NCERT Science Notes - Class 8
Chapter 8 - Force and Pressure

Welcome to AJs Chalo Seekhen. This webpage is dedicated to Class 8 | Science | Chapter 8 - Force and Pressure. The chapter delves into the fundamental concepts of force and pressure. It explains that a force is a push or pull that can change the state of motion or shape of an object. The chapter covers different types of forces, including contact forces like muscular and frictional forces, and non-contact forces such as gravitational and magnetic forces. It also explores the concept of pressure, defined as force per unit area, and discusses how pressure is exerted by liquids and gases. Understanding these principles is crucial for grasping more complex scientific concepts later on.

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NCERT Science Notes - Class 8
Chapter 8 - Force and Pressure

8.1 : Force – A Push or a Pull

The terms like picking, opening, shutting, kicking, hitting, lifting, flicking, pushing, and pulling can indeed be replaced or described using the more general terms "push" and "pull" in many cases. Here's how:

1. Picking: This involves lifting an object, often upward, and can be categorized as a combination of "pull" (towards yourself or up) and "lifting" (an upward pull).
2. Opening: When you open something (like a door or a drawer), you typically use a "pull" motion towards yourself or a "push" if it's away from you.
3. Shutting: This generally involves a "push" to close something, like a door or a lid.
4. Kicking: When you kick something (like a ball), you're applying a "push" with your foot to propel it forward.
5. Hitting: This also applies a "push", as it involves applying force to an object, like striking a ball with a bat.
6. Lifting: Lifting involves a "pull" in the upward direction, often against gravity.
7. Flicking: This is a quick motion that often involves a combination of "push" and "pull", but it can generally be categorized as a "push" when propelling something away.
8. Pushing: This is straightforwardly a "push", where force is applied to move something away from oneself.
9. Pulling: This is simply a "pull", where force is applied to bring something towards oneself.

Table 8.1: Identifying Actions as Push or Pull based on common examples of motion:

8.2 - Forces are due to an Interaction

1. Interaction Between Objects:
   • A force acts on an object due to its interaction with another object.
   • Interaction is necessary for the force to come into play.
2. Example of a Car:
   • A man standing behind a stationary car (Fig. 8.2(a)) doesn’t cause the car to move.
   • The car moves only when the man pushes it, thus applying a force on the car (Fig. 8.2(b)).
   • This shows that force causes motion in the direction of the applied force.
3. Push and Pull Examples:
   • Fig. 8.3(a): Two girls are pushing each other. This action represents force by pushing.
   • Fig. 8.3(b): Two girls are pulling each other, demonstrating force by pulling.
   • Fig. 8.3(c): A man and a cow are pulling each other, illustrating that force can be exerted in opposite directions.
4. Summary of Force and Interaction:
   • Force results from interactions between objects.
   • Both objects in the interaction (whether pushing or pulling) experience a force, which may cause them to move or change their state of motion.

• At least two objects must interact for force to be exerted.
• Force can be either a push or a pull, depending on the nature of interaction between the objects.

8.3 - Exploring Forces

Concept of Force:

• Force can be applied in different ways and directions, influencing an object's movement.
• The magnitude (strength) and direction of the applied force are important in determining the effect.
  
  

1. Pushing an Object Alone:
   • If you try to push a heavy object like a table or box by yourself, it may be difficult to move.
2. Pushing with a Friend in the Same Direction (Fig. 8.4(a)):
   • When you and your friend push the object in the same direction, it becomes easier to move.
   • This demonstrates that forces in the same direction add up, increasing the overall force and making it easier to move the object.
3. Pushing in Opposite Directions (Fig. 8.4(b)):
   • If you push the object in one direction and your friend pushes from the opposite side, the object may not move or it will move in the direction of the larger force.
   • This shows that forces in opposite directions are subtracted from each other, and the net force acting on the object depends on which side is applying a larger force.
     
     

• In a game of tug-of-war, two teams pull a rope in opposite directions.
• The team that applies the larger force wins, pulling the rope toward them.
• If both teams pull with equal force, the rope stays in place, illustrating that when forces are equal and opposite, the net force is zero, and no movement occurs.
  
  

• Forces in the Same Direction: Add up, resulting in a larger net force.
• Forces in Opposite Directions: Subtract from one another, and the direction of movement depends on the larger force.
• Net Force: The overall force acting on an object, considering all forces in various directions.
• Magnitude of Force: The strength of a force, which, combined with direction, determines its effect.
  
  

• Yes, if two equal forces act in opposite directions, the net force is zero, meaning no movement occurs.

8.4 - A Force Can Change the State of Motion

Concept of Force and Motion:

• A force can alter the motion of an object, including starting its movement, stopping it, or changing its speed.

• Pushing a Ball on a Surface:
  • When you push a rubber ball placed on a level surface, the ball starts to move.
  • If you push the ball again while it is moving, its speed increases.
  • Placing your palm in front of the moving ball causes the speed to decrease when the ball touches your palm. If you hold the ball, it will stop moving entirely.

• In football, when a player takes a penalty kick, the ball is at rest, and the applied force causes it to move toward the goal.
• The goalkeeper applies a force to stop or deflect the ball, reducing its speed and potentially bringing it to a complete stop.

These examples demonstrate the effects of force on an object's speed:

• When force is applied in the direction of motion, the object's speed increases.
• When force is applied in the opposite direction of motion, the object's speed decreases.

• Children moving a rubber tyre or a ring push it repeatedly to increase its speed.
• This explains why force applied in the same direction as the movement results in an increase in speed.

• Force can change the state of motion of an object.
• Force applied in the direction of motion increases the speed.
• Force applied in the opposite direction decreases the speed, and can even stop the object.

• Paheli wonders if force can only change the speed of an object. This leads to exploring the next concept—whether force can also change the direction of an object's motion.

   State of Motion:

• The state of motion of an object is defined by its speed and direction.
• If an object is at rest, it is in a state of zero speed.
• An object can be either in motion or at rest—both are considered different states of motion.
  
  

Does Force Always Change the State of Motion?

• No, the application of force doesn’t always change the state of motion.
  • Example 1: Pushing a heavy box may not move it even if you exert maximum force.
  • Example 2: Pushing a wall doesn’t result in motion, as the wall remains stationary.

8.5 - Force can Change the Shape of an Object

In this section, we explore how applying force to objects can alter their shape, even when they are not free to move. Various activities demonstrate that different situations allow us to observe the effects of force in real-time.

Key Points:

• Change in Shape: Applying force can change the shape of objects without causing them to move.
• Applications of Force: The manner in which force is applied affects the outcome on the object.
• Experimental Observations: Conducting experiments with different materials can provide insight into how force impacts both shape and motion.

Table 8.2: Studying the Effect of Force on Objects

Conclusions from Observations of Table 8.2

1. Effect of Force on Objects:
   • Applying a force can result in various outcomes, including motion, change in speed, change in direction, and alteration of shape.
   • Each of these effects depends on the magnitude and direction of the force applied.
2. Specific Examples:
   • Inflated Balloon: When you press an inflated balloon between your palms, the balloon compresses and changes its shape. The air inside tries to push back against the applied force, illustrating how internal pressure interacts with external force.
   • Ball of Dough: Rolling a ball of dough to make a chapati changes its shape into a flat disc. The applied force evenly distributes the dough, demonstrating how force can reshape an object.
   • Rubber Ball on a Table: Pressing a rubber ball placed on a table compresses it, altering its shape temporarily. Once the force is removed, the ball typically returns to its original shape, indicating elastic deformation.

Summary of Effects of Force 

Through the activities performed, we understand that:

• A force can initiate movement in an object that is at rest.
• It can change the speed of a moving object, either increasing or decreasing it.
• The direction of motion can also be altered through the application of force.
• The shape of an object can change when a force is applied, as seen in the examples with dough and balloons.
• These effects may occur individually or simultaneously when force is applied to an object.

• None of these actions (movement, change in speed, direction, or shape) can occur without the application of a force.
• An object does not have the capability to move, change its speed, alter its direction, or change shape independently; these actions are solely the result of external forces acting upon them.

8.7 - Non-contact Forces

Electrostatic Force

Activity 8.7: Exploring Electrostatic Charge

1. Materials Needed:
   • A plastic straw
   • A piece of thread
   • A sheet of paper
2. Setup:
   • Cut the straw into two equal pieces.
   • Suspend one piece from the edge of a table using the thread.
3. Experiment:
   • Step 1: Take the other piece of straw and rub one end with the sheet of paper.
   • Step 2: Bring the rubbed end of this straw close to the suspended straw without letting them touch. What do you observe?
   • Step 3: Now rub the free end of the suspended straw with the sheet of paper.
   • Step 4: Bring the previously rubbed straw near the free end of the suspended straw again. What do you observe now?
4. Observations:
   • First Case: When you bring the rubbed straw near the suspended straw, they attract each other.
   • Second Case: After rubbing both straws, bringing them near each other may cause them to repel.

• Charged Straws: Rubbing a straw with paper gives it an electrostatic charge, making it a charged body.
• Electrostatic Force: This is the force that a charged body exerts on another charged or uncharged body.
• Non-Contact Force: The electrostatic force can act even when the objects are not touching, making it another example of a non-contact force.

Gravitational Force

• What Happens When You Release an Object: When you drop a coin or a pen, it falls to the ground. Similarly, leaves and fruits fall off plants when detached. This happens because they are pulled downwards.
• Why Do Objects Fall?: When you hold an object, it stays at rest. But when you release it, it starts moving downward. This change in motion is due to a force acting on the object.
• What is this Force?: The force that pulls objects toward the Earth is called gravity. It is an attractive force that acts on all objects, regardless of their size.
• Everyday Examples:
  • When you open a tap, water flows down because of gravity.
  • Rivers flow downward due to the force of gravity.
• Universal Force: Gravity is not just a property of the Earth; every object in the universe, big or small, exerts a gravitational force on every other object. This is known as gravitational force.

• Gravity causes objects to fall toward the Earth.
• It acts on all objects all the time.
• Every object in the universe has a gravitational effect on others.

8.8 - Pressure

• Understanding Pressure: Pressure is related to force. When you apply a force to an object, the way that force is distributed over an area matters.
• Example with a Nail:
  • When you push a nail into a wooden plank by its head, it’s difficult.
  • However, if you push the nail by its pointed end, it goes in easily.
• Sharp vs. Blunt Knives: Cutting vegetables is easier with a sharp knife than with a blunt one. This is because a sharp knife has a smaller area in contact with the vegetable, increasing the pressure.
• Definition of Pressure:
  • Pressure is defined as the force acting on a unit area of a surface:
  $$\text{Pressure}=\frac{\text{Force}}{\text{Area <span style="font-size: 1.07rem;"><br></span>}}$$
• Key Point: The smaller the area, the higher the pressure for the same amount of force. For example, the pointed end of a nail has a much smaller area than its head, allowing it to push into the wood more easily.
  
  
• Practical Examples:
  • Porters use a round piece of cloth on their heads to carry heavy loads. This increases the area in contact with their head, reducing pressure and making it easier to carry the load.
  • Shoulder bags have broad straps instead of thin ones to distribute the weight over a larger area, reducing pressure on the shoulders.
• Cutting Tools: Tools designed for cutting and piercing have sharp edges to apply pressure effectively with less force.

• Do liquids and gases exert pressure?: Yes, they do exert pressure, and it also depends on the area on which the force acts.

8.9 Pressure Exerted by Liquids and Gases

Activity 8.8: Exploring Liquid Pressure

• Materials Needed:
  • A transparent glass tube or plastic pipe (about 25 cm long and 5-7.5 cm in diameter).
  • A thin sheet of rubber (like a balloon) to stretch over one end of the pipe.
• Instructions:
  1. Setup: Stretch the rubber sheet tightly over one end of the pipe. Hold the pipe vertically in the middle.
  2. Pour Water: Ask a friend to pour water into the pipe.
  3. Observe:
     • Note if the rubber sheet bulges out as you pour water.
     • Measure the height of the water column in the pipe.
  4. Repeat: Continue pouring more water and observe:
     • The bulge in the rubber sheet.
     • The height of the water column each time.
• Observations:
  • As you increase the amount of water, observe the relationship between the height of the water column and the bulge in the rubber sheet.
  • Generally, the more water you pour, the higher the water column, which leads to a greater bulge in the rubber sheet.

• Relationship: The amount of bulge in the rubber sheet is related to the height of the water column in the pipe. This demonstrates how liquids exert pressure at the bottom of a container, which increases with the height of the liquid column.

Exploring Gas Pressure

• Do Gases Exert Pressure?:
  • Yes, gases also exert pressure.
  • They exert pressure on the walls of their containers.

• Fountains: When water leaks from holes in pipes, the pressure from the water inside causes the water to shoot out.
• Inflating Balloons:
  • When you inflate a balloon, you must close its mouth to keep the air inside.
  • If you open the mouth of an inflated balloon, the air escapes rapidly because it is under pressure.
  • If a balloon has holes, you cannot inflate it because the air will escape, indicating that air exerts pressure in all directions.

Air Pressure in Tires

• When a bicycle tire has a puncture, the air escapes because the pressure inside the tire is greater than the pressure outside.
• This shows that air exerts pressure on the inner walls of an inflated balloon or tire.

• Liquids and gases both exert pressure on the walls of their containers, confirming that pressure is a fundamental property of fluids (both liquid and gas).

8.10 - Atmospheric Pressure

Definition:
Atmospheric pressure is the pressure exerted by the weight of the air surrounding the Earth. This air, known as the atmosphere, extends many kilometers above the Earth's surface.Understanding Atmospheric Pressure:

• Atmospheric pressure is calculated as the force per unit area exerted by the weight of air.
• Visualize a unit area with a long cylinder filled with air above it; the force of gravity acting on this air is what we refer to as atmospheric pressure.

Activity 8.11: Exploring Atmospheric Pressure with a Rubber Sucker

Materials Needed:

• A good-quality rubber sucker (like a small rubber cup).

1. Press the Sucker:
   • Firmly press the rubber sucker onto a smooth surface.
   • Observe whether it sticks to the surface.
2. Attempt to Pull it Off:
   • Try to pull the sucker off the surface.
   • Note how difficult it is to remove.

• When you press the sucker, most of the air between the sucker and the surface escapes.
• The sucker sticks to the surface due to atmospheric pressure acting on it.
• To remove the sucker, you need to exert a force that is strong enough to overcome the atmospheric pressure.

• This activity illustrates the magnitude of atmospheric pressure.
• Without air, the sucker would not stick, indicating that atmospheric pressure is significant.

Boojho's Question:

• Force Exerted by Air:
  If the area of Boojho's head is 15 cm × 15 cm, the force exerted by air on his head can be calculated using the concept of atmospheric pressure.The pressure exerted by the air in a column of height equal to the atmosphere (around 1 atm) is roughly equal to the force of gravity acting on a mass of about 225 kg (or approximately 2250 N).

• The reason we don’t feel crushed by this force is that our bodies exert an equal internal pressure that balances the external atmospheric pressure.

• Atmospheric pressure is a significant force acting on all objects, balanced by the internal pressure of our bodies, allowing us to function normally without feeling the immense weight of the air around us.

Did You Know?

Otto von Guericke:

• Otto von Guericke was a German scientist in the 17th century known for his experiments with air pressure.

• He invented a pump designed to extract air from a vessel, demonstrating the power of atmospheric pressure.

• Guericke joined two hollow metallic hemispheres, each with a diameter of 51 cm, and used his pump to remove the air inside them.
• After creating a vacuum, he employed eight horses to pull the two hemispheres apart.

• Despite the strength of the horses, they could not separate the hemispheres, showcasing the incredible force exerted by atmospheric pressure. This experiment highlighted the significance of air pressure in a dramatic way.

NCERT Science Notes - Class 8 | Science | Chapter 8 - Force and Pressure

NCERT Science Notes - Class 8 | Science | Chapter 8 - Force and Pressure