road due to friction.

Example:

Cars and bicycles move because of friction between tyres and roads.

3. Helps Us Write

Friction between the pencil tip and paper allows writing.

Example:

Writing with a pencil.

4. Helps in Holding Objects

Friction provides a firm grip.

Example:

Holding a pen without slipping.

Quick Revision

✅ Force is a push or a pull.

✅ Force can:

  1. Move a body.

  2. Stop a moving body.

  3. Increase speed.

  4. Reduce speed.

  5. Change direction.

  6. Change shape and size.

✅ Types of Force:

  • Contact Force

  • Non-Contact Force

✅ Friction is a force that opposes motion.

✅ Types of Friction:

  • Static Friction

  • Sliding Friction

  • Rolling Friction

✅ Friction can be reduced by:

  • Smoothing surfaces

  • Using lubricants

  • Using ball bearings

  • Streamlining

✅ Friction is useful because it helps us walk, write, hold objects, and move vehicles. 📚✨