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Notes Chapter 5 Science Curiosity Class 8 CBSE Board

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Exploring Forces


1. What is a Force?

A force is a push or pull on an object when it interacts with another object.

Examples: Pushing a box, pulling a rope, kicking a football.

Forces can:

  • Make an object move from rest.
  • Change the speed of a moving object.
  • Change the direction of a moving object.
  • Change the shape of an object.
  • Cause a combination of these effects.

2. Types of Forces

Forces are divided into two main types: Contact Forces and Non-Contact Forces.

2.1 Contact Forces

These forces need physical contact between objects.

Muscular Force:

  • Produced by muscles in humans or animals.
  • Used for walking, running, lifting, pushing, or jumping.
  • Examples: Lifting a school bag, animals pulling carts, chewing food, or blood circulation in the body.

Frictional Force (Friction):

  • Acts when an object moves or tries to move over another surface.
  • Always opposes the direction of motion.
  • Caused by tiny irregularities on surfaces (even smooth ones).
  • More friction on rough surfaces (e.g., a ball stops faster on a rough road than a smooth one).
  • Friction also occurs in liquids and gases (e.g., air resistance on airplanes).

2.2 Non-Contact Forces

These forces act without physical contact.

Magnetic Force:

  • Exerted by magnets on other magnets or magnetic materials (e.g., iron).
  • Like poles (N-N or S-S) repel; unlike poles (N-S) attract.
  • Example: A magnet attracts a paperclip without touching it.

Electrostatic Force:

  • Caused by electric charges built up after rubbing objects.
  • Example: Rubbing a plastic scale with polythene attracts paper pieces.
  • Similarly charged objects repel; differently charged objects attract.

Gravitational Force:

  • The force with which Earth (or other planets) pulls objects toward itself.
  • Always attractive (never pushes away).
  • Example: A fruit falling from a tree.

3. Friction in Detail

What is Friction?

  • A force that opposes motion when an object moves over another surface.
  • Acts opposite to the direction of movement.
  • Caused by surface irregularities locking into each other.

Factors Affecting Friction:

  • Surface Type: Rough surfaces have more friction than smooth ones.
  • Example: A ball stops faster on a rough road than a smooth one.
  • Friction varies on different surfaces like glass, cloth, wood, or sand.

Friction in Liquids and Gases:

  • Objects moving through air or water face friction.
  • Example: Airplanes and ships are designed to reduce air or water friction.

4. Weight and Mass

Mass:

  • The amount of matter in an object.
  • Measured in grams (g) or kilograms (kg).
  • Stays the same everywhere.

Weight:

  • The gravitational force exerted by Earth (or another planet) on an object.
  • Measured in newtons (N) using a spring balance.
  • Varies slightly depending on location (e.g., less on the Moon).
  • Example: A 1 kg object weighs 10 N on Earth, 1.6 N on the Moon.

Spring Balance:

  • A device to measure weight (force) in newtons.
  • A spring stretches when an object is hung from it, showing the weight.
  • Example: A spring balance with a 0-10 N range, where each small division is 0.2 N.
  • Used to measure weights of objects like a pencil box or water bottle.

5. Floating and Sinking

Why do objects float or sink?

  • Gravitational Force: Pulls objects downward.
  • Buoyant Force (Upthrust): The upward force exerted by a liquid on an object.
  • If gravitational force > buoyant force, the object sinks.
  • If buoyant force = gravitational force, the object floats.
  • Example: A plastic bottle floats due to buoyant force pushing it up.

Archimedes’ Principle:

  • An object in a liquid feels an upward force equal to the weight of the liquid displaced.
  • If the displaced liquid’s weight equals the object’s weight, it floats; if less, it sinks.
  • Example: Pumice rock floats on water because it’s less dense due to air pockets.

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