Force Chapter Notes | Electricity & Magnetism for PAT PDF Download

Introduction

Force is something we use every day to make things move, stop, or change shape. Imagine pushing a swing to make it go, pulling a door to open it, or kicking a football to score a goal. All these actions need a force! In this chapter, we will learn what force is, how it can change the way things move or look, and the different kinds of forces around us. We will also explore how friction, a special force, helps or stops things from moving and how it works in our daily life.

Force

• Picking up, opening, shutting, clicking, twisting, hitting, lifting, flicking, pushing, and pulling are actions that make things move or change.
• We say that an object needs a force to start moving, stop moving, change its speed, direction, or shape.
• Force is something that can change or try to change the state of rest or motion of an object.
• Force can also change the shape or size of an object.
• We call force a pull or a push that acts on an object to change how it moves or looks.
• The state of motion of an object means its speed and direction of movement.
• If an object’s speed is zero, it is at rest, which means it does not move or change its position over time.
• Force is shown by the letter "F".
• The direction of a force is the direction in which the object is pushed or pulled.
• For a force to work, two objects must interact with each other in some way.
  • For example, a door does not open on its own. You need to apply force to pull it, so it opens.
• Interaction between objects is needed for force to work.

Examples of Push and Pull in Daily Life

• Two teams pulling a rope in a tug-of-war game.
• Pulling a chair to sit on it.
• Pulling a rubber band to stretch it.
• Pushing a wheelbarrow.
• Pushing a pram (baby cart) to move it ahead.
• Pushing the refrigerator door to close it.

Stretching and Squeezing

• Terms like stretch and squeeze also mean force.
• When you apply force on an object and its length increases, the object is stretched.
  • For example, when you stretch a rubber band, its length increases because of the force you apply.
• When you apply force on an object and its length, width, or volume decreases, the object is squeezed.
  • For example, when you squeeze a tube of cream, its shape changes, and the cream comes out because its size gets smaller.

Unit of Force

• Force is measured in gram-force (gf), kilogram-force (kgf), dyne, or newton (N).
• The SI unit of force is newton (N).
• We usually measure force using a device called a spring balance.

Did You Know?

Sir Isaac Newton (1643–1727) was an English physicist, mathematician, and astronomer. He discovered the laws of motion and universal gravitation. It is said that he got the idea of gravitation by watching an apple fall from a tree.

Effects of Force

Force can change the state, shape, speed, or direction of an object it acts on. Now, let’s learn about the different effects of force in detail.

Effect of Force on Mass

Let’s understand how force affects the mass of an object with a simple activity:

Activity 3.1

Aim: To study the effect of force on the mass of an object.

Material required: Football and electronic balance.

Procedure:

• Take a football and measure its mass using an electronic balance. Let it be m₁.
• Now, apply some force on it by kicking it or pressing it between your hands.
• After doing this, measure its mass again and record it as m₂.

Observations: You will see that there is no change in the mass of the football after applying force on it.

Conclusion: The mass of an object stays the same even when force is applied on it.

The activity shows that force does not change the mass of an object. This is because the applied force does not change the material that makes up the object.

Force Can Produce Motion

• Force can make an object move by pushing, pulling, or hitting it.
  • For example, you can push a heavy box to move it, pull a door to open it, or hit a golf ball with a club to make it move.
• In these examples, the objects move in the direction of the force applied.
• However, if the force is not strong enough, the object may not move.
  • For example, if you try to push a tree or a wall, it will not move because the force is not enough.

Force Can Stop Motion

• Force can stop a moving object by applying force in the opposite direction of its motion.
• For example, you can stop a moving pram by pulling it from behind, or stop a moving ball with your hands by applying force.

Force Can Change the Speed of Motion

• Force can increase the speed of a moving object by applying force in the direction it is moving.
  • For example, by kicking a moving football in the direction it is going, its speed increases.
• Force can also decrease the speed of a moving object by applying force in the opposite direction of its motion.
  • For example, if you apply force on a moving swing in the opposite direction, its speed reduces.

Force Can Change the Direction of Motion

• Force can change the direction of a moving object by applying force in the desired direction.
• For example, in cricket, when a batsman hits a moving ball with a bat, the direction of the ball changes.

Force Can Change the Shape and Size of an Object

• Force can change the shape and size of an object by applying force on it.
• For example, if you take a ball of cotton or clay and press it between your hands, its shape changes.
• Similarly, you can change the shape and size of a spring, rubber band, sponge, dough, and many other things by applying force on them.

Types of Forces

• If the changes discussed earlier are not caused by a force, then we can say that an equal force is acting from the opposite direction, which cancels the first force.
• Hence, we can say that the two forces cancel each other and a balanced force acts on the object, which does not bring any change.
• On the other hand, if changes occur, the forces acting on the object are not equal and are unbalanced forces.
• This unbalanced force causes the object to move, stop, or change its shape, size, etc.
• The two objects must interact with each other in some way for the forces to be effective.

We can divide forces into two types: contact forces and non-contact forces.

Contact Forces

Contact forces are forces where the two objects are physically touching each other. These forces happen when the objects are in contact, like a stick, a piece of rope, etc. There are different kinds of contact forces, such as applied force, normal force, tension, muscular force, collision force, and friction.

Applied Force

• Applied force is the force applied on an object to change its state of rest or motion by a person, animal, or another object.
• For example, if you push a chair across the room, the force you apply on the chair is the applied force acting on the chair.

Normal Force

• Normal force is the support force that a stable object gives to another object that is touching it.
• For example, if a glass of water is kept on a table, the table pushes upward on the glass to support its weight.

Tension

• Tension is the force that a string or rope applies when it is stretched.
• For example, when you pull a cart with a rope, you apply force on the rope, and the rope in turn applies a force on the cart to make it move.
• Another example is when two teams pull a rope in a tug-of-war game; the rope applies tension force on the teams.

Muscular Force

• Muscular force is the force that comes from the muscles of our body or an animal’s body.
• We use muscular force to do work like carrying a load, walking, running, lifting things, ploughing a field, etc.
• Some examples are lifting a bag, kicking a football, opening a door, pushing a chair, or a horse pulling a cart.

Collision Force

• Collision force happens when two objects bump into each other.
• For example, when two cars collide, they push against each other, and this force is called collision force.

Friction

• Friction is the force that slows down or stops the movement of two objects that are touching each other.
• It acts in the opposite direction of the movement of the object.
• Friction happens when two surfaces are in contact, like a ball and the ground.
• For example, when you hit a ball, it slowly slows down and finally stops because of friction.
• Similarly, a car or scooter also comes to a stop after the engine is turned off because of friction.
• The wear and tear in the moving parts of a machine, like a car, is also because of friction.
• We can walk properly on rough surfaces because of friction, but it is hard to walk on smooth or wet surfaces because friction is less on them.

Mechanical Force

• Mechanical force is the force produced by a machine, not by itself, but by using external sources like electricity, petrol, diesel, etc.
• For example, a vehicle runs with the help of fuel, or the wind makes the blades of a windmill move.

Non-Contact Forces

Non-contact forces are forces that do not need the two objects to touch each other to work. They act at a distance. The different kinds of non-contact forces are magnetic force, electrostatic force, and gravitational force.

Magnetic Force

• Magnetic force is the force that a magnet uses to pull or push certain objects like iron, nickel, or cobalt, even without touching them.
• For example, if you bring a magnet close to some iron pins, the pins get pulled toward the magnet and stick to its surface.
• This pull happens because of the magnetic force, even though the magnet does not touch the pins directly.
• Magnetic force can work on objects like iron, nickel, or cobalt, but not on other materials like plastic or wood.
• A magnet has two poles: a north pole and a south pole.
• Like poles of a magnet (north-north or south-south) push each other away, while unlike poles (north-south) pull each other closer.

Electrostatic Force

• Electrostatic force is the force that happens between a charged object and another charged or uncharged object.
• It works just like magnets: charges can pull or push each other.
• For example, when you rub a comb on dry hair, the comb gets charged.
• If you bring this charged comb near tiny pieces of paper, the pieces get pulled toward the comb.
• This pulling happens because of the electrostatic force, even though the comb and paper pieces are not touching each other directly.

Gravitational Force

• Gravitational force is the force that pulls objects toward each other without them touching.
• For example, when you throw a ball up in the air, it comes back down to the ground.
• Similarly, when you drop an object from a height, it falls to the ground because of gravitational force.
• The earth pulls all objects toward its center with this force, which is why things fall down.
• Gravitational force works between any two objects or masses and pulls them closer to each other.
• For example, the earth has a gravitational force that pulls all objects down to the ground.
• Gravitational force also keeps the moon near the earth.
• The movement of the moon, planets, and satellites around the sun is also because of gravitational force.
• Unlike magnetic and electrostatic forces, gravitational force only pulls objects closer and never pushes them away.

Friction

• Friction is a force that stops or slows down the movement between two objects that are touching each other.
• When an object moves or tries to move, friction acts in the opposite direction to stop or slow it down.
• For example, when a bucket of water is moved on a table, friction acts between the bucket and the table to slow it down.
• Friction happens because no surface is perfectly smooth; even smooth surfaces have tiny bumps and grooves when seen closely.
• These tiny bumps and grooves on two surfaces get locked with each other, which causes friction and makes it harder to move.
• If the applied force is strong enough to overcome friction, the object starts moving.

Properties of Friction

Friction has the following characteristics:

• Friction opposes motion, slowing down and eventually stopping a moving object.
• Friction generates heat, which is why rubbing hands together warms them, and why tires or machine parts heat up during use.
• Friction leads to surface wear, causing the soles of shoes, vehicle tires, and machine components to degrade over time, requiring periodic replacement.

Factors Affecting Friction

Friction depends on three main things: the nature of surfaces, the weight of the object, and the area of surfaces touching each other.

• Nature of surfaces in contact:
  • Friction happens because of tiny bumps on the surfaces of objects.
  • Surfaces that have more bumps (rough surfaces) create more friction.
  • Surfaces that have fewer bumps (smooth surfaces) create less friction.
  • For example, a rough surface like sandpaper has more friction than a smooth surface like glass.
• Weight of the body:
  • The weight of an object affects the normal force (the force pressing the object down).
  • Normal force is the force that presses two surfaces together, and it is equal to the weight of the object.
  • If an object is heavier, the normal force is more, so friction is also more.
  • For example, it takes more force to move a box of 80 kg than a box of 40 kg because the heavier box has more friction.
• Area of surfaces in contact:
  • The area or shape of the surfaces that are touching does not change friction.
  • For any two surfaces, friction depends only on the nature of the surfaces and the weight, not on how much area is touching.
  • For example, if you push a box on its wide side or narrow side, the friction stays the same.

Types of Friction

We can divide friction into two main types: static friction and kinetic friction.

1. Static friction:

• Static means not moving or staying still.
• Static friction is the friction that acts when two objects are touching but not moving.
• For example, if you try to push a heavy table, you start with a small force.
• The table does not move at first because static friction is balancing your push and keeping the table still.
• As you push harder, the static friction increases to match your force and keep the table from moving.
• Static friction has a limit (maximum value); if you push harder than this limit, the table starts to move.
• If your push is less than or equal to the static friction, the table will not move.
• Kinetic friction:
  • Kinetic means moving.
  • Kinetic friction is the friction that acts when two objects are moving against each other.
  • Once the applied force is more than the static friction, the object starts moving, and kinetic friction takes over.
  • Kinetic friction is of two types: sliding friction and rolling friction.
• Sliding friction:
  • Sliding friction happens when an object slides over a surface.
  • For example, when you push a box on the floor, the box slides, and the friction between the box and the floor is sliding friction.

2. Rolling friction:

• Rolling friction happens when an object rolls over a surface.
  • For example, when a ball rolls on the ground, the friction between the ball and the ground is rolling friction.
• Rolling friction is less than sliding friction, which means it is easier to roll an object than to slide it.
  • For example, it takes less force to roll a heavy object with wheels (like a car) than to slide it.
• Static friction is always more than kinetic friction, so it takes more force to start moving an object than to keep it moving.

Advantages of Friction

Friction is very important in our daily life; it helps us in many ways:

• Friction between our feet and the ground helps us walk on smooth or icy floors without slipping.
• Friction between the tyres of vehicles and the road helps cars, bikes, and buses move safely.
• On wet or greasy roads, friction is less, so vehicles can skid more easily, which is why accidents happen more on rainy days.
• Friction between the brake and the wheel of a vehicle helps slow down or stop the vehicle when we press the brake.
• Friction between a pencil or pen and paper helps us write; it is hard to write on smooth surfaces like glass or plastic because there is less friction.
• Friction also helps teachers write on the blackboard with chalk.
• Friction helps us light a matchstick, cut things, tie knots, or fix a nail in the wall.
• Friction helps us hold objects in our hands without dropping them.
• In winter, we rub our hands together, and friction produces heat to make our hands warm.

Methods of Increasing Friction
The easiest way to increase friction is by making surfaces rougher, because rough surfaces have more friction. Some of the methods of increasing friction are as follows:

1. By grooving:

• The soles of shoes have grooved designs to increase friction and stop us from slipping.
• For example, sportspersons, trekkers, or people walking on icy ground wear shoes with spikes to increase friction.
• Tyres of cars and buses have grooves on their surface to increase friction with the road.
• Grooved tyres reduce the risk of skidding on wet roads by increasing friction.

2. By making the surfaces rough:

• When the ground is slippery after rain, people spread sand or gravel on it to make it rough and increase friction.
• The handle covers of two-wheelers are made of spiked or treaded rubber, or sometimes covered with cloth, to provide a good grip and increase friction.

Disadvantages of Friction

• Friction causes problems too; it is not always helpful.
• Friction causes wear and tear of moving parts, like the rubbing surfaces of machines.
• For example, the soles of our shoes wear out due to friction with the ground.
• Friction produces heat, which can damage the moving parts of machines over time.
• Some parts of machines need to be replaced often because of the heat and wear caused by friction.
• Friction wastes energy; we have to use extra energy in machines to overcome friction, which increases the cost of running machines.

Methods of Reducing Friction
We can reduce friction by using different methods to make things move more easily.

1. By polishing:

• Polishing a surface makes it smooth by reducing the tiny bumps on it, which lowers friction.
• For example, polishing the floor makes it smoother, so things slide more easily on it.

2. By greasing:

• We can reduce friction by using lubricants like oil, grease, or powder between two surfaces that are touching.
• Lubricants form a thin layer between the surfaces, which reduces the direct contact between them and lowers friction.
• For example, oil is used in machines to make moving parts slide more easily.

3. Streamlining:

• Streamlining means shaping a body so that it moves easily through air or water with less resistance (friction).
• For example, birds and fish have a streamlined shape (narrow in the front and broad at the back) to move easily through air or water.
• We make boats, ships, aeroplanes, and rockets streamlined to reduce friction with air or water.

4. Using rollers or ball bearings:

• Rolling friction is less than sliding friction, so we use rolling instead of sliding to reduce friction.
• For example, we use wheels in vehicles or ball bearings in machines to make things roll instead of slide, which lowers friction.
• Even with all these methods, we can never reduce friction to zero; some friction will always remain.

Points To Remember

• For force to act, two bodies must interact with each other in some way.
• The force does not affect the mass of the body.
• Force, if applied sufficiently, can make an object move.
• Force can change the speed of motion; the speed can be increased by applying force in the direction of motion.
• Force can also decrease the speed or stop the object by applying force in the opposite direction of the motion.
• Force can also change the shape and size of an object.
• There are two types of force: contact forces and non-contact forces.
• The types of contact forces are applied force, normal force, tension, muscular force, collision force, friction, and mechanical force.
• The types of non-contact forces are magnetic force, gravitational force, and electrostatic force.
• Friction always opposes the relative motion.
• Friction produces heat and it also causes wear and tear of the surfaces.
• The factors that affect friction are nature of surfaces in contact, weight of the body, and area of surface in contact.
• There are two types of friction: static friction and kinetic friction.
• Kinetic friction is of two types: sliding friction and rolling friction.
• Static friction is more than kinetic friction.
• Sliding friction is more than rolling friction.
• Friction plays an important role in our lives.
• Friction can also be reduced by using methods such as polishing, lubricating, streamlining, and use of rollers or ball bearings.

Glossary

• Force: Push or pull acting on a body that tends to change its state of rest, motion, direction, shape, or size.
• Contact forces: Forces where the two interacting bodies are in physical contact with each other.
• Applied force: Force applied to an object by a person, animal, or another object to change its state of rest or motion.
• Normal force: Support force exerted on a body in contact with another stable body.
• Tension: Force exerted by a string or rope when it is stretched.
• Muscular force: Force exerted by the muscles of the body.
• Collision force: Contact force that occurs when two bodies collide with each other.
• Friction: Force that opposes the relative motion of two bodies.
• Mechanical force: Force produced by a machine.
• Non-contact forces: Forces that act at a distance without physical contact between the two interacting bodies.
• Magnetic force: Force exerted by a magnet.
• Electrostatic force: Force exerted by a charged body on another charged or uncharged body.
• Gravitational force: Force with which any two masses or objects pull each other.
• Static friction: Friction that exists when the bodies in contact are at rest relative to each other.
• Kinetic friction: Friction that exists between two bodies when the applied force overcomes static friction, causing movement.
• Sliding friction: Friction that exists when a body slides over a surface.
• Rolling friction: Friction that exists when a body rolls over a surface.