1.0 Introduction to Force
In daily life, we push and pull many things. We push a door to open it. We pull a drawer to take something out. We kick a football to make it move. We press brakes to stop a bicycle. In Physics, a push or pull acting on an object is called force.
Force is very important because it can change the condition of an object. It can make a stationary object move, stop a moving object, change the speed of an object, change its direction, or change its shape.
Statement: Force is a push or pull acting on an object.
A force is usually produced when one object acts on another object. When you push a table, your hand applies force on the table. When you pull a school bag, your hand applies force on the bag. When Earth pulls objects downward, gravity applies force on them.
Can an object move by itself without any force? Usually, an object at rest needs a force to start moving. For example, a football lying on the ground will not start moving on its own. When someone kicks it, force is applied and the ball starts moving.
When you open a door, push a shopping trolley, pull a suitcase, kick a ball, squeeze a sponge or stretch a rubber band, you are applying force. Force is involved in almost every action we do.
Force has two important basic features: strength and direction. A gentle push moves an object slowly, while a strong push may move it faster. Also, the direction in which force is applied matters. If you push a box to the right, it may move to the right. If you push it to the left, it may move to the left.
| Action | Type of Force | Effect |
|---|---|---|
| Pushing a door | Push | Door opens or moves |
| Pulling a drawer | Pull | Drawer comes out |
| Kicking a football | Push by foot | Ball starts moving |
| Pulling a rope | Pull | Rope moves towards us |
Do not write that force only means pushing. Force can be either a push or a pull. Pulling a bag, lifting a bucket and stretching a rubber band are also examples of force.
Force = P + P
P = Push
P = Pull
Any push or pull is a force.
Force is not visible directly, but we can observe its effect. We cannot see the force applied by a hand on a ball, but we can see the ball moving after the force is applied.
- Force is a push or pull acting on an object.
- Force can make objects move, stop or change motion.
- Force has strength and direction.
A football is lying still on the ground. What force is needed to make it move?
2.0 Effects of Force
We cannot see force directly, but we can understand force by observing its effects. Whenever a force acts on an object, it may change the object's motion, speed, direction, shape or size. This is why force is called an important cause of change in Physics.
Statement: Force can change the state of rest or motion of an object, change its speed, change its direction, or change its shape and size.
The first effect of force is that it can move a stationary object. A football lying on the ground does not move by itself. When a player kicks it, the applied force makes the ball move.
The second effect of force is that it can stop a moving object. When a goalkeeper catches a moving ball, the hands apply force and stop the ball. When brakes are applied on a bicycle, frictional force helps stop the bicycle.
Why does a moving bicycle stop when brakes are applied? The brake pads press against the wheel and produce friction. This friction acts opposite to the motion of the wheel and gradually stops the bicycle.
Force can also change the speed of an object. If you push a swing gently, it moves slowly. If you push it strongly, it moves faster. Similarly, a cyclist can increase speed by pedalling harder.
Force can change the direction of a moving object. In cricket, when a batsman hits a moving ball, the bat applies force and changes the direction of the ball. In football, players change the direction of the ball by kicking it from different sides.
When you squeeze a sponge, stretch a rubber band, press clay or bend a plastic scale slightly, force changes the shape or size of the object. Some objects return to their original shape, while some remain changed.
| Effect of Force | Meaning | Example |
|---|---|---|
| Starts Motion | Moves a stationary object | Kicking a football |
| Stops Motion | Stops a moving object | Catching a ball |
| Changes Speed | Makes an object faster or slower | Pedalling a bicycle harder |
| Changes Direction | Turns a moving object | Bat hitting a cricket ball |
| Changes Shape | Changes form or size | Pressing clay or squeezing sponge |
Force does not always produce visible motion. When you press a wall, the wall may not move, but force is still applied. Sometimes force changes shape slightly or is balanced by another force.
MSDS helps you remember the effects of force:
M = Move or stop
S = Speed changes
D = Direction changes
S = Shape changes
In sports, players control force very carefully. A small force may pass the ball slowly, while a large force may send it far away. This is why force, direction and timing are important in games.
- Force can start or stop motion.
- Force can change speed or direction.
- Force can change the shape or size of an object.
When you squeeze a rubber ball, which effect of force do you observe?
3.0 Contact and Non-Contact Forces
Forces can be grouped into two main types: contact forces and non-contact forces. The difference is simple. Some forces act only when objects touch each other, while some forces can act even without touching.
Statement: A contact force acts only when two objects are physically touching each other. A non-contact force can act even when objects are not touching.
Contact force is the force that needs direct touch. When you push a chair, pull a bag, kick a football or write with a pencil, contact force is acting. Your hand, foot or pencil must touch the object for the force to act.
Muscular force is a contact force because it is produced by muscles and usually acts through touch. When you lift a bucket, push a table or throw a ball, your muscles apply force directly on the object.
Another important contact force is frictional force. Friction acts when two surfaces touch each other. It opposes motion. For example, when a moving ball rolls on the ground, friction between the ball and the ground slows it down.
Non-contact force does not need direct touch. The force can act from a distance. Examples include gravitational force, magnetic force and electrostatic force.
A magnet can attract iron pins without touching them. Earth pulls a falling mango downward without touching it directly. A rubbed plastic comb can attract small bits of paper without touching them first. These are examples of non-contact forces.
| Type of Force | Touch Needed? | Examples | Daily-Life Situation |
|---|---|---|---|
| Contact Force | Yes | Muscular force, frictional force | Pushing a table, writing with a pen |
| Non-Contact Force | No | Gravitational, magnetic, electrostatic force | Magnet attracting iron, fruit falling down |
Do not think that all forces need touch. Magnetic force, gravitational force and electrostatic force can act without direct contact.
Contact = Touch
Non-contact = No Touch
If touching is needed, it is contact force. If touching is not needed, it is non-contact force.
Gravity is a non-contact force that acts everywhere on Earth. It pulls objects towards the centre of Earth. That is why objects fall down when dropped.
- Contact force acts only when objects touch.
- Non-contact force can act without direct touch.
- Muscular force and friction are contact forces, while gravity and magnetic force are non-contact forces.
How can a magnet pull an iron nail without touching it?
4.0 Frictional Force and Gravitational Force
Two very important forces in our daily life are frictional force and gravitational force. Friction helps us walk, write and stop vehicles. Gravity keeps us on Earth and makes objects fall down when dropped.
Statement: Friction is the force that opposes the motion of one surface over another surface in contact.
Friction acts when two surfaces rub against each other. It always acts opposite to the direction of motion. If a ball rolls on the ground, friction acts between the ball and the ground and slowly stops the ball.
Why does a moving object slow down on its own? When an object moves on a surface, friction acts against its motion. This opposing force reduces the speed of the object. That is why a rolling ball finally stops after some time.
Friction can be useful. We can walk because there is friction between our shoes and the ground. A pencil writes on paper because friction allows graphite to leave a mark. Brakes work because friction helps stop the wheels.
Friction can also be harmful. It can wear out shoes, tyres and machine parts. It also produces heat in moving parts of machines. That is why oil and grease are used in machines to reduce friction.
When you walk, your foot pushes the ground backward, and friction helps you move forward. Without friction, walking would be very difficult, like trying to walk on slippery ice.
| Friction | Example | Result |
|---|---|---|
| Useful Friction | Walking on the ground | Helps us move without slipping |
| Useful Friction | Writing with a pencil | Graphite leaves marks on paper |
| Harmful Friction | Machine parts rubbing | Produces heat and wear |
| Harmful Friction | Tyres rubbing on road | Tyres wear out slowly |
Friction can be reduced by using oil, grease, polishing surfaces or using wheels. Oil and grease make machine parts slide more easily. Wheels reduce friction because rolling is easier than sliding.
Statement: Gravitational force is the force by which Earth pulls objects towards itself.
Gravity is a non-contact force. It acts even when Earth is not touching the object directly. When we throw a ball upward, it comes back down because Earth pulls it. When leaves fall from a tree, gravity pulls them towards the ground.
Do not write that friction is always bad. Friction is useful for walking, writing, gripping and stopping vehicles. It is harmful only when it causes unwanted wear, heat or loss of energy.
Friction Fights Motion and Gravity Goes Down. Friction opposes motion, while gravity pulls objects towards Earth.
Gravity also keeps the Moon moving around Earth and keeps planets moving around the Sun. At Class 6 level, remember that gravity is the force of attraction that pulls objects.
- Friction opposes motion between two surfaces in contact.
- Friction can be useful as well as harmful.
- Gravity pulls objects towards Earth.
Why is it difficult to walk on a very smooth or slippery floor?
5.0 Measurement, Applications and Exam Readiness
Force is used in almost every action around us. We use force to walk, write, lift, push, pull, throw, catch, open, close and move objects. In Physics, force is studied carefully because it can change motion, speed, direction, shape and size.
Statement: The SI unit of force is newton, written as N. Force is measured using a spring balance.
A spring balance is an instrument used to measure force. It contains a spring that stretches when force is applied. The greater the force, the more the spring stretches. The reading is shown on a scale, usually in newton (N).
Force = Push or Pull
- Force: A push or pull acting on an object
- Unit: newton (N)
- Instrument: spring balance
Why do we need to measure force? In daily life, we may say "small force" or "large force", but in science we need exact values. For example, a weak force may move a light toy car, but a stronger force is needed to move a heavy table. Measuring force helps us compare pushes and pulls accurately.
Force helps us walk because our feet push the ground. Force helps us write because our fingers press the pen on paper. Force helps cyclists move because they push pedals. Force helps machines lift heavy objects, cut materials and move goods from one place to another.
Do not write the unit of force as kg. Kilogram is a unit of mass. The SI unit of force is newton (N).
| Topic | Quick Revision Point | Example |
|---|---|---|
| Force | A push or pull acting on an object | Pushing a door, pulling a bag |
| Effects of Force | Can move, stop, speed up, slow down, turn or change shape | Kicking a ball, squeezing clay |
| Contact Force | Acts when objects touch | Muscular force, frictional force |
| Non-Contact Force | Acts without touching | Gravity, magnetic force |
| Friction | Opposes motion between surfaces in contact | Brakes stopping a bicycle |
| Gravity | Pulls objects towards Earth | A fruit falling from a tree |
| Unit of Force | newton | Symbol: N |
Force = Push or Pull
SI unit of force = newton (N)
- Force can be contact or non-contact.
- Friction: Opposes motion.
- Gravity: Pulls objects towards Earth.
F-E-T for Force Revision:
F = Force is push or pull
E = Effects change motion or shape
T = Types are contact and non-contact
- Define force.
- Write any four effects of force.
- Differentiate between contact and non-contact force.
- Why is friction useful for walking?
- Name the force that pulls objects towards Earth.
- What is the SI unit of force?
- Force is a push or pull that can change motion or shape.
- Forces may be contact forces or non-contact forces.
- The SI unit of force is newton (N), and force is measured using a spring balance.
Why is force important in almost every action we do?