Force and Pressure class 8th Notes/ Numerical/download pdf
Force and Pressure Notes, Formulae, pdf download
Class:-8th
Introduction
In our daily life, we often push or pull objects. These actions are examples of force. Force can change the state of motion, shape, or direction of an object. This chapter deals with the basic concepts of force and pressure, and their applications in real life.
Force
- A force is a push or pull acting on an object.
- It is a vector quantity, meaning it has both magnitude and direction.
Effects of Force
Force can:
- Change the shape of an object.
- Change the speed of an object.
- Change the direction of motion of an object.
- Bring an object in motion to rest and rest to motion.
Types of Forces
1. Contact Force:
Types of Contact Force:
1) Muscular Force:
Definition:
- Muscular force is the force exerted by the muscles of humans or animals to
perform physical tasks.
Key Points:
- It is a contact force, meaning it acts only when
there is physical contact.
- It is used in daily activities like pushing,
pulling, lifting, and walking.
- Both humans and animals use this force to move
or hold things.
Examples:
- Lifting a school bag
- Pedaling a bicycle
- Kicking a football
Definition:
- Friction force is the resisting force that occurs when two surfaces are in
contact and one tries to move over the other.
- It is also a contact force.
- It opposes motion.
- Friction helps in walking, writing, driving, etc.
- It produces heat and causes wear and tear.
Examples:
- A ball slowing down on the ground.
- Rubbing hands to produce heat.
- Brakes stopping a bicycle.
2. Non-contact Force – Acts without contact (e.g., gravitational force, magnetic force, electrostatic force).
Types of Non-contact Force:
1) Gravitational Force:
- Definition: The force of attraction between two masses.
- Example: Earth pulling objects toward itself (falling of an apple).
- Acts on: All objects with mass.
- Direction: Always attractive, toward the center of the massive body (e.g., Earth)
2) Magnetic Force:
- Definition: The force exerted by magnets on other magnets or magnetic materials (like iron).
- Example: A magnet attracting iron nails.
- Acts on: Magnetic materials or moving electric charges.
- Can be: Attractive or repulsive depending on the poles.
3) Electrostatic Force:
- Definition: The force between two charged objects.
- Example: A rubbed comb attracting tiny bits of paper.
- Acts on: Charged particles.
- Can be: Attractive or repulsive depending on the charges (like charges repel, unlike charges attract).
Pressure
- Pressure is the force acting perpendicularly per unit area.
- It is given by the formula:
Pressure = Force / Area
Unit of Pressure
- SI unit of pressure is Pascal (Pa), where 1 Pa = 1 N/m².
Pressure Exerted by Liquids and Gases
- Liquids and gases exert pressure on the walls of the container.
- Pressure in liquids increases with depth.
- Air pressure decreases with height.
Applications of Pressure in Daily Life
- Sharp knives have small surface area to increase pressure and cut easily.
- Broad camel feet reduce pressure on sand.
- Syringes and hydraulic lifts work using liquid pressure.
Solved Numerical
Q1: A body of mass 5 kg is moving with an
acceleration of 3 m/s². Calculate the force applied.
Solution:
Formula: F = m × a
Solution: F = 5 × 3 = 15 N
Q2: A force of 100 N is applied over an area
of 5 m². What is the pressure exerted?
Solution:
Formula: P = F / A
Solution: P = 100 / 5 = 20 Pa
Q3: The pressure at the bottom of a water
tank is 4000 Pa. If the area of the base is 2 m², calculate the force exerted
by the water on the base.
Solution:
Formula: F = P × A
Solution: F = 4000 × 2 = 8000 N
Q4: A brick of weight 30 N has dimensions 20
cm × 10 cm. Calculate the pressure when it rests on the smaller face.
Solution:
Area: A = 0.2 × 0.1 = 0.02 m²
Pressure: P = F / A
Solution: P = 30 / 0.02 = 1500 Pa
Q5: A 60 N force is applied on two different
surfaces:
- Surface A: 0.5 m²
- Surface B: 0.2 m²
Calculate the pressure on both.
Solution:
For A: P = 60 / 0.5 = 120 Pa
For B: P = 60 / 0.2 = 300 Pa
Conclusion: Pressure increases when area decreases.
Practice Numerical:
(Answer: 2500 N)
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