We are familiar with the movement of objects from previous studies. We know how to determine if an object is moving faster than another based on distance covered per unit time. We also observe that moving objects, like a ball rolling on the ground, eventually slow down or change direction.

These changes in motion – speeding up, slowing down, or changing direction – don't happen by themselves. They are caused by something acting on the object.

Consider everyday actions like kicking a football, pushing a swing, throwing a ball, or hitting a shuttlecock. In all these instances, we are doing something to the object to make it move, change speed, or change direction.

These actions are often described using terms like kicking, pushing, throwing, flicking, lifting, etc. What do these terms represent in science? What effects do they have on the objects they act upon?

Force – A Push Or A Pull

Many everyday tasks involve actions like picking up, opening, shutting, kicking, hitting, lifting, flicking, pushing, and pulling objects. Each of these actions typically results in some alteration to the object's state of motion.

Let's consider various situations and identify the actions involved:

S. No.	Description of the situation	Action (Examples: pushing, pulling, picking, hitting, lifting, etc.)	Action can be grouped as a Push	Action can be grouped as a Pull
1.	Moving a book placed on a table	Pushing or pulling	Yes	Yes
2.	Opening or shutting a door	Pushing or pulling	Yes	Yes
3.	Drawing a bucket of water from a well	Lifting / Pulling the rope	No	Yes
4.	A football player taking a penalty kick	Kicking / Hitting	Yes	No
5.	A cricket ball hit by a batsman	Hitting	Yes	No
6.	Moving a loaded cart	Pushing or pulling	Yes	Yes
7.	Opening a drawer	Pulling	No	Yes

Looking at these examples, we can see that actions like picking, opening, shutting, hitting, lifting, flicking, kicking, pushing, and pulling can generally be classified as either a push or a pull, or a combination of both.

This leads to the fundamental concept in science: a push or a pull on an object is called a force.

Therefore, when we say an object's motion changed, it was due to the application of a force (a push or a pull). But when does a force actually come into play?

Forces Are Due To An Interaction

For a force to exist and act on an object, there must be an interaction between at least two objects.

Consider a stationary car with a person standing behind it. The presence of the person alone does not cause the car to move.

However, if the person starts pushing the car, they are applying a force on it. This interaction (the person pushing the car) can cause the car to move in the direction of the push.

Think about a game of tug-of-war or two people pushing/pulling each other. In these situations, each person is applying a force on the other through interaction (pulling the rope, pushing against each other). The force arises because of the interaction between the participants.

Thus, a force is not a property of a single object; it emerges from the interaction between two or more objects.

Exploring Forces

Forces have characteristics and effects that are important to understand.

When multiple forces act on an object, the overall effect on the object is determined by the net force, which is the combined effect of all individual forces.

Consider pushing a heavy box. If you push it alone, it might not move. If a friend joins and pushes in the same direction, it becomes easier to move. This is because forces applied in the same direction add up. The net force is the sum of the individual forces.

If you and your friend push the same box from opposite directions, the effect depends on the strength of your pushes. If you push with equal strength, the box might not move at all (net force is zero). If one person pushes harder, the box will move in the direction of the stronger push. In this case, when forces act in opposite directions, the net force is the difference between the two forces.

Similarly, in tug-of-war, if both teams pull with equal force in opposite directions, the rope doesn't move. The net force on the rope is zero. The team that applies a larger force (pulls harder) creates a non-zero net force in their direction, causing the rope to move and winning the game.

From these examples, we learn that forces can be larger, smaller, or equal to each other. The strength of a force is its magnitude. When describing a force, both its magnitude and direction are important. Changing either the magnitude or the direction of a force will change its effect on the object.

A Force Can Change The State Of Motion

One of the primary effects of a force is its ability to change the state of motion of an object.

State Of Motion

The state of motion of an object is described by its speed and its direction of motion. If an object is not moving, it is considered to be in the state of rest, which is defined as having zero speed. Both being at rest and being in motion are considered states of motion.

Applying a force can cause an object that is at rest to start moving. For example, pushing a stationary rubber ball makes it move.

A force can also change the speed of a moving object. If a force is applied in the same direction as the object's motion, its speed increases (e.g., pushing a moving ball further). If a force is applied in the opposite direction to the object's motion, its speed decreases (e.g., stopping a moving ball with your palm).

Force can also change the direction of motion of an object. When a moving ball hits a ruler placed in its path, the force exerted by the ruler on the ball changes the ball's direction.

In sports like volleyball or cricket, players apply forces on the ball with their hands or bats to change both the speed and direction of the ball's motion.

Therefore, a change in the state of motion of an object means a change in its speed, or its direction of motion, or both. Applying a force can cause such a change in the state of motion.

However, it is important to note that applying a force does not always guarantee a change in the state of motion. For example, pushing a very heavy box or a wall might not cause them to move, even though force is being applied. The effect of the force depends on its magnitude, the object's mass, and other forces acting on the object.

Force Can Change The Shape Of An Object

Apart from changing the state of motion, applying a force can also cause a change in the shape of an object.

Consider the following situations where force is applied to objects that are not free to move easily:

Description of Situation	How to Apply Force	Diagram/Action	Change in State of Motion	Change in Shape
A lump of dough on a plate.	Pressing it down with your hands. Rolling it into a chapati.		No	Yes
Spring fixed to the seat of a bicycle.	By sitting on the seat (compressing the spring).		No	Yes
A rubber band suspended from a hook/nail fixed on a wall.	By hanging a weight or by pulling its free end (stretching).		No	Yes
A plastic or metal scale placed between two bricks.	By putting a weight at the centre of the scale (bending).		No	Yes
An inflated balloon.	Pressing it between your palms.		No	Yes

In all these examples, applying force causes the object to deform or change its shape. Pressing a ball of dough, stretching a rubber band, or bending a scale all demonstrate how force can alter an object's form.

In summary, applying a force on an object can potentially:

• Start motion in an object at rest.
• Change the speed of a moving object.
• Change the direction of motion of a moving object.
• Change the shape of an object.
• Cause one or more of these effects simultaneously.

It's crucial to remember that these changes (motion from rest, change in speed/direction, change in shape) do not happen spontaneously. They always require the application of a force.

Contact Forces

Forces can be classified based on whether the objects exerting and receiving the force are in direct physical contact.

Contact forces are those that require the objects to be in physical contact for the force to be exerted.

Muscular Force

To push a table, lift a book, or draw water from a well, you need to touch or hold the object. The force you apply comes from the action of your muscles. The force exerted by the muscles of the body is called muscular force.

Muscular force is what allows us to move, lift, push, pull, bend, and perform various physical activities. Even internal bodily processes like the movement of food through the digestive tract and the expansion/contraction of lungs during breathing are driven by muscular forces.

Animals also use muscular force for their activities, such as walking, running, and performing tasks like pulling carts or ploughing fields when harnessed.

Since muscular force requires contact (either direct body contact or contact via an object like a rope or stick), it is categorised as a contact force.

Friction

Observe what happens when a ball rolls on the ground: it gradually slows down and stops. A bicycle stops after you stop pedalling. A boat stops when you stop rowing. In these situations, even though no obvious push or pull is applied to stop the objects, they come to rest.

This slowing down and stopping is due to a force called friction. Friction is a force that opposes the motion of an object when it is in contact with another surface. It acts in the direction opposite to the direction of motion.

Friction exists between the surface of the moving object and the surface it is moving on (e.g., between the ball and the ground, the bicycle wheels and the road, the boat and the water). Since friction arises from the contact between surfaces, it is also an example of a contact force.

Non-Contact Forces

Sometimes, a force can be exerted on an object without direct physical contact between the objects. These are called non-contact forces.

Magnetic Force

Magnets can attract or repel other magnets or magnetic materials (like iron) without touching them. If you bring the North pole of one magnet near the North pole of another, they repel each other (pushing away). If you bring the North pole near the South pole, they attract each other (pulling towards). These forces of attraction and repulsion act even when there is a distance between the magnets.

The force exerted by a magnet is therefore a non-contact force.

Electrostatic Force

When certain materials are rubbed together, they acquire an electric charge. For instance, rubbing a plastic straw with paper can charge the straw. A charged object can exert a force on another charged object or an uncharged object without physical contact. This force is called electrostatic force.

Like magnetic force, electrostatic force acts at a distance and is thus a non-contact force.

Gravitational Force

When you drop a coin, it falls to the ground. Leaves and fruits fall from trees. These events happen because the Earth pulls objects towards itself. This force of attraction exerted by the Earth is called the force of gravity or simply gravity.

Gravity is always an attractive force. It acts on all objects near the Earth's surface, pulling them downwards. This is why rivers flow downwards and water comes out of a tap and falls towards the ground.

Gravity

The concept of gravity is not unique to Earth. In fact, every object in the universe exerts a gravitational force of attraction on every other object. This fundamental force is known as the gravitational force. While the gravitational force between everyday objects is usually very weak, the gravitational force exerted by massive bodies like planets and stars is significant.

Gravity is a non-contact force because it acts even when objects are not touching each other. The Earth's gravity acts on objects even when they are high above the ground.

Pressure

When a force is applied to a surface, the effect can also depend on the area over which the force is distributed. This concept is described by pressure.

Pressure is defined as the force acting per unit area of a surface.

The formula for pressure is:

$\text{Pressure} = \frac{\text{Force}}{\text{Area on which it acts}}$

Typically, we consider the force acting perpendicular to the surface for calculating pressure.

From the formula, it is clear that for a given amount of force, pressure is inversely proportional to the area. This means:

• If the area is smaller, the pressure exerted is larger.
• If the area is larger, the pressure exerted is smaller.

Consider trying to push a nail into wood. Pushing the head (larger area) is difficult. Pushing the pointed end (much smaller area) is easy. The same force applied to the smaller area of the pointed end creates a much larger pressure, enough to penetrate the wood.

This principle explains why tools designed for cutting or piercing (like knives, needles) have sharp, thin edges – they concentrate the applied force onto a small area, creating high pressure. Similarly, shoulder bags have broad straps to distribute the weight (force) over a larger area, reducing the pressure on the shoulders and making them more comfortable to carry.

Porters carrying heavy loads place a round piece of cloth on their heads. This increases the contact area between the load and their head, thereby reducing the pressure and making it easier to bear the weight.

Pressure Exerted By Liquids And Gases

Liquids and gases also exert pressure. They exert pressure not only downwards (like solids due to gravity) but also sideways on the walls of their container and upwards at the bottom.

Experiments with a glass tube covered at one end with a rubber sheet demonstrate that water exerts pressure at the bottom of the container. As more water is poured (increasing the height of the water column), the rubber sheet bulges out more, indicating increased pressure.

Similar experiments show that liquids also exert pressure on the walls of their containers. If a rubber sheet is fixed to the side of a bottle and the bottle is filled with water, the rubber sheet bulges outwards. This indicates pressure exerted by the water on the side wall.

Furthermore, liquids exert equal pressure at the same depth in all directions. If a bottle with holes drilled at the same height is filled with water, the streams of water coming out of all holes will travel approximately the same horizontal distance.

These observations confirm that liquids exert pressure on the bottom and the walls of their containers, and this pressure increases with the depth of the liquid column.

Do gases also exert pressure? Yes, gases also exert pressure on the walls of their containers. When you inflate a balloon, the air inside pushes outwards on the balloon's elastic surface. If there's a hole, the air escapes due to this pressure. This shows that air (a gas) exerts pressure on the inner walls of the balloon. Similarly, air in a bicycle tube exerts pressure on the inner walls of the tube. Thus, gases exert pressure on the walls of their containers in all directions.

Atmospheric Pressure

The Earth is surrounded by a vast layer of air, known as the atmosphere, which extends many kilometres above the surface. This atmospheric air has weight, and this weight exerts a force on the Earth's surface and everything on it.

The pressure exerted by this atmospheric air is called atmospheric pressure.

Pressure is force per unit area. If we imagine a column of air standing on a unit area on the Earth's surface, extending up to the top of the atmosphere, the total force exerted by the weight of this air column on that unit area is the atmospheric pressure.

The magnitude of atmospheric pressure is surprisingly large. An experiment with a rubber sucker pressed onto a smooth surface demonstrates this. When the sucker is pressed, air is expelled from underneath, creating a region of lower pressure inside. The higher atmospheric pressure outside then pushes the sucker onto the surface, making it stick. To pull the sucker off requires a force strong enough to overcome this atmospheric pressure.

For instance, the force exerted by atmospheric pressure on an area of $15 \text{ cm} \times 15 \text{ cm}$ is roughly equivalent to the gravitational force on an object with a mass of $225 \text{ kg}$ (approximately $2250 \text{ N}$). We are not crushed by this enormous pressure because the fluids and gases inside our bodies also exert pressure that balances the external atmospheric pressure.

Historically, experiments like the Magdeburg hemispheres (performed by Otto von Guericke in the 17th century) dramatically illustrated the power of atmospheric pressure. He used a vacuum pump to remove air from between two large metal hemispheres joined together. The external atmospheric pressure was so strong that even eight horses pulling on each hemisphere could not separate them.

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