Exploring Forces — Class 8 Science
Curiosity · Chapter 5
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6- Mathematics
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- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
2- Mathematics
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
Chapter summary
Key ideas & terms
- Force
- A push or pull on an object resulting from its interaction with another object; its SI unit is the newton (N).
- Contact force
- A force that acts only when two objects are in physical contact, such as muscular force and friction.
- Friction
- The contact force that opposes the motion of one surface over another, arising from surface irregularities.
- Non-contact force
- A force that acts even without physical contact, such as magnetic, electrostatic, and gravitational forces.
- Gravitational force
- The always-attractive force with which the Earth pulls objects towards itself, giving them weight.
- Buoyant force
- The upward force, also called upthrust, that a liquid exerts on an object placed in it.
Extra questions & answers
What is a force, and what is its SI unit?
A force is a push or pull resulting from the interaction between objects, and its SI unit is the newton (N).
List the effects a force can have on an object.
It can make a resting object move, change the speed or direction of a moving object, or change the shape of an object.
How do contact and non-contact forces differ?
Contact forces act only when objects touch, while non-contact forces act from a distance without contact.
In which direction does friction act, and what causes it?
Friction acts opposite to the direction of motion and is caused by irregularities on the two surfaces in contact.
What is observed when two similarly charged balloons are brought together?
They repel each other, showing that like charges repel while unlike charges attract.
Why is gravitational force called a non-contact force, and is it attractive or repulsive?
It acts without contact across a distance, and it is always attractive, unlike magnetic or electrostatic forces.
What is the difference between mass and weight?
Mass is the amount of matter in an object, measured in kg and unchanging; weight is the gravitational force on it, measured in newtons and able to vary from place to place.
Which instrument measures weight, and on what principle?
A spring balance, which stretches in proportion to the weight of the object hung from it.
What is buoyant force?
The upward force, or upthrust, exerted by a liquid on an object placed in it.
When does an object float or sink in a liquid?
It floats when the buoyant force equals the gravitational force, and sinks when the gravitational force is greater than the buoyant force.
Keep the curiosity alive
(i) Muscular force
Matches (b) A child lifting a school bag. Lifting the bag needs pull from the muscles of the arms and body contracting, which is a contact muscular force.
(ii) Magnetic force
Matches (e) A compass needle pointing North. The needle is a small magnet and it turns because it experiences a non-contact magnetic force from the Earth's magnetic field.
(iii) Frictional force
Matches (a) A cricket ball stopping on its own just before touching the boundary line. The ball slows down due to force of friction between the ball and the ground acting opposite to its motion, which is a contact force.
(iv) Gravitational force
Matches (c) A fruit falling from a tree. The Earth pulls the fruit towards its centre with gravitational force, a non-contact attractive force, so the fruit falls down.
(v) Electrostatic force
Matches (d) Balloon rubbed on woollen cloth attracting hair strands. Rubbing creates static charges on the balloon and the charged balloon exerts a non-contact electrostatic force on uncharged hair strands.
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.
True. A change in speed means acceleration. A net unbalanced force is needed to increase or decrease the speed. Without a force, a moving object keeps its speed.
2. State whether the following statements are True or False. (ii) Due to friction, the speed of the ball rolling on a flat ground increases.
False. Friction always acts opposite to the direction of motion or tendency to move, so it decreases speed and eventually stops the ball.
2. State whether the following statements are True or False. (iii) There is no force between two charged objects placed at a small distance apart.
False. Charged objects exert an electrostatic force on each other even without contact. Like charges repel and unlike charges attract.
3. Two balloons rubbed with a woollen cloth are brought near each other. What would happen and why?
The two balloons will repel each other and move away. When both balloons are rubbed with the same woollen cloth they acquire the same kind of static charge, like charges repel. The electrostatic force between them is repulsive.
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.
Whether an object sinks or floats depends on its weight
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
During upward motion the main force is the gravitational force due to the Earth acting vertically downwards. In addition, a small frictional force due to air also acts opposite to motion, i.e., downwards while going up. Both forces oppose the upward motion, so the speed decreases.
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. (ii) During its downward motion
During downward motion the gravitational force continues to act vertically downwards, now in the same direction as the motion, so the speed increases. Air friction still opposes motion, so it acts vertically upwards, but it is much smaller than gravity.
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. (iii) At its topmost position
At the topmost point the velocity is momentarily zero but the gravitational force still acts vertically downwards. Air friction is momentarily zero because speed is zero. Due to the downward gravitational pull the ball then starts to fall down.
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
To make it stop before point A, increase the force of friction on the horizontal surface. For example, spread a rough cloth, sand or sandpaper on the surface, which increases irregularities and friction, so the ball loses energy sooner and stops earlier.
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 (ii) after crossing the point A
To make it go beyond point A, decrease the force of friction. Make the horizontal surface smoother, for example by using a polished glass sheet, waxed wood or by applying a thin oil layer, or by using a smoother ball. Less friction means the ball travels a longer distance before stopping.
7. Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain.
Walking needs friction between our feet and the ground to push forward. On smooth surfaces like ice or polished floors the friction is very small. Without enough frictional force to oppose slipping, our foot slides forward or backward and we lose balance.
8. Is any force being applied to an object in a non-uniform motion?
Yes. Non-uniform motion means the speed or direction is changing, which means the object is accelerating. A net force must be acting to cause that acceleration. Even if the change is small, at least one of the forces like friction, gravity, muscular push or drag is causing it.
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?
Weight is the gravitational force
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
The weight of each floating object is balanced by the buoyant upthrust, which equals the weight of the water it displaces. The object that sinks deepest displaces the most water, so it must be the heaviest, and the one that dips least is the lightest. From Fig. 5.17, object 2 is submerged deepest and object 1 least, so the correct order is (iii)
Discover, design, and debate
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.
Procedure: make a table with columns material 1, material 2, rubbed, attracts paper? For example, rub plastic scale with polythene, woollen cloth with balloon, silk with glass rod, rubber with wool, etc. Observation: plastic-polythene, balloon-wool, and polythene-paper combinations generally get charged and attract small paper pieces due to electrostatic force, while metal-metal rubbing does not show attraction because charges flow away. Conclusion: only certain non-conducting pairs develop static charges and exert a non-contact electrostatic force. A research paper would present aim, method, table, and inference that like charges repel and charged bodies attract uncharged light objects.
Imagine a scenario where the gravity disappears. Develop a story. Create a cartoon strip to present your story.
Story idea: One morning Sonali wakes up and finds water floating in blobs, her bicycle floats away without pedalling, and she cannot stay on the ground. Food does not fall into her plate and the river does not flow. She and Ragini learn that without gravity nothing has weight, everything would drift. Cartoon strip: Panel 1 - alarm floats, Panel 2 - Sonali jumps and keeps going up, Panel 3 - fruits hang in air, Panel 4 - they hold ropes to come back, Panel 5 - they realise gravity keeps life anchored and wish for it back. The moral shows gravity is essential for holding atmosphere, water and for walking.
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.
Notes: Friction is a necessity when we need grip. Examples: walking without slipping, writing with a pencil, holding objects, tyres gripping the road, brakes stopping vehicles, lighting a match. Friction is a problem when it causes wear and loss of energy. Examples: wear of shoe soles and machine parts, heating of moving parts, extra fuel needed to overcome air and water friction, difficulty in sliding heavy boxes. Conclusion: friction is both useful and harmful; we increase it where needed with rough surfaces and treads, and reduce it where not needed with lubricants, polished surfaces and streamlined shapes.
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?
Method: fix a helical spring at top, attach a hook and pointer, hang known masses like
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.
Construction: a glass jar with a lid, a copper wire passing through the lid, aluminium foil leaves hanging inside. Test: rub a balloon with wool and bring near the top wire without touching; the leaves diverge because charges spread to the leaves and like charges repel, proving the balloon is charged. Other uses: test if different rubbed materials are charged, check whether charge is positive or negative by observing collapse or more divergence when a known charged object is brought near, and distinguish conductors from insulators by seeing how quickly the leaves collapse when touched.