Class 8 Science Chapter 5 – Exploring Forces from NCERT Curiosity with easy Notes and complete Book Solutions. Get simple explanations, important concepts, questions and answers, and revision material for effective learning, practice and exam preparation.
Exploring Forces
Class 8 Science – Curiosity
Probe and Ponder
Why does it feel harder to pedal a bicycle when going uphill than on flat ground?
Why is it easier to slip on a wet surface?
Why do we feel ‘light’ or like we are ‘floating’ just after our swing reaches its highest point and begins to come down?
Share your questions
Short Notes and Easy Concepts
5.1 What Is a Force?
- Push and pull are forms of force.
- Force can make a stationary object move.
- Force can change the speed of a moving object.
- Force can change the direction of motion.
- Force can change the shape of an object.
- The SI unit of force is newton (N).
5.2 What Can a Force Do?
- make an object move from rest;
- change the speed of a moving object;
- change the direction of motion;
- change the shape of an object;
- produce more than one of these effects.
5.3 Force as an Interaction
A force comes into play when two or more objects interact.
For example, when we push a table, our hand and the table interact.
5.4 Types of Forces
| Type | Meaning | Examples |
|---|---|---|
| Contact force | Acts when objects are physically in contact. | Muscular force, friction |
| Non-contact force | Acts without physical contact. | Magnetic, electrostatic, gravitational force |
Muscular Force
Examples include walking, running, lifting, pushing, jumping and stretching.
Friction
- Friction is a contact force.
- It acts opposite to the direction of motion.
- Friction is greater on rough surfaces.
- Friction is smaller on smoother surfaces.
- Air and water can also produce friction on moving objects.
Magnetic Force
- Magnetic force is a non-contact force.
- Like poles repel.
- Unlike poles attract.
Electrostatic Force
- It is a non-contact force.
- There are two types of static charges: positive and negative.
- Like charges repel.
- Unlike charges attract.
Gravitational Force
- Gravity is a non-contact force.
- Gravity always attracts objects.
- Gravity pulls objects towards the Earth.
5.5 Weight and Its Measurement
- SI unit of weight = newton (N)
- Weight is a force.
- Weight can be measured using a spring balance.
Mass and Weight
| Mass | Weight |
|---|---|
| Amount of matter in an object. | Gravitational force acting on an object. |
| Measured in g or kg. | Measured in N. |
| Remains the same from place to place. | Can change from place to place. |
Spring Balance
A spring balance is a simple device used to measure weight or force.
5.6 Floating and Sinking
- If gravitational force is greater than buoyant force, the object sinks.
- If the two forces are equal, the object floats.
- Buoyant force depends partly on the density of the liquid.
Snapshots – Quick Revision
- A force is a push or pull resulting from interaction between objects.
- The SI unit of force is newton (N).
- Forces can act with or without contact.
- Muscular force and friction are contact forces.
- Magnetic, gravitational and electrostatic forces are non-contact forces.
- Force can change speed, direction or shape.
- Friction opposes motion.
- Gravity is always attractive.
- Weight is the force with which Earth pulls an object.
- Weight is measured in newtons.
- Mass remains unchanged whereas weight may vary from place to place.
- Upthrust is the upward force applied by a liquid on an object.
Keep the Curiosity Alive
1. Match items in Column A with the items in Column B.
| Column A (Type of force) | Column B (Example) |
|---|---|
| (i) Muscular force | (a) A cricket ball stopping on its own just before touching the boundary line |
| (ii) Magnetic force | (b) A child lifting a school bag |
| (iii) Frictional force | (c) A fruit falling from a tree |
| (iv) Gravitational force | (d) Balloon rubbed on woollen cloth attracting hair strands |
| (v) Electrostatic force | (e) A compass needle pointing North |
(i) – (b)
(ii) – (e)
(iii) – (a)
(iv) – (c)
(v) – (d)
2. State whether the following statements are True or False.
(i) A force is always required to change the speed of motion of an object.
(ii) Due to friction, the speed of the ball rolling on a flat ground increases.
(iii) There is no force between two charged objects placed at a small distance apart.
(i) True
(ii) False
(iii) False
3. Two balloons rubbed with a woollen cloth are brought near each other. What would happen and why?
4. When you drop a coin in a glass of water, it sinks, but when you place a bigger wooden block in water, it floats. Explain.
5. If a ball is thrown upwards, it slows down, stops momentarily, and then falls back to the ground. Name the forces acting on the ball and specify their directions.
(i) During its upward motion
(ii) During its downward motion
(iii) At its topmost position
(i) Gravitational force acts downward.
(ii) Gravitational force acts downward.
(iii) Gravitational force still acts downward.
6. A ball is released from the point P and moves along an inclined plane and then along a horizontal surface as shown in the Fig. 5.16. It comes to stop at the point A on the horizontal surface. Think of a way so that when the ball is released from the same point P, it stops (i) before the point A (ii) after crossing the point A.
(i) To make it stop before A, increase friction on the horizontal surface, for example by making the surface rougher.
(ii) To make it stop after A, reduce friction by making the surface smoother.
7. Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain.
8. Is any force being applied to an object in a non-uniform motion?
9. The weight of an object on the Moon becomes one-sixth of its weight on the Earth. What causes this change? Does the mass of the object also become one-sixth of its mass on the Earth?
10. Three objects 1, 2, and 3 of the same size and shape but made of different materials are placed in the water. They dip to different depths as shown in Fig. 5.17. If the weights of the three objects 1, 2, and 3 are w1, w2, and w3, respectively, then
(i) w1 = w2 = w3
(ii) w1 > w2 > w3
(iii) w2 > w3 > w1
(iv) w3 > w1 > w2
Reflect on the Questions Framed by Your Friends
I think …
But we thought … Maybe …
Shouldn’t it be …
Why? When? How long? Where? Why not?
Prepare some questions based on your learnings so far …
Examples:
- Why does friction act opposite to motion?
- Why does an object float in water?
- When does a force change the shape of an object?
- Why does weight change from one planet to another?
- Why does mass remain unchanged?
Discover, Design, and Debate
Activity 1
Collect objects made of different materials, such as plastic, wool, silk, rubber, polythene sheet, paper, and metals. Rub one material with another and check if it attracts small pieces of paper or not, that is, whether it gets charged or not. Record your observations in a systematic manner and write a research paper.
Activity 2
Imagine a scenario where the gravity disappears. Develop a story. Create a cartoon strip to present your story.
Activity 3
Organise a discussion in your class on the topic: Friction—a necessity or a problem? Make a note of the discussion and state where friction is a necessity and when it is a problem.
Activity 4
Make your own spring balance with the help of your teacher and calibrate it using standard weights. Now measure the weights of different objects and calculate the ratio of the weight and mass of different objects. Do you observe a pattern?
Activity 5
An electroscope is a device which can determine whether an object is electrically charged. You can make your own electroscope (Fig. 5.18) in your class with the help of your teacher, test the device. Explore in what other ways you may use this electroscope.
Quick Practice – Important Concepts
1. What is force?
2. What is the SI unit of force?
3. Name two contact forces.
4. Name three non-contact forces.
5. What does friction do?
6. What is weight?
7. What is the SI unit of weight?
8. What is upthrust?
9. Does mass change from Earth to Moon?
10. Does weight change from Earth to Moon?
Prepare with this NCERT Curiosity worksheet for Chapter 5. The worksheet includes important questions and revision exercises to help students understand key concepts, practise what they have learned, and prepare effectively for class tests and exams.
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