Class 8 Science Notes Chapter 11 (Force and Pressure) – Science Book

Let's get straight into the important points from Chapter 11: Force and Pressure. Pay close attention, as these concepts are fundamental and often tested.

Chapter 11: Force and Pressure - Key Notes for Exams

1. What is Force?

• Think simple: A push or a pull on an object is called a force.
• Interaction is Key: Forces only come into play when there's an interaction between at least two objects. You can't apply a force without something to push or pull against, and something to receive that push or pull. (Example: Pushing a box, a magnet attracting a nail).

2. Effects of Force:

What can a force do to an object? It can:

• Change the State of Motion:
  • Make a stationary object move (e.g., kicking a football).
  • Stop a moving object (e.g., catching a ball).
  • Change the speed of a moving object (e.g., pushing a moving swing to make it go faster).
  • Change the direction of motion of an object (e.g., hitting a moving cricket ball with a bat).
• Change the Shape of an Object: (e.g., squeezing a sponge, stretching a rubber band, hammering metal).

Important Note: A force may cause these changes. Sometimes, a force isn't strong enough to overcome other forces (like friction or the object's weight) to cause a visible change in motion.

3. Magnitude and Direction of Force:

• Force has both magnitude (how strong the push or pull is) and direction (which way it's acting). This makes it what we call a vector quantity (though you might not need that term specifically, understand the concept).
• Net Force: When multiple forces act on an object:
  • Forces in the same direction add up. (e.g., Two people pushing a heavy box together).
  • Forces in opposite directions subtract. The object moves (or tends to move) in the direction of the larger force. (e.g., Tug-of-war).
  • If forces are equal and opposite, the net force is zero, and there's no change in the state of motion (the object stays still or continues moving at a constant velocity - this is balanced forces).

4. Types of Forces:

We broadly classify forces into two main types based on whether contact is needed:

• (A) Contact Forces: These forces act only when the objects are in direct physical contact.

  • Muscular Force: Force exerted by the muscles of living beings (humans, animals). (e.g., lifting a bag, chewing food, bullocks pulling a cart). This is always a contact force.
  • Frictional Force (Friction): This is a very important force that opposes motion between surfaces in contact. It always acts in the direction opposite to the intended or actual motion. (e.g., Ball rolling on the ground eventually stops, difficulty sliding a heavy box).

• (B) Non-Contact Forces: These forces can act even when objects are not in direct contact (they act over a distance).

  • Magnetic Force: The force exerted by magnets. (e.g., A magnet attracting iron pins, repulsion between like poles of two magnets).
  • Electrostatic Force: The force exerted by a charged body on another charged or uncharged body. (e.g., A plastic straw rubbed with paper attracting small bits of paper, lightning).
  • Gravitational Force (Gravity): The force of attraction that objects with mass exert on each other. The Earth pulls everything towards its center – this is the force of gravity we experience daily. (e.g., A ball thrown up falls down, leaves falling from a tree, rivers flowing downhill).
    • Key Point: Gravity acts on all objects in the universe. It's not just the Earth! You exert a tiny gravitational pull on everything around you, but it's usually too small to notice compared to Earth's gravity.
    • Weight: The force of gravity acting on an object is its weight.

5. Pressure:

• Now, let's relate force to the area over which it acts.
• Definition: Pressure is the force acting perpendicularly on a unit area of a surface.
• Formula:
  Pressure (P) = Force (F) / Area (A)
• Units: The standard unit is Pascal (Pa), which is Newton per square meter (N/m²).

6. Pressure and Area Relationship (Crucial Concept!):

• From the formula (P = F/A), you can see:
  • For the same force (F), if the area (A) is smaller, the pressure (P) is larger.
  • For the same force (F), if the area (A) is larger, the pressure (P) is smaller.
• Everyday Examples:
  • Sharp Knife vs. Blunt Knife: A sharp knife has a very small edge area, so even a small force creates high pressure, making cutting easy.
  • Nails/Pins have Pointed Ends: Small area concentrates the force, creating high pressure to easily penetrate surfaces.
  • Wide Straps on School Bags: Larger area distributes the weight (force) over your shoulder, reducing pressure and making it comfortable.
  • Wide Foundations for Buildings/Dams: Distributes the huge weight over a large area of ground, reducing pressure to prevent sinking.
  • Tractor/Tank Tyres: Wide tyres/tracks reduce pressure on soft ground. Camels have wide feet for the same reason on sand.

7. Pressure Exerted by Liquids:

• Liquids exert pressure on the bottom and the walls of their container.
• Key Properties:
  • Pressure exerted by a liquid increases with depth. The deeper you go, the more weight of the liquid is above you, hence more pressure. (Think of divers).
  • Liquid exerts pressure sideways as well (on the walls of the container).
  • At any given depth, a liquid exerts equal pressure in all directions.

8. Pressure Exerted by Gases:

• Gases also exert pressure on the walls of their container.
• Atmospheric Pressure: The air around us forms the atmosphere. This air has weight and exerts pressure on everything within it. This pressure is called atmospheric pressure.
  • It acts in all directions.
  • It's quite large! We don't get crushed because the pressure inside our bodies (e.g., blood pressure) balances the atmospheric pressure outside.
  • Examples demonstrating atmospheric pressure: Rubber suckers sticking to surfaces, drinking with a straw (you reduce pressure inside the straw, atmospheric pressure outside pushes the liquid up), collapsing tin can experiment.

Exam Focus Points:

• Know the definitions of Force and Pressure.
• Be able to list the effects of force with examples.
• Differentiate clearly between contact and non-contact forces and give examples of each type (especially Muscular, Friction, Magnetic, Electrostatic, Gravitational).
• Understand the P=F/A relationship and explain everyday examples based on it.
• Know the key properties of pressure exerted by liquids (increases with depth, acts sideways, equal at same depth).
• Understand the concept of atmospheric pressure and how it explains phenomena like using a straw or a sucker.

Make sure you understand these concepts thoroughly. Don't just memorize definitions; try to visualize the examples. Good luck with your preparation!