Exploring Forces - Chapter 5 Class 8 (Curiosity)

Q 1
Question
Match items in Column A (type of force) with the items in Column B (example). Column A: (i) Muscular force, (ii) Magnetic force, (iii) Frictional force, (iv) Gravitational force, (v) Electrostatic force. Column B: (a) A cricket ball stopping on its own just before the boundary line, (b) A child lifting a school bag, (c) A fruit falling from a tree, (d) Balloon rubbed on woollen cloth attracting hair strands, (e) A compass needle pointing North.
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Q 2
Question
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 a ball rolling on a flat ground increases. (iii) There is no force between two charged objects placed at a small distance apart.
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Q 3
Question
Two balloons rubbed with a woollen cloth are brought near each other. What would happen and why?
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Q 4
Question
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.
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Q 5
Question
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.
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Q 6
Question
A ball is released from point P and moves down an inclined plane and then along a horizontal surface (Fig. 5.16). It stops at point A. Think of a way so that, released from the same point P, it stops (i) before point A, (ii) after crossing point A.
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Q 7
Question
Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain.
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Q 8
Question
Is any force being applied to an object in a non-uniform motion?
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Q 9
Question
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?
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Q 10
Question
Three objects 1, 2, and 3 of the same size and shape but different materials are placed in water and dip to different depths (Fig. 5.17). If their weights are w1, w2, and w3, then which is correct? (i) w1 = w2 = w3, (ii) w1 > w2 > w3, (iii) w2 > w3 > w1, (iv) w3 > w1 > w2.
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Project 1
Project
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.
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Key Points
Rubbing certain material pairs builds static charge, which then attracts small paper pieces.
Key Points / Good Practices
Try many pairs — plastic with wool, polythene with paper, rubber with silk, and so on.
After rubbing, hold each item near small paper bits without touching them.
Note "charged" if the paper is attracted and "not charged" if nothing happens.
Make a neat table of each material pair and its result.
Conclude which pairs get charged most strongly and write it up as a short report.
Some rubbed pairs get charged and attract paper; record and report the pattern.

Project 2
Project
Imagine a scenario where gravity disappears. Develop a story. Create a cartoon strip to present your story.
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Key Points
Without gravity, nothing would stay down — people, water and objects would float away.
Key Points / Good Practices
Show people, furniture and water floating up off the ground.
Rain would not fall; food and drink would drift around.
Add funny moments — a cricket ball that never comes down, hair standing on end.
Use a few cartoon panels with speech bubbles to tell the story.
End by showing why gravity is so important for everyday life.
A no-gravity world has everything floating — a fun way to show gravity's importance.

Project 3
Project
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.
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Key Points
Friction is both helpful and harmful, depending on the situation.
Key Points / Good Practices
Friction as a necessity: it lets us walk, grip objects, write, and brake a vehicle.
Friction as a problem: it wears out machine parts, wastes energy as heat, and slows motion.
Let one group argue "necessity" and another argue "problem".
Note examples each side gives and write a short summary of the discussion.
Conclude that friction is needed in many places but unwanted in others.
Friction helps us grip and stop, but also wastes energy and wears parts.

Project 4
Project
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?
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Key Points
The ratio of weight to mass comes out nearly the same for every object.
Key Points / Good Practices
Make a simple spring balance and mark its scale using known standard weights.
Hang different objects and read each weight in newton.
Note the mass of each object in kilograms.
Work out the ratio weight ÷ mass for each object.
You will find the ratio is about the same (close to 10 N/kg) for all objects.
Weight ÷ mass is nearly constant (about 10 N/kg) for all objects on Earth.

Project 5
Project
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, and test the device. Explore in what other ways you may use this electroscope.
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Key Points
A simple electroscope shows charge when its foil leaves move apart.
Key Points / Good Practices
Build it from a jar, a lid, a straw, a copper wire and two aluminium foil strips.
Bring a charged object (like a rubbed scale) near the top wire.
The foil strips gain like charges and spread apart — showing the object is charged.
Test different rubbed objects to see which are charged.
Explore using it to compare how strongly different materials get charged.
The foil leaves spreading apart tell you an object is electrically charged.