1. Force: A Push or a Pull

In our daily life, we perform various actions like pushing, pulling, lifting, stretching, twisting and pressing. For example, to open a door we either push or pull it. To draw water from a well, we pull the rope. To kick a football, we push it with our foot.

Forces are due to an interaction. At least two objects must interact for a force to come into play. For example, if a man stands behind a stationary car, there is no interaction, and no force is applied. But if he pushes the car, he applies a force on it due to the interaction between him and the car.

2. Effects of Force

A force can produce the following effects:

• Force can change the state of motion: A force can move a stationary object (e.g., kicking a football). It can stop a moving object (e.g., a goalkeeper catching a ball). It can change the speed of a moving object (e.g., pushing a moving swing). It can change the direction of motion (e.g., hitting a cricket ball with a bat).
• Force can change the shape of an object: Squeezing a toothpaste tube, stretching a rubber band, pressing a lump of dough, sitting on a sofa with springs.

3. Types of Forces

Forces can be broadly classified into two categories:

A. Contact Forces

Contact forces come into play only when two objects are in physical contact with each other.

• Muscular Force: The force resulting from the action of muscles is known as muscular force. Animals like bullocks, horses, donkeys and camels use muscular force to perform various tasks for us. Since muscular force can be applied only when it is in contact with an object, it is called a contact force.
• Friction: The force responsible for changing the state of motion of objects in contact with a surface is friction. It always acts on all the moving objects and its direction is always opposite to the direction of motion. For example, a ball rolling on the ground gradually slows down and comes to rest due to the force of friction between the ball and the ground.

B. Non-Contact Forces

Non-contact forces can act even when the objects are not in physical contact with each other.

• Magnetic Force: The force exerted by a magnet is a magnetic force. Like poles repel each other and unlike poles attract each other. A magnet can attract an iron object from a distance.
• Electrostatic Force: The force exerted by a charged body on another charged or uncharged body is known as electrostatic force. For example, a straw rubbed with paper attracts another straw.
• Gravitational Force: Objects or things fall towards the earth because it pulls them. This force is called the force of gravity, or just gravity. It is an attractive force. Every object in the universe exerts a force on every other object. This force is known as the gravitational force.

4. Pressure

Have you ever wondered why school bags have wide straps? Or why a sharp knife cuts better than a blunt one?

The SI unit of pressure is Pascal (Pa). From the formula, it is clear that for the same force, if the area is smaller, the pressure is more. This is why a sharp knife (small area of contact) cuts easily, and a nail has a pointed end to be driven into the wall easily. Conversely, wide straps of school bags increase the area of contact, reducing the pressure on shoulders.

5. Pressure Exerted by Liquids and Gases

Liquids and gases also exert pressure.

• Liquids exert pressure on the walls of the container: If you take a plastic bottle and make holes at the same height, water comes out with the same force. This shows that liquids exert equal pressure at the same depth.
• Pressure increases with depth: The pressure exerted by a liquid increases with depth. That is why dams have thicker walls at the bottom.
• Gases exert pressure: Gases also exert pressure on the walls of their container. An inflated balloon stays inflated due to the pressure exerted by the air inside.

6. Atmospheric Pressure

Our earth is surrounded by a layer of air called atmosphere using extends up to many kilometres above the surface of the earth. The air in the atmosphere has weight. The pressure exerted by this air is known as atmospheric pressure.

Applications:

• Rubber Sucker: When pressed against a smooth surface, it sticks because the air between the sucker and the surface escapes, and atmospheric pressure presses it down.
• Drinking Straw/Dropper: When we suck air out, the atmospheric pressure acting on the liquid surface pushes the liquid up into the straw or dropper.

7. Magnitude and Direction of Force

The strength of a force is usually expressed by its magnitude. We also have to specify the direction in which a force acts. If the direction or magnitude of the applied force changes, its effect also changes.

• Forces applied in the same direction add to one another. (Net force = sum of forces)
• Forces applied in opposite directions subtract from one another. (Net force = difference of forces, acts in direction of larger force)

A force is a push or pull acting on an object. It arises due to the interaction between at least two objects.

A force can change the state of motion of an object (speed, direction) or change its shape.

The state of motion of an object is described by its speed and the direction of motion. The state of rest is considered to be the state of zero speed.

Forces that come into play only when two objects are in physical contact are called contact forces. Examples are muscular force and friction.

The force resulting from the action of muscles is called muscular force. It is used for lifting, pushing, pulling, and all bodily movements.

Friction is an opposing force that comes into play when two surfaces are in contact and moving relative to each other. It acts opposite to the direction of motion.

Forces that can act from a distance without physical contact are called non-contact forces. Examples are magnetic force, electrostatic force, and gravitational force.

The force exerted by a magnet is called magnetic force. Like poles repel each other, while unlike poles attract each other.

The force exerted by a charged body on another charged or uncharged body is called electrostatic force. It can be attractive or repulsive.

Every object in the universe exerts an attractive force on every other object. This force is called gravitational force. It is the force that holds us to the earth.

Gravity is the gravitational force exerted by the earth. It pulls all objects towards the centre of the earth. Leaves fall and water flows down due to gravity.

Pressure is the force acting normally (perpendicularly) on a unit area of a surface. Pressure = Force / Area.

The SI unit of pressure is Pascal (Pa). One Pascal is equal to a force of 1 Newton acting on an area of 1 square metre (1 N/m²).

For the same applied force, pressure is inversely proportional to the area. Smaller area results in large pressure (sharp knife), while larger area results in less pressure (wide straps).

Liquids exert pressure on the walls and bottom of the container. The pressure exerted by a liquid increases with depth.

At the same depth, a liquid exerts equal pressure in all directions. This is why water spurts out equally from holes at the same level in a bottle.

The envelope of air surrounding the earth is called the atmosphere. The weight of this air column exerts a pressure on the earth surface, known as atmospheric pressure.

Atmospheric pressure is immense. At sea level, it is about 101,325 Pascals. We don't feel it because our body's internal pressure balances it.

A rubber sucker sticks to a smooth surface because pressing it expels air from the cup, creating a vacuum. Atmospheric pressure pushes it firmly against the surface.

When you suck on a straw, you reduce the air pressure inside it. The higher atmospheric pressure on the drink's surface pushes the liquid up into the straw.

A sharp knife has a very thin edge (small area). When force is applied, it exerts high pressure, allowing it to cut through objects easily.

School bags have wide straps to increase the area of contact with the shoulder. This reduces the pressure exerted by the weight of the bag, making it comfortable to carry.

The pointed tip of a sewing needle has a tiny surface area. This allows a small force to create enough pressure to pierce through fabric.

Porters place a round piece of cloth (pagri) on their heads. This increases the area of contact with the load, reducing pressure and making it easier to carry heavy loads.

If two equal forces act on an object in opposite directions, the net force is zero. The object stays at rest or continues moving at constant speed.

Net force is the vector sum of all forces acting on an object. Forces in same direction add up; forces in opposite directions subtract.

A spring balance is a device used for measuring the force acting on an object. It works on the principle that extension of spring is proportional to force.

A barometer is an instrument used to measure atmospheric pressure. The mercury barometer was invented by Torricelli.

A manometer is a device used to measure liquid pressure or pressure difference. A simple U-tube manometer is commonly used in labs.

Walking on sand is difficult with high heels because the small area sinks into the sand due to high pressure. Flat shoes distribute weight over a larger area.

Camels have broad feet which exert less pressure on the ground. This prevents them from sinking into the sand and helps them walk easily in deserts.

Heavy tanks run on continuous broad caterpillar tracks. These tracks increase the surface area enormously, reducing pressure on the ground to prevent sinking.

A German scientist who demonstrated the strength of atmospheric pressure using two hollow metallic hemispheres (Magdeburg hemispheres) that could not be pulled apart by horses.

Tools meant for cutting and piercing always have sharp edges or points to maximize pressure for a given force.

Particles of a gas collide with the walls of their container randomly. The force exerted by these collisions per unit area is the gas pressure.

Atmospheric pressure decreases as we go up (altitude increases). At high altitudes, nose bleeding may occur because blood pressure becomes higher than outside pressure.

Force is a vector quantity, meaning it has both magnitude (strength) and direction. Both must be specified to fully describe a force.

Interaction is necessary for force. A magnet cannot attract an iron nail unless there is a magnetic field interaction between them.

Weight is the force with which the earth attracts an object. It is a force, so its unit is newton (N). Weight = Mass x Gravity.

Mass is the amount of matter in an object (kg), while weight is the force of gravity on it (N). Mass is constant, weight varies with gravity.

1 Newton is the force required to accelerate a mass of 1 kg by 1 m/s². Roughly, it is the force required to hold a 100g mass against gravity.

A syringe works on atmospheric pressure. Pulling the plunger creates low pressure, and atmospheric pressure pushes the liquid medicine in.

Walls of a dam are made thicker at the bottom than at the top to withstand the high pressure exerted by water at greater depths.

Deep sea divers wear special suits to protect themselves from the crushing pressure of water at great depths.

In a tug of war, if both teams pull with equal force, the rope does not move. The forces are balanced.

If the forces acting on an object are unequal, there is a net non-zero force. This unbalanced force causes a change in motion.

Friction produced by rubbing hands together creates heat. This is an effect of force converting mechanical energy to heat energy.

The force applied to stretch a spring changes its shape (length). When released, the restoring force brings it back to original shape.

When kneading dough, muscular force is applied to change the shape of the dough ball into a flat chapati.

Sir Isaac Newton formulated the laws of motion and universal gravitation, laying the foundation for understanding force and motion.