Case 1: Kicking a Football

Rohan and his friends are playing football in the school playground. The ball is lying at rest on the ground. Rohan runs towards the ball and kicks it. The ball starts moving. After some time, the ball slows down and finally comes to rest. When the ball is moving, another player hits it from the side and the ball changes its direction.

1. Why did the football start moving when Rohan kicked it?
2. Which two effects of force are observed when the second player hits the moving ball?
3. Why did the ball slow down and stop after some time even when no one touched it?

Answers:

1. The football started moving because Rohan’s kick applied a force on the ball. This force changed the state of the ball from rest to motion.
2. Two effects of force are seen: (i) change in speed (the speed of the ball changes) and (ii) change in direction of motion (the ball moves in a new direction after being hit).
3. The ball slowed down and stopped due to friction between the ball and the ground, and air resistance. These forces act opposite to its motion and gradually reduce its speed to zero.

Case 2: Pushing a Heavy Wardrobe

Two students, Aisha and Meena, are trying to push a heavy wardrobe across the room. When both push from one side in the same direction, the wardrobe starts moving slowly. Later, as a joke, they decide to push from opposite sides with equal force and the wardrobe does not move at all.

1. What type of forces act on the wardrobe when both girls push it in the same direction?
2. Why does the wardrobe remain at rest when they push from opposite sides with equal force?
3. In which situation does the wardrobe experience unbalanced forces?

Answers:

1. When they push in the same direction, both forces act as unbalanced forces in one direction, and the net force is the sum of the two forces.
2. When they push with equal forces in opposite directions, the forces cancel each other. The net force becomes zero, so the wardrobe remains at rest. These are balanced forces.
3. The wardrobe experiences unbalanced forces when both girls push in the same direction and the wardrobe starts moving.

Case 3: Pulling a Loaded Cart

A vegetable vendor pulls a loaded handcart along a rough road. He uses his muscular force to pull the handle of the cart. The wheels of the cart rub against the rough surface of the road. The vendor notices that it is easier to pull the cart on a smooth cement road than on a muddy kachcha road.

1. Which type of force does the vendor use to pull the cart? Is it a contact or non-contact force?
2. Name the force that opposes the motion of the cart on the road.
3. Why is it easier to pull the cart on a smooth cement road than on a muddy road?

Answers:

1. The vendor uses muscular force to pull the cart. It is a contact force because it acts through direct contact between his hands and the cart handle.
2. The force that opposes the motion of the cart is friction between the wheels of the cart and the road.
3. On a smooth cement road, friction is less, so less opposing force acts on the cart and it moves more easily. On a muddy road, friction and resistance are higher, so more force is needed to pull the cart.

Case 4: Walking Without Slipping

Riya walks normally on a dry rough floor but finds it difficult to walk on a newly polished floor or a wet bathroom floor. Her mother warns her to walk carefully on slippery surfaces to avoid falling. Riya observes that her shoes with patterned soles give better grip than smooth sandals.

1. Which force helps Riya to walk safely on a dry rough floor?
2. Why does she find it difficult to walk on a wet or polished floor?
3. How do patterned soles on shoes help in safe walking?

Answers:

1. Friction between her shoes and the floor helps Riya to walk safely without slipping.
2. On wet or polished floors, friction between the floor and her feet is reduced. With less friction, her feet tend to slip, making it difficult to walk safely.
3. Patterned soles increase the roughness and the effective area of contact, which increases friction. This extra grip prevents slipping and helps in safe walking.

Case 5: Lubricants in Machines

In the school workshop, the teacher shows the students how oil is applied between the moving parts of a machine. She explains that without oil, the machine gets heated, makes noise and its parts wear out quickly. With oil, the machine runs smoothly and quietly.

1. What causes the machine to become hot and noisy when no oil is used?
2. What is the role of oil when applied between the moving parts?
3. Suggest two other ways (besides oiling) to reduce friction in machines.

Answers:

1. When no oil is used, there is large friction between the moving parts. This friction produces heat and sound, making the machine hot and noisy.
2. Oil acts as a lubricant. It fills the gaps between surfaces and makes them smooth, thereby reducing friction and helping the machine to run smoothly.
3. Two other ways: (i) Using grease instead of oil, and (ii) using ball bearings to change sliding friction into rolling friction.

Case 6: Playing with Magnets

In a science activity, students are given bar magnets and iron nails. They place nails at different distances from the magnet. The nails near the magnet move towards it and stick to it, while nails kept very far do not move at all. When they bring two bar magnets close, sometimes the magnets pull each other and sometimes they push each other away.

1. What kind of force is exerted by a magnet on the iron nails?
2. Why do some nails move towards the magnet while others kept far away do not move?
3. Why do two magnets sometimes attract and sometimes repel each other?

Answers:

1. The magnet exerts a magnetic force on the iron nails. It is a non-contact force.
2. Nails placed near the magnet are within the magnetic field and experience enough magnetic force to move. Nails kept far away experience very weak or negligible magnetic force, so they do not move.
3. Magnets have two poles: north (N) and south (S). Unlike poles (N–S) attract each other and like poles (N–N or S–S) repel each other. Hence attraction or repulsion is seen.

Case 7: Charged Comb and Paper Pieces

After combing dry hair, Sneha brings the plastic comb near small pieces of paper. She is surprised to see that the paper pieces jump up and stick to the comb. After some time, the paper pieces fall back and no longer get attracted to the comb.

1. Which type of force is responsible for the attraction of paper pieces to the comb?
2. Why do the paper pieces get attracted only after Sneha has rubbed the comb on her hair?
3. Why do the paper pieces stop getting attracted after some time?

Answers:

1. The attraction is due to electrostatic force, which is a non-contact force between charged bodies.
2. Rubbing the comb on dry hair charges the comb with static electricity. A charged comb can exert electrostatic force and attract light objects like paper pieces.
3. With time, the charge on the comb leaks away into the surroundings and disappears. When the comb is no longer charged, it cannot exert electrostatic force, so the paper pieces stop getting attracted.

Case 8: Falling Objects

During a classroom demonstration, the teacher drops a chalk and a duster from the same height. Both fall down towards the floor. She explains that whatever object is thrown upwards eventually comes back down unless some force keeps it up.

1. Which force is responsible for pulling the chalk and the duster towards the Earth?
2. Is this force a contact or non-contact force? Give a reason.
3. What will happen if you throw a ball upwards? Name the force that brings it back.

Answers:

1. The force responsible is the gravitational force of the Earth.
2. It is a non-contact force because the Earth attracts objects towards itself even when there is no physical contact between them.
3. When a ball is thrown upwards, it slows down, stops for a moment, and then falls back. The gravitational force of the Earth pulls it downward and brings it back.

Case 9: Cutting Vegetables with Different Knives

Ankit tries to cut vegetables using a blunt knife and finds it difficult. Later, he uses a sharp knife and easily cuts the same vegetables. His mother explains that both knives are pressed with almost the same force, yet the sharp one cuts better.

1. Define pressure in terms of force and area.
2. Why does the sharp knife cut vegetables more easily than the blunt knife?
3. Write the formula for pressure and name its SI unit.

Answers:

1. Pressure is the force acting per unit area of a surface. Pressure = Force ÷ Area.
2. The sharp knife has a thinner edge, so the area of contact is smaller. For the same force, smaller area produces greater pressure, helping it to cut more easily.
3. Formula: P = F / A, where P is pressure, F is force and A is area. The SI unit of pressure is pascal (Pa) or N/m².

Case 10: Porters and the Cloth Pad

At a railway station, porters carry heavy loads on their heads. Most of them place a folded cloth or round ring-shaped pad on their head and then keep the load on it. A student wonders why they do not place the load directly on their bare head.

1. What happens to pressure on the head when area of contact is increased?
2. How does the cloth pad help the porter carry the load more comfortably?
3. Relate this situation to the formula of pressure.

Answers:

1. When the area of contact increases for the same force, the pressure decreases.
2. The cloth pad increases the area of contact between the load and the porter’s head. For the same weight, increased area reduces the pressure, making it more comfortable and less painful.
3. According to P = F / A, when A (area) increases and F (force, i.e., weight) remains the same, P (pressure) decreases. This explains why the pad reduces pressure on the head.

Case 11: Camels in the Desert

On a desert trip, students notice that camels walk easily on soft sand whereas people wearing normal shoes sink into the sand and find walking difficult. The guide tells them that camels are known as “ships of the desert” because they can walk long distances on sand without sinking.

1. Why do human feet sink into soft sand more easily than camel feet?
2. How are camel feet adapted to walk on sand?
3. State the relation between pressure and area that explains this adaptation.

Answers:

1. Human feet have relatively smaller area of contact with the sand. For the same body weight, smaller area causes greater pressure, so the feet sink more.
2. Camel feet are broad and wide. The large area spreads the body weight over more sand, reducing pressure and preventing the feet from sinking too much.
3. For a constant force (weight), pressure is inversely proportional to area: P ∝ 1/A. Larger area gives smaller pressure, helping camels walk easily on sand.

Case 12: High Heels and Floor Damage

A newly built hall has a soft wooden floor. The caretaker requests people not to wear high-heeled shoes inside, but to use flat footwear. He explains that high heels can damage the floor by leaving deep marks.

1. Why can high-heeled shoes damage a soft floor more easily than flat shoes?
2. What can you say about the pressure produced by high heels compared to flat shoes?
3. How is this situation related to the concept of pressure = force/area?

Answers:

1. High-heeled shoes have a very small area in contact with the floor, so the same body weight acts on a smaller area, producing larger pressure which can damage the soft floor.
2. The pressure produced by high heels is much greater than that by flat shoes, since the area of contact of high heels is much smaller.
3. For the same body weight (force), reducing area (A) increases pressure (P) as P = F / A. High heels reduce area, so pressure increases and can damage the floor.

Case 13: Water Fountain from a Bottle

A teacher takes a plastic bottle and makes three small holes on one side at different heights: one near the top, one in the middle and one near the bottom. She covers the holes, fills the bottle with water and then removes the covers. The students see water jets coming out of all three holes, but the bottom jet goes the farthest.

1. What does this activity show about the pressure of water at different depths?
2. Why does the water jet from the lowest hole travel the farthest?
3. How does this help us understand why dam walls are thicker at the bottom?

Answers:

1. The activity shows that water pressure increases with depth. The deeper the hole, the greater the pressure and the stronger the water jet.
2. The lowest hole is at the greatest depth, so water pressure there is highest. High pressure pushes water out more strongly, causing the jet to travel the farthest.
3. In dams, water at greater depth exerts higher pressure on the walls. To bear this high pressure safely, the walls are made much thicker at the bottom than at the top.

Case 14: Bursting of a Weak Dam

A news report shows that a small earthen dam broke after heavy rains. The water level in the reservoir had risen very high. The dam wall gave way at its lower part and a large amount of water rushed out.

1. Why does the water at the bottom of the dam exert more pressure than at the top?
2. Why did the wall break first at the lower part and not near the surface?
3. What precaution should engineers take while designing strong dams?

Answers:

1. Water pressure increases with depth due to the weight of the water column above. Therefore, water at the bottom experiences maximum pressure.
2. The lower part of the wall faces the highest pressure, especially when the water level is high. If the wall is weak, it is more likely to break at the bottom where pressure is greatest.
3. Engineers must design dams with thicker and stronger walls at the bottom to withstand high water pressure and ensure safety.

Case 15: Inflated Tyres and Air Pressure

A cycle mechanic checks the air pressure in bicycle tyres. When there is less air, the tyre feels soft and gets pressed easily. After pumping air, the tyres become firm and can support the rider’s weight. The mechanic warns that overfilling air can burst the tyre.

1. What causes a soft tyre to become firm when air is pumped into it?
2. Why can overfilling air cause the tyre to burst?
3. What does this show about the pressure exerted by gases?

Answers:

1. When air is pumped into the tyre, the number of air molecules inside increases. Their collisions with the inner walls increase, so the air pressure inside rises and makes the tyre firm.
2. Overfilling air increases internal pressure beyond the tyre’s capacity to withstand. If the pressure becomes too high, the tyre material cannot bear the force and it bursts.
3. This shows that gases like air exert pressure on the walls of their container. Increasing the amount of gas or compressing it in a fixed volume increases gas pressure.

Case 16: Drinking with a Straw

In the school canteen, students drink juice using straws. When a student simply puts a straw in the glass and does nothing, the juice level in the straw does not rise. However, when he sucks air out from the straw, the juice rises and enters his mouth. A classmate explains this using the concept of atmospheric pressure.

1. Why does the juice not rise in the straw when the student does not suck?
2. What happens inside the straw when the student sucks air out?
3. Explain how atmospheric pressure helps juice to rise in the straw.

Answers:

1. When the student does not suck, the air pressure inside the straw and the atmospheric pressure on the juice surface are equal. There is no pressure difference, so the juice does not rise.
2. When he sucks, he removes some air from the straw, reducing the air pressure inside. The pressure inside the straw becomes lower than the atmospheric pressure outside.
3. The atmospheric pressure on the juice surface in the glass is now greater than the pressure inside the straw. This pressure difference pushes the juice up into the straw and then into the student’s mouth.

Case 17: Using a Syringe

A doctor uses a syringe to take blood from a patient’s arm. She inserts the needle into a vein and slowly pulls back the plunger. Blood rises through the needle and fills the syringe. Later, she pushes the plunger to inject medicine into another patient.

1. Why does blood enter the syringe when the plunger is pulled back?
2. What role does atmospheric pressure play in this process?
3. How is the same syringe used to inject medicine into the body?

Answers:

1. When the plunger is pulled back, the space inside the syringe increases and the pressure inside becomes lower than the blood pressure in the vein. The higher pressure in the vein pushes blood into the syringe.
2. Atmospheric pressure helps in similar situations when liquid is drawn from an open container. In the case of a vein, it is mainly the blood pressure that pushes blood into the region of lower pressure created inside the syringe.
3. To inject medicine, the plunger is pushed in. This increases the pressure inside the syringe, forcing the medicine out through the needle into the patient’s body.

Case 18: Rubber Suction Hook on a Tile

A plastic hook with a rubber suction cup is used to hang towels on smooth bathroom tiles. When the suction cup is pressed firmly against the tile, it sticks and supports the towel’s weight. After a few days, the hook sometimes falls off on its own.

1. How does pressing the suction cup against the tile make it stick?
2. What role does atmospheric pressure play in holding the suction cup in place?
3. Why does the suction hook fall off after some time?

Answers:

1. Pressing the suction cup squeezes out most of the air between the cup and the tile, creating a region of lower air pressure inside the cup.
2. Atmospheric pressure outside the cup is greater than the reduced pressure inside. This higher outside pressure pushes the suction cup strongly against the tile, making it stick.
3. With time, air slowly leaks into the space under the suction cup, equalising the pressure inside and outside. When the pressure difference disappears, the cup can no longer stick and falls off.

Case 19: Heavy Truck on Soft Ground

A heavy truck gets stuck when driven onto soft, muddy ground. Its narrow tyres sink deeply into the mud. A tractor with very broad tyres is able to move more easily on the same ground and helps to pull the truck out.

1. Why do the narrow tyres of the truck sink into the soft ground?
2. How do the broad tyres of the tractor help it move easily on the same ground?
3. Which concept of force and pressure explains this difference?

Answers:

1. The narrow tyres have a smaller area of contact with the ground. The heavy truck’s weight produces large pressure on the soft mud, causing the tyres to sink.
2. The tractor’s broad tyres have a larger area of contact, so the same or even greater weight produces less pressure on the mud. This prevents deep sinking and helps it move easily.
3. This is explained by pressure = force/area. For a given force (weight), smaller area gives higher pressure and larger area gives lower pressure.

Case 20: Balloon Bursting at High Pressure

Two balloons are inflated: one is filled with little air and feels soft, while the other is filled completely and feels very tight. When even a small extra puff of air is blown into the tight balloon, it suddenly bursts with a loud sound.

1. Why does the balloon feel tight when it is filled with a lot of air?
2. What happens to the pressure inside the balloon when more air is added?
3. Why does the balloon burst when the internal pressure becomes too high?

Answers:

1. When much air is filled, a large number of air molecules are trapped inside the balloon. Their collisions with the inner walls increase, so the internal air pressure becomes high and the balloon feels tight.
2. Adding more air in the same volume further increases the number of molecules and the internal pressure rises even more.
3. Every balloon can bear pressure only up to a certain limit. When internal pressure becomes higher than what the balloon material can withstand, the rubber tears and the balloon bursts.