Class 8 Science Notes Chapter 11 (Chapter 11) – Examplar Problems Book

Alright class, let's focus on Chapter 11: Force and Pressure from your NCERT Class 8 Science Exemplar. This chapter is crucial for understanding fundamental physics concepts often tested in government exams. Pay close attention to the definitions, types, effects, and units.

Chapter 11: Force and Pressure - Detailed Notes for Exam Preparation

1. Force:

• Definition: A push or a pull on an object is called a force.
• Interaction: Forces arise due to the interaction between at least two objects. If there's no interaction, there's no force.
• Nature: Force has both magnitude (strength) and direction. This makes it a vector quantity (though the term 'vector' might not be explicitly used in Class 8, understanding direction is important).
• Unit: The SI unit of force is the Newton (N).

2. Effects of Force:

A force applied to an object can:

• Change the state of motion:
  • Make a stationary object move.
  • Stop a moving object.
  • Change the speed of a moving object (increase or decrease).
  • Change the direction of motion of a moving object.
  • Example: Kicking a football (starts moving), applying brakes on a bicycle (stops moving), accelerating a car (speed increases), hitting a cricket ball with a bat (direction changes).
• Change the shape or size of an object:
  • Example: Squeezing a sponge, stretching a rubber band, hammering a metal sheet.
• Note: A force may cause these changes; it doesn't always succeed (e.g., pushing a heavy wall doesn't move it).

3. Net Force:

• If multiple forces act on an object:
  • Forces in the same direction add up.
  • Forces in opposite directions result in a net force equal to the difference between the two forces, acting in the direction of the larger force.
  • If forces are equal and opposite, the net force is zero, and there is no change in the state of motion (object remains at rest or continues moving at a constant velocity - this is a balanced force scenario).

4. Types of Forces:

Forces are broadly classified into two types:

• A. Contact Forces: Forces that act only when the objects are in direct physical contact.

  • i. Muscular Force: The force exerted by the muscles of living beings (humans or animals).
    • Examples: Lifting a book, pulling a cart (by a bullock), chewing food, breathing (diaphragm muscle). It's always a result of muscle action.
  • ii. Frictional Force (Friction): A force that opposes the motion of an object over a surface or through a fluid (like air or water). It always acts in the direction opposite to the direction of motion or intended motion.
    • Examples: A ball rolling on the ground eventually stops due to friction. We can walk because of friction between our shoes and the ground. Friction causes wear and tear.

• B. Non-Contact Forces: Forces that can act even when the objects are not in direct physical contact (act over a distance).

  • i. Magnetic Force: The force exerted by magnets on magnetic materials (like iron, nickel, cobalt) or other magnets.
    • Examples: A magnet attracting iron pins, repulsion between like poles of two magnets, attraction between unlike poles.
  • ii. Electrostatic Force: The force exerted by a charged body on another charged or uncharged body.
    • Examples: A plastic comb rubbed with dry hair attracts small pieces of paper. A charged balloon sticking to a wall.
  • iii. Gravitational Force (Gravity): The force of attraction exerted by objects possessing mass. The Earth exerts a gravitational force on all objects near it, pulling them towards its center. This force exists between any two objects with mass in the universe.
    • Examples: A fruit falling from a tree, a ball thrown upwards eventually comes down, planets revolving around the sun. Weight of an object is the gravitational force exerted by the Earth on it.

5. Pressure:

• Definition: The force acting perpendicularly on a unit area of a surface is called pressure.
• Formula: Pressure (P) = Force (F) / Area (A)
• Unit:
  • The SI unit is Pascal (Pa). 1 Pascal = 1 Newton per square meter (1 Pa = 1 N/m²).
  • Other units like N/cm², atmosphere (atm), bar are also used.
• Relationship between Pressure, Force, and Area:
  • Pressure is directly proportional to the force applied (for a constant area). More force = More pressure.
  • Pressure is inversely proportional to the area over which the force acts (for a constant force). Smaller area = More pressure; Larger area = Less pressure.
• Applications/Examples based on Area:
  • Sharp knives cut better (small area, high pressure).
  • Nails have pointed tips (small area, high pressure).
  • Foundation of buildings is wide (large area, low pressure).
  • Tractor tyres are broad (large area, low pressure on soft ground).
  • Shoulder bags have broad straps (large area, low pressure on the shoulder).

6. Pressure Exerted by Liquids:

• Liquids exert pressure on the base and walls of their container.
• Pressure exerted by a liquid at a point increases with the depth of the point from the free surface.
• Liquid exerts equal pressure at the same depth.
• Liquids exert sideways pressure on the walls of the container.
• Example: Water leaks out faster from a hole near the bottom of a tank than from a hole near the top. Dams have thicker walls at the bottom.

7. Pressure Exerted by Gases:

• Gases also exert pressure on the walls of their container.
• Example: Inflating a balloon – the air inside exerts pressure on the balloon walls.

8. Atmospheric Pressure:

• Definition: The pressure exerted by the weight of the air in the atmosphere surrounding the Earth is called atmospheric pressure.
• Nature: It acts in all directions.
• Magnitude: It is quite large, but we don't usually feel it because the pressure inside our bodies (e.g., blood pressure) balances the external atmospheric pressure.
• Variation: Atmospheric pressure decreases as we go up to higher altitudes (less air above).
• Applications/Examples: Drinking with a straw (pressure difference), rubber suckers sticking to surfaces, droppers working.
• Measurement: Measured using a barometer.

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on Chapter 11 for your practice:

1. Which of the following is an example of a non-contact force?
   (a) Frictional force
   (b) Muscular force
   (c) Gravitational force
   (d) Force exerted by a spring

2. A ball rolling on the ground slows down and finally stops. This is because of:
   (a) Magnetic force
   (b) Electrostatic force
   (c) Frictional force
   (d) Gravitational force

3. The SI unit of pressure is:
   (a) Newton (N)
   (b) Kilogram (kg)
   (c) Pascal (Pa)
   (d) Meter per second (m/s)

4. Pressure exerted by a liquid at the bottom of a container depends on:
   (a) The shape of the container
   (b) The area of the bottom of the container
   (c) The height of the liquid column
   (d) The colour of the liquid

5. If the force applied to an object is doubled, keeping the area of contact constant, the pressure will:
   (a) Become half
   (b) Remain the same
   (c) Be doubled
   (d) Be quadrupled

6. Why are the foundations of high-rise buildings made wide?
   (a) To increase the force on the ground
   (b) To decrease the pressure on the ground
   (c) To increase the pressure on the ground
   (d) For aesthetic reasons

7. Which force is responsible for a charged balloon sticking to a wall?
   (a) Magnetic force
   (b) Gravitational force
   (c) Muscular force
   (d) Electrostatic force

8. Atmospheric pressure decreases as we go:
   (a) Deeper into the sea
   (b) To higher altitudes
   (c) Closer to the Earth's core
   (d) Towards the equator

9. Two forces of 10 N and 7 N are acting on an object in opposite directions. The net force acting on the object is:
   (a) 17 N
   (b) 3 N
   (c) 10 N
   (d) 7 N

10. Which of the following actions involves the application of muscular force?
    (a) A magnet attracting iron nails
    (b) An apple falling from a tree
    (c) Pushing a cart
    (d) Rubbing a comb with dry hair

Answers to MCQs:

1. (c) Gravitational force
2. (c) Frictional force
3. (c) Pascal (Pa)
4. (c) The height of the liquid column
5. (c) Be doubled (Pressure = Force/Area)
6. (b) To decrease the pressure on the ground (Pressure = Force/Area; larger area reduces pressure)
7. (d) Electrostatic force
8. (b) To higher altitudes
9. (b) 3 N (Net force = 10 N - 7 N = 3 N)
10. (c) Pushing a cart

Revise these notes thoroughly. Understanding the core concepts and the relationship between force, area, and pressure is key. Good luck with your preparation!