Class 8 Force - Notes

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CBSE Class 8 - Force and Pressure Notes

CBSE Class 8 – Force and Pressure (FORCE Notes)

1. Force – A Push or a Pull

Definition: A push or a pull acting on a body which tends to change its state of rest or of motion is called a force.

Symbol: F

Standard unit: Newton (N)

Two things required to describe force:

  • Magnitude (how strong the push/pull is)
  • Direction of force

Example: You push a box, but it may not move if the applied force is smaller than opposing friction.

Spring Balance : Device used to measure force

2. Force Comes into Play Only Due to Interaction

  • Force is the cause, and change in motion is the effect.
  • For a force to act: two objects must interact.
  • Force may be:
    • Contact force: requires physical contact (e.g., pushing a car).
    • Non-contact force: acts from a distance (e.g., magnet attracting pins).

3. Types of Forces

(A) Contact Forces

Forces which act only when objects are in direct physical contact.

Examples: muscular force, frictional force.

1. Muscular Force

  • Force exerted by muscles of humans/animals.
  • Activities: walking, running, lifting, ploughing, pulling carts.

2. Frictional Force

  • The opposing force when two surfaces are in contact.
  • Examples: rolling a ball on the ground (slows and stops due to friction).

Properties of Friction:

  • Always opposes the motion of an object.
  • Acts along surfaces in contact.
  • Due to roughness of surfaces.
  • Lesser on smooth surfaces, higher on rough ones.
  • Friction can be reduced by lubrication in machines.

(B) Non-Contact Forces (Action-at-a-distance Forces)

Forces which act even without contact.

1. Magnetic Force

  • Attraction/repulsion between magnets.
  • Opposite poles attract, like poles repel.
  • Works from a distance.

2. Electrostatic Force

  • Force between charged objects.
  • Like charges repel, unlike charges attract.
  • Formula (Coulomb's Law):
F = k · (q1q₂ / r²)

where q1, q₂ are charges, r is distance.

Example: rubbed plastic comb attracting tiny paper bits.

3. Gravitational Force

  • Force of attraction between any two masses.
  • Universal (acts between all objects having mass).
  • Example: Earth attracts objects → fruit falls down, ball comes back after being thrown up.
  • Earth's gravity = force of gravity keeping us on the ground.

4. Units of Force

  • SI Unit: 1 Newton (N).
  • 1 Newton = a force is said to be 1N if it produces an acceleration of 1 m/s² in a body of mass 1 kg.
  • 1 Newton = force required to hold 100 g mass on palm.
  • Other Units:
    • Gram force (gf): force to lift 1 g mass.
    • Kilogram force (kgf): force to lift 1 kg mass.

Relation:

  • 1 kgf = 9.8 N (approximated as 10 N for ease)
  • 1 kgf = 1000 gf

5. Resultant Force

When several forces act on a body, their combined effect is replaced by a single force called the resultant force.

1. If two forces act in the same direction:

FR = F1 + F2

Example: Shyam pushes a cart (100 N) and Arun pushes with 150 N.

Resultant = 100 + 150 = 250 N.

2. If two forces act in opposite directions:

FR = |F1 - F2|

Example: Shyam pushes with 250 N, Arun opposes with 150 N → Resultant = 100 N.

3. If equal forces act opposite along same line:

FR = 0

(forces are balanced)

6. Balanced and Unbalanced Forces

1. Balanced Forces

  • Resultant = 0
  • Effects:
    • Cannot start motion of a resting body.
    • Cannot change velocity of moving body.
    • Can change shape/size of soft bodies (e.g. pressing a balloon).

Example: Tug of war, both teams pulling with equal force → rope doesn't move.

2. Unbalanced Forces

  • Resultant ≠ 0
  • Effects:
    • Can set a stationary body into motion.
    • Can bring a moving body to rest.
    • Can change direction of moving object.

7. Effects of Force

Force acting on a body may cause:

  • Change in state of motion/rest
    • Stationary object moves (kicking football).
    • Moving object stops (fielder catches ball).
    • Increase in speed (pushing car forward).
    • Decrease in speed (brakes stop cycle).
  • Change in direction of motion
    • Cricket bat changing ball's direction.
    • Hands waving smoke away.
  • Change in shape/size
    • Pressing balloon, squeezing toothpaste tube, stretching spring.
  • Stop a moving object
    • Applying brakes, catching a cricket ball.
  • Change the speed
    • Force along motion → speed increases.
    • Force opposite to motion → speed decreases.

📘 Summary – Key Points

Force = Push/Pull → may change motion, shape, speed, or direction.

Types of Force → Contact (muscular, friction) & Non-contact (magnetic, electrostatic, gravitational).

Units of Force → SI unit is Newton (N); 1 kgf ≈ 10 N.

Resultant Force → vector sum of all forces acting.

Balanced Force → resultant = 0, no change in motion.

Unbalanced Force → resultant ≠ 0, causes motion/change.

Effects of Force → motion change, speed change, direction change, shape/size change.

CBSE Class 8 Science Notes - Force and Pressure | Horizon Academy

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