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Exploring Forces – Class 8 Science notes, important questions and MCQs

NCERT Class 8 Science, chapter 5. Notes, key terms, important questions with answers and MCQs.

Chapter overview

A force is a push or a pull that can change an object's motion, direction or shape. Forces can act by contact or from a distance.

Types of forces

Contact forces: muscular force, friction.
Non-contact forces: gravitational, magnetic, electrostatic.
Forces in the same direction add; in opposite directions they subtract.

Friction

Friction opposes motion between surfaces; rough surfaces have more friction.
Static friction > sliding friction > rolling friction.
Reduce friction with lubricants, ball bearings, polishing; increase it with treads on tyres and shoes.
Weight is the force of gravity on an object; it is measured in newtons with a spring balance.

Key terms

Force: a push or pull on an object
Friction: force that opposes motion between touching surfaces
Weight: force with which the Earth pulls an object

Formulas

Net force = sum of forces in one direction − sum in the opposite direction
Weight W = m × g

Important questions with answers

Name two contact forces and two non-contact forces.2 marks

Contact: muscular force, friction. Non-contact: gravitational force, magnetic force (also electrostatic force).

Friction is both a friend and a foe. Explain with examples.3 marks

Friend: it lets us walk without slipping, helps brakes stop vehicles, and lets us hold things and write. Foe: it wears out shoes, tyres and machine parts and wastes energy as heat.

What are the effects of force on an object? Give an example of each, and explain why the weight of an object on the Moon is less than on Earth though its mass is the same.5 marks

A force can move a still object (kicking a ball), stop a moving object (catching a ball), change speed (pedalling harder), change direction (hitting a ball with a bat), and change shape (pressing clay). Weight is the force of gravity on an object. The Moon's gravity is about one-sixth of Earth's, so weight there is about one-sixth, while mass (amount of matter) stays the same everywhere.

Quick revision

Force can change speed, direction or shape.
Gravity is a non-contact force.
Friction produces heat.

Exam tips

Rolling is easier than sliding — that is why wheels and ball bearings are used.
Mass stays the same everywhere; weight changes with gravity.

Practice MCQs

Which is a non-contact force?
1. friction
2. magnetic force
3. muscular force
4. push of a hand
Show answer
Answer: (b) magnetic force. Acts from a distance.
The SI unit of force is
1. kilogram
2. newton
3. metre
4. joule
Show answer
Answer: (b) newton. Newton.
Ball bearings are used to
1. reduce friction
2. increase friction
3. stop motion
4. add weight
Show answer
Answer: (a) reduce friction. Rolling friction is small.
Two forces of 10 N and 4 N act in opposite directions. Net force is
1. 6 N
2. 14 N
3. 40 N
4. 2.5 N
Show answer
Answer: (a) 6 N. 10 − 4.
The SI unit of force is
1. joule
2. newton
3. watt
4. pascal
Show answer
Answer: (b) newton. Newton.
Which friction is the least?
1. static
2. rolling
3. sliding
4. all equal
Show answer
Answer: (b) rolling. Rolling friction.
An object weighs 60 N on Earth. On the Moon (gravity about one-sixth) it weighs about
1. 60 N
2. 6 N
3. 360 N
4. 10 N
Show answer
Answer: (d) 10 N. 60 ÷ 6.
Ball bearings are used in machines to
1. increase friction
2. reduce friction by rolling
3. increase weight
4. produce heat
Show answer
Answer: (b) reduce friction by rolling. Rolling friction is smaller.

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Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.