Lesson 4. Force and Pressure Notes

Introduction

"Force is the heartbeat of all motion in the universe. From the fall of an apple to the launch of a rocket, every movement is governed by forces acting upon objects. It is one of the most fundamental concepts in physics, described by Newton's laws and central to understanding everything from everyday mechanics to the vast dynamics of galaxies. This paragraph explores the nature of force, its types, and how it shapes the physical world around us. In common men language force is a push or pull of an object. In most cases, the interaction between two objects leads to force, however, in some cases, force happens without a contact between 2 bodies. . Force has both magnitude(amount) and direction. The strength of a force is expressed in magnitude. In this lesson, we are going to discuss about various aspects of force such as , types of force, characteristics of force, and measurement.

Push

A push is a force exerted away from the body, for example, Hitting a snooker ball, or kicking a football. The force of attraction or repulsion between two magnetic bodies due to their poles is known as a magnetic force.

Pull

A pull is a force exerted towards the body, for example, drawing a bucket of water from a well or playing tug of war.

 "Push and pull forces are the simplest, but most essential actions to produce movement of an object. When you are opening a door, kicking a ball, or pulling a drawer, you're applying a force that either pushes or pulls an object. These forces can start motion, stop it, change its direction, or even change the shape of an object. Understanding push and pull helps us make sense of how things move and interact in our daily lives.

Net force

Net force is the total force acting on an object. Net force determines in which direction the force is more and in which it is less. When multiple forces act on an object at the same time, they can either balance each other out or combine to create movement. If the forces are balanced, the object stays still or moves at a constant speed. But if they are unbalanced, the object will accelerate in the direction of the greater force.

Balanced and unbalanced forces

Balanced and unbalanced forces explain why objects stay still, move, or change direction. When forces acting on an object are balanced, they cancel each other out, and the object stays at rest or keeps moving at the same speed. But when the forces are unbalanced, one force is stronger than the others, causing the object to start moving, stop, or change direction. These concepts are essential in physics because they help us understand how and why things move the way they

Frictional force

"Frictional force is the resistance or the oppositional force on an object against the direction of moving object. It plays a crucial. Friction is a friend as well as foe, Friction can help in stopping the moving vehicle. Without friction, we wouldn’t be able to grip objects, write with a pencil, or even stand still. Although it can sometimes slow things down, friction is essential for control and stability in motion." On the other side, friction causes damage to goods and parts of engine due to heavy resistance.

Effects of force

"Force can do many things — it can move objects. Some of the effects of force includes

• Force can stop moving objects.
• It can change their direction
• Force can change the objects shape.
• Forces can also twist, stretch, or squash things, like when you squeeze a sponge or pull on a rubber band.

Vector quantities

Vector quantities are physical quantities that have both magnitude and direction. This means they not only tell us how much of something there is, but also which way it's going. For example, velocity is a vector because it describes how fast something is moving and in what direction. Other examples include force, acceleration, and displacement. Understanding vector quantities is important in physics because they help us describe motion and interactions in a more complete and accurate. Some examples of vector quantities are ; velocity, displacement, weight, momentum, force, acceleration etc.

When multiple forces act on a body, they can be resolved into one component known as the net force acting on the object.
Example:

State of Motion

"The state of motion of an object refers to whether it is at rest or in movement. An object is said to be in motion if it changes its position over time compared to a reference point. If it does not change its position, it is considered to be at rest or the state of inertia. The state of motion can also include the speed and direction in which an object is moving. Forces can change an object’s state of motion by making it start moving, stop, speed up, slow down, or change direction." An object at rest has zero velocity which happens due to the absence of an unbalanced force. 

Inertia = tendency of an object to resist changes in its velocity.

Types of Forces

Forces are majorly classified as contact and noncontact forces. When force is applied on an object that cause a change in the state with a physical contact, this is known as contact force. Touch or contact is required to do most of our everyday activities. For example, lifting, pulling, kicking, pushing etc. Unlike contact forces, which need direct interaction (like pushing or pulling), non-contact forces can affect objects from a distance. These forces occur due to invisible fields like gravity, magnetism, and electricity. Even though we can’t see these forces, we can observe their effects—like when a magnet pulls a paperclip or when the Earth pulls objects downward." The table below illustrates different types of forces.

Type of Force	Definition	Example
Gravitational Force	The force of attraction between two masses, usually between Earth and objects on it.	A dropped ball falling to the ground.
Frictional Force	The force that opposes the motion of objects sliding or moving past each other.	Rubbing hands together, or a book sliding on a table slowing down.
Muscular force	The force applied by the effort of our muscles.	lifting a heavy box, pulling a bucket of water or pedalling a cycle
Tension Force	The force transmitted through a string, rope, or cable when it is pulled tight.	A tug of war rope being pulled by teams.
Normal Force	The support force exerted by a surface perpendicular to an object resting on it.	A book resting on a table. The table pushes up on the book.
Air Resistance Force	A type of frictional force that acts against objects moving through air.	A parachute slowing down a falling person.
Magnetic Force	The attraction or repulsion between magnetic objects or fields.	Magnets sticking to a fridge or repelling each other.
Electrostatic Force	The force between charged particles or objects.	Clothes sticking together after coming out of the dryer.

Thrust and Pressure

It is commonly produced by engines or propellers and is essential in moving vehicles like rockets, airplanes, and boats. Thrust works by pushing air or gas in the opposite direction — a principle explained by Newton’s Third Law of Motion: for every action, there is an equal and opposite reaction. For example, when a rocket expels hot gases downward, the rocket is pushed upward with equal force — this upward force is called thrust."

 Pressure, on the other hand, is the force applied per unit area on a surface. It tells us how concentrated a force is. For example, when a person stands on one foot instead of two, the pressure on the ground increases because the same force (weight) is applied over a smaller area." Pressure depends on two factors: the amount of force applied and the area over which it is applied. When the same force is applied over a smaller area, the pressure increases. On the other hand, if the same force is spread over a larger area, the pressure decreases. This is why sharp objects like knives cut easily (small area = high pressure), while snowshoes help people walk on snow without sinking (large area = low pressure). Understanding how pressure distributes helps in designing everything from tools to buildings."

Pressure Distribution:

Situation	Force	Area	Pressure Effect
Walking in snow with regular shoes	Body weight	Small foot area	High pressure — feet sink into snow
Walking in snow with snowshoes	Body weight	Large foot area	Low pressure — feet stay on top of snow
Cutting with a sharp knife	Hand force	Tiny edge area	High pressure — cuts easily
Sitting on a soft cushion	Body weight	Wide seat area	Low pressure — comfortable sitting experience

Fluid pressure

It is the force exerted by a fluid (liquid or gas) on the walls of its container or on any object in the fluid. This pressure is caused by the constant movement of the fluid’s particles, which collide with surfaces. Fluid pressure increases with depth in liquids because the weight of the fluid above adds more force. For example, when you dive deeper into a swimming pool, you feel more pressure in your ears — that’s fluid pressure at work. It also explains how things like hydraulic systems, water supply, and air pressure function."

Key Points on Fluid Pressure:

Factor	Effect on Fluid Pressure	Example
Depth	Pressure increases with depth	Deep-sea divers feel greater water pressure
Density of fluid	Higher density = greater pressure	Pressure in saltwater is higher than in freshwater
Gravity	More gravity = more weight = higher pressure	Pressure on Earth is higher than on the Moon
Container shape	Does not affect pressure at a given depth	Tall or wide tanks have the same pressure at the bottom (if depth is equal)

Upthrust The upward force exerted by a fluid on an object is known as upthrust or buoyant force.

Atmospheric Pressure

Gaseous pressure

Gases exert the same pressure on the walls of the container in all directions.

Atmospheric pressure

• Our atmosphere extends to several kilometres above sea level. The weight of the air acts as pressure known as atmospheric pressure.
• The pressure in our body balances the atmospheric pressure and that is why we do not feel it.

Question 1.Give two examples each of situations in which you push or pull to change the state of motion of objects.

Answer: Two examples in which push changes the state of motion of objects are:

• The football player kicking the football.
• The hockey player hitting the ball.

Two examples in which pull changes the state of motion of objects are:

• Pulling out water from the well by pulling rope.
• Opening of a box in the table by pulling it.

Question 2.Give two examples of situations in which applied force causes a change in the shape of an object.

Answer: We can change the shape of a lump of dough by pressing it down with our hands.

We can change the shape of the seat of a bicycle by sitting on it.

Question 3. Fill in the blanks in the following statements.

Answer:

1. To draw water from a well we have to pull at the rope.
2. A charged body attracts an uncharged body towards it.
3. To move a loaded trolley we have to pull
4. The north pole of a magnet repels the north pole of another magnet.

Question 4. An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move towards the target. Based on this information, fill up the gaps in the following statements using the following terms. (muscular, contact, non-contact, gravity, friction, shape, attraction)

 

Answers :

To stretch the bow, the archer applies a force that causes a change in its shape.

The force applied by the archer to stretch the bow is an example of muscular force.

The type of force responsible for a change in the state of motion of the arrow is an example of a contact force.

While the arrow moves towards its target, the forces acting on it are due to gravity and that due to friction of air.

Question 5. In the following situations identify the agent exerting the force and the object on which it acts. State the effect of the force in each case.

(а) Squeezing a piece of lemon between the fingers to extract its juice.

(b) Taking out paste from a toothpaste tube.

(c) A load suspended from a spring while its other end is on a hook fixed to a wall.

(d) An athlete making a high jump to clear the bar at a certain height.

Answer:

(а) The agent exerting the force is our muscles (muscular force). The force is exerted on a lemon. The effect of force is that it changes the shape of the lemon and juice is extracted.

(b) The agent exerting the force is our muscles muscular force). The force is exerted on the toothpaste tube. The effect of force is that it changes the shape of the toothpaste tube due to which paste comes out.

(c) The agent exerting the force is the load (non-contact force). The force is exerted on spring and the effect is that the spring expands.

(d) The agent exerting the force is the speed of an athlete. The force is exerted on the body of the athlete. The effect is that he can make his body jump the bar.

Question 6. A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?

Answer:A blacksmith hammers a hot piece of iron while making a tool. The muscular force applied changes the shape of piece of iron. It flatters and becomes thinner than earlier.

Question 7. An inflated balloon was pressed against a wall after it has been rubbed with a piece of synthetic cloth. It was found that the balloon sticks to the wall. What force might be responsible for the attraction between the balloon and the wall?

Answer: The electrostatic force is responsible for the attraction between the balloon and the wall.

Question 8. Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.

Answer: The forces acting on the bucket are:

1. The pressure of water contained in it exerted on its wall and on the base of the bucket.
2. Force of gravity by the earth.
3. The forces acting on the bucket do not bring a change in its state of motion because the net effect of force on it is zero.

Question 9. A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad

Answer: The two forces that are acting on the rocket immediately after leaving the launching pad are: Force of gravity of earth acting downward, and the frictional force produced by air particles.

Question 10. When we press the bulb of a dropper with its nozzle kept in water, the air in the dropper is seen to escape in the form of bubbles Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to

Answer:

(d) Atmospheric pressure