Force – A Push Or A Pull

Objects are set into motion or their motion is altered by certain actions. When we make a football move, a goalkeeper stops a ball, a hockey player changes the direction of a ball, or a fielder stops a cricket ball, these actions involve making the object move faster, slower, or changing its direction. These actions can be described using terms like kicking, pushing, throwing, flicking, picking, opening, shutting, hitting, or lifting.

In the context of science, these actions can fundamentally be described as either a push or a pull on an object. To initiate movement in a stationary object or alter the movement of a moving object, we essentially apply a push or a pull on it.

Therefore, in scientific terms, a force is defined as a push or a pull applied to an object. The motion or change in motion observed in objects is a result of the application of a force.

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Forces Are Due To An Interaction

For a force to come into play, there must be an interaction between at least two objects. A force is not an inherent property of a single object in isolation; it arises when one object acts upon another.

For example, a car will not move just because a person is standing near it. The person must interact with the car by pushing it to apply a force that can potentially move the car. Similarly, when two people push or pull each other, or when a man pulls a cow, there is an interaction between the two entities, resulting in forces being applied on both.

Thus, the interaction between one object and another object leads to the exertion of a force between them.

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Exploring Forces

When multiple forces act on an object, the resulting effect is determined by the net force.

• If two forces are applied on an object in the same direction, they add up. The net force is the sum of the individual forces. This makes it easier to move an object when pushed or pulled by multiple people in the same direction.
• If two forces are applied on an object in opposite directions, the net force is the difference between the magnitudes of the two forces. The object will move in the direction of the larger force. If the forces are equal in magnitude, the net force is zero, and the object remains stationary or continues its existing state of motion. This is seen in a tug-of-war where if both teams pull with equal force, the rope doesn't move.

Forces have both magnitude (strength) and direction. The overall effect of a force depends on both how strong it is and in which direction it is applied. Changing either the magnitude or the direction of an applied force will change its effect on the object.

An object can have more than one force acting on it simultaneously. The cumulative effect on the object depends on the overall or net force.

A Force Can Change The State Of Motion

When a force is applied to an object, it can cause changes in its state of motion. The state of motion of an object is defined by its speed and the direction it is moving. An object at rest is considered to be in a state of zero speed. Both being at rest and being in motion are states of motion.

Applying a force can:

• Make a stationary object move: Pushing a ball at rest will set it in motion.
• Change the speed of a moving object:
  • If the force is applied in the same direction as the motion, the object's speed will increase. Pushing a rolling ball or a moving tyre further in the direction they are already moving will make them go faster.
  • If the force is applied in the opposite direction of the motion, the object's speed will decrease, potentially bringing it to rest. A goalkeeper stopping a ball or a fielder stopping a cricket ball applies force opposite to the ball's motion, reducing its speed.
• Change the direction of motion: When a moving ball strikes a ruler placed in its path, the force from the ruler changes the ball's direction. Similarly, in sports like volleyball or cricket, players apply force to change the direction of the ball.

Therefore, a change in speed, a change in direction, or both, is a change in the object's state of motion. Applying a force can bring about such a change.

However, it is important to note that applying a force does not always result in a change in the state of motion. For example, pushing against a sturdy wall or trying to move a very heavy box might not cause any visible movement or change in speed, even though force is being applied.

Force Can Change The Shape Of An Object

In addition to affecting motion, applying a force can also cause an object to change its shape. This effect is observable even when the object is not free to move.

Examples where force changes shape include:

• Pressing down on a lump of dough with hands changes its shape.
• Sitting on the seat of a bicycle with springs causes the springs to compress and change shape.
• Hanging a weight on a suspended rubber band stretches and changes the band's shape.
• Placing a weight at the center of a plastic or metal scale supported at both ends causes it to bend and change shape.
• Squeezing an inflated balloon between palms deforms its shape.
• Rolling a ball of dough to make a chapati changes its shape.
• Pressing a rubber ball placed on a table flattens or deforms it.

These observations demonstrate that force is capable of altering the form or structure of an object.

In summary, applying a force can:

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

Crucially, none of these changes (moving from rest, changing speed, changing direction, or changing shape) can happen to an object by itself; they require the action of a force.

Contact Forces

Forces can be categorised based on whether the object applying the force is in direct physical contact with the object on which the force is applied. Forces that act only when there is physical contact are called contact forces.

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Muscular Force

The force exerted by the muscles of our body is known as muscular force. We use muscular force for actions like pushing, lifting, carrying, walking, bending, or any movement of our limbs or body parts. For muscular force to be applied, there must be direct contact between the body (or an extension like a stick or rope held by the body) and the object.

Muscular force is also used internally within our bodies, such as the movement of food through the digestive tract or the expansion and contraction of lungs during breathing, which are facilitated by muscular actions.

Animals also utilise muscular force for their activities, including tasks like pulling carts, ploughing fields, or carrying loads, often performed by animals like bullocks, horses, donkeys, and camels for human benefit.

Since muscular force requires physical contact between the source (muscles) and the object being acted upon, it is classified as a type of contact force.

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Friction

Friction is a force that opposes motion between surfaces that are in contact. It always acts in the direction opposite to the direction of motion. Friction is why objects moving on a surface, like a ball rolling on the ground or a bicycle when you stop pedalling, gradually slow down and eventually come to rest, even when no other force seems to be actively stopping them.

Friction arises from the interaction between the surfaces of objects in contact. For example, friction between the ball and the ground, or between the tires of a bicycle and the road, or between a boat and the water. Since friction occurs due to the contact between surfaces, it is another important example of a contact force.

Non-contact Forces

Forces that can be applied to an object even without direct physical contact are called non-contact forces. The object applying the force does not need to touch the object it acts upon.

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Magnetic Force

Magnets exert a force on other magnets or magnetic materials (like iron) even when there is a distance between them. This force can be attractive (between unlike poles) or repulsive (between like poles).

Bringing one magnet near another magnet placed on rollers shows that the second magnet moves without the first one touching it, demonstrating that a force is being exerted across a distance. This force exerted by a magnet is an example of a non-contact force.

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Electrostatic Force

When certain materials are rubbed together (like rubbing a plastic straw with paper), they acquire an electric charge. An object that has acquired such a charge is called a charged body. A charged body can exert a force on another charged body (either attractive or repulsive, depending on the type of charge) or even on an uncharged body (attractive force). This force acts across a distance without physical contact.

For example, a charged straw can attract or repel another charged straw, or attract an uncharged straw, without touching it. This force exerted by charged bodies is known as electrostatic force and is another example of a non-contact force.

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Gravitational Force

Objects fall towards the Earth when released. Leaves and fruits detach from plants and fall to the ground. This happens because the Earth pulls objects towards itself with a force. This force is called the force of gravity, or simply gravity.

Gravity is an attractive force. It's not just the Earth that exerts this force; every object in the universe exerts a gravitational force of attraction on every other object. The strength of this force depends on the masses of the objects and the distance between them. The force of gravity acts constantly on all objects, including ourselves, even though we are not always aware of it.

The flow of water downwards from a tap or in rivers is also due to the Earth's gravitational pull. Since gravity acts without physical contact between the Earth and the object (like a falling coin or the water), it is a non-contact force.

Pressure

When a force is applied to a surface, the effect also depends on the area over which the force is distributed. This concept is captured by the term pressure.

Pressure is defined as the force acting on a unit area of a surface. When calculating pressure in simple cases, we usually consider the force that acts perpendicularly to the surface.

The formula for pressure is:

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

From the formula, it is clear that for a given force, the pressure is inversely proportional to the area. This means if the area over which the force acts is smaller, the pressure exerted is larger, and if the area is larger, the pressure is smaller.

This principle explains why:

• Pushing a nail by its pointed end into a wooden plank is easier than pushing by its head. The pointed end has a much smaller area, so the same force creates a much larger pressure.
• Shoulder bags have broad straps. Broad straps increase the contact area with the shoulder, distributing the weight over a larger area and reducing the pressure felt on the shoulder. Thin straps would exert higher pressure, causing discomfort.
• Cutting and piercing tools (like knives, needles) have sharp edges. Sharp edges have a very small area, allowing even a moderate force to exert high pressure for cutting or piercing.
• Porters carrying heavy loads often place a round piece of cloth on their heads. This increases the contact area between the load and their head, reducing the pressure on the head and making it easier to carry the weight.

Pressure Exerted By Liquids And Gases

Liquids and gases, collectively called fluids, also exert pressure. Unlike solids, fluids exert pressure not just downwards but also on the walls of the container they are in, and in all directions.

• Liquids Exert Pressure:
  • Liquids exert pressure on the bottom of their container. This pressure increases with the height of the liquid column. An experiment using a glass tube with a rubber sheet stretched over the bottom shows that the rubber bulges outwards, and the bulge increases as more water is poured into the tube (increasing the height of the water column).
  • Liquids also exert pressure on the walls of their container. An experiment with a bottle having a rubber sheet covering a hole in its side shows the rubber bulging outwards when the bottle is filled with water. This bulge increases with the depth of the hole from the water surface.
  • Liquids exert equal pressure at the same depth in all directions. This is shown by observing streams of water coming out of holes drilled at the same height on the side of a container; the streams of water shoot out to roughly the same distance.
  • Leaking joints or holes in water supply pipes demonstrate the pressure exerted by water on the pipe walls.
• Gases Exert Pressure:
  • Gases, like air, also exert pressure on the walls of their container. When you inflate a balloon, the air inside exerts pressure on the rubber walls, causing it to expand. If there are holes, the balloon cannot be inflated because the air escapes due to this internal pressure.
  • The air inside a bicycle tube also exerts pressure on its inner walls. A puncture allows this pressurised air to escape.
  • Gases exert pressure in all directions.

In summary, both liquids and gases exert pressure on the surfaces they are in contact with, including the walls and bottom of their containers.

Atmospheric Pressure

The Earth is surrounded by a vast envelope of air extending many kilometres upwards, known as the atmosphere. The air in the atmosphere 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.

Considering the definition of pressure as force per unit area, the atmospheric pressure on a given area can be thought of as the force of gravity (weight) acting on a column of air of unit area extending from the surface up to the top of the atmosphere.

The magnitude of atmospheric pressure is surprisingly large. An experiment using a rubber sucker pressed firmly onto a smooth surface demonstrates this. By pressing the sucker, most of the air between the sucker and the surface is expelled. The atmospheric pressure acting on the outside of the sucker then pushes it firmly against the surface. Pulling the sucker off requires a force strong enough to overcome this atmospheric pressure.

For example, the force exerted by the atmospheric pressure on an area of $15 \text{ cm} \times 15 \text{ cm}$ is equivalent to the force of gravity on an object with a mass of about 225 kg (which is approximately $2250 \text{ N}$).

We are not crushed by this immense pressure because the fluids (blood and other liquids) inside our bodies also exert pressure that balances the external atmospheric pressure.

Otto von Guericke's historical demonstration with two large metallic hemispheres from which air was pumped out showed the immense force of atmospheric pressure. Even eight horses pulling on each hemisphere could not separate them.

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