0- Explain in brief why birds that walk on the leaves floating on the surface of water have wide feet.
Answer:
Birds that walk on leaves floating on the surface of the water have wide feet. This is because wide feet increase the area of contact with the leaves. So, they exert less pressure on the leaves, allowing them to walk on these leaves.
- Explain why the wall of a dam is made thicker at the bottom.
Answer:
The wall of a dam is made thicker at the bottom because of the following reasons:
a. A dam has a massive weight and thus, applies a large force on the ground. In order to reduce the pressure exerted by it on the ground, its area of contact with the ground is increased by making its base broad. So, a broad base supports the weight of the dam.
- Moreover, the pressure of water increases with depth. So, it is greater at the base than at the top. So, the base is broad to withstand the huge pressure of the water.
- When the nozzle of a syringe is dipped in a liquid and its piston is withdrawn, the liquid pushes up into the syringe. Give a reason.
Answer:
When the nozzle of a syringe Is dipped in a liquid and its piston is withdrawn, a region of low pressure is created in the syringe. The atmospheric pressure acting on the surface of the liquid is greater and hence forces the liquid to rise in the syringe.
- Why do the pillars of a bridge have a broader base than the top?
Answer:
Pillars of a bridge have a broader base than the top. This is done to reduce the pressure exerted on the ground by the enormous weight of the bridge. As pressure is inversely proportional to the area of contact, so if the base is broader, the area of the contact increases, reducing the pressure on the ground due to the weight of the bridge.
- Where is the pressure of the sea water greater : 5 m below the surface of the sea or 10 m below the surface of the sea? Why?
Answer:
Pressure is greater at 10 m below the surface of the sea as compared to that at 5 m below the surface of the sea. This is because pressure exerted by a liquid increases with depth.
- Why is water not used as a barometric liquid?
Answer:
Water is not used as a barometric fluid as it has less density due to which it will have to rise to a height of 10.5 m to balance the atmospheric pressure. Moreover, it vaporizes in vacuum and thus, never records the true atmospheric pressure.
- Two women are of the same weight. One of them wears sandals with pointed heels, while the other woman wears sandals with flat soles. Which one of them would feel more comfortable while walking on a sandy beach? Also, give reasons for your answer.
Answer:
Although both women’s heights are the same, they also carry the same weight when they walk. But the woman wearing the sandal with flat heels will be more comfortable than the woman wearing sandals with pointed heels while walking on a sandy beach. This is because the flat soles have a larger surface area as compared to sandals with pointed heels. Also, the pressure exerted by the woman with pointed heels will be greater as compared to that of the woman wearing sandals with flat soles. This increased pressure will make walking on sand in sandals with pointed soles difficult.
- Why do safety pins and drawing pins have narrow and sharp tips?
Answer:
Safety pins and drawing pins have narrow and sharp tips because the smaller surface area of the tip causes a high pressure and, therefore, makes it easier for these objects to pierce through a given material.
- The hand flour grinder ¡s provided with a handle near its rim. Give reasons why.
Answer:
The hand flour grinder is provided with a handle near its rim to increase the moment arm so that even a small force applied can produce a large turning effect.
- How are force, area and pressure related to each other?
Answer:
Force, area and pressure are related by the following expression.
If the force applied is doubled for the same area, then the pressure will also double.
- Why is it easier to walk on soft sand if we have flat shoes rather than shoes with sharp heels (or pencil heels)?
Answer:
A flat shoe has a greater area in contact with the soft sand and exerts less pressure on the soft ground. Due to this the ‘flat’ shoes do not sink much in soft sand and it is easy to walk on it. On the other hand, a sharp heel has a small area in contact with the soft sand and exerts a greater pressure on the sand. Due to this, the sharp heels sink deep into soft sand making it difficult for the wearer to walk on it.
- We know that there is a huge amount of atmospheric pressure on us. But we do not experience its effect why?
Answer:
The pressure of air inside our body is the same as that of the atmosphere. Therefore, we do not experience its effect.
- Why do sea divers wear specially designed suits?
Answer:
Since the pressure of liquid increases with the increasing depth of sea. Specially designed suits protect scuba divers from the huge pressure of the water underneath.
- An archer shoots an arrow in the air horizontally. However, after moving some distance, the arrow falls to the ground. Name the initial force that sets the arrow in motion. Explain why the arrow ultimately falls down.
Answer:
The initial force is muscular force which sets the arrow in motion. The force of gravity that acts on the arrow in the downward direction brings it to the ground.
- Two rods: A and B, having same weight and equal length have different thickness. Rod A is thinner while Rod B is thicker. They are held vertically on the surface of sand. Which one of them will sink more? Why?
Answer:
Rod A will go deeper as it has a smaller area of contact, therefore the same force produces more pressure. In case of rod B the same force produces less pressure.
- It is difficult to cut cloth using a pair of scissors with blunt blades. Explain.
Answer:
Blunt blades have a larger area compared to the sharp-edged blades. Thus, the applied force produces a lower pressure in case of blunt blades, which makes it difficult to cut the cloth.
- Observe the figures given below carefully.
Volume of water in each vessel is shown above. Arrange them in order of decreasing pressure at the base of each vessel. Explain the reason.
Answer:
Pressure of a liquid column depends upon the height of the liquid column and not on volume of the liquid. Decreasing order of pressure at the base of each vessel is B > D > A > C.
- On Lokesh’s birthday Shreya was given charge to amuse children with some little tricks. Shreya stuck balloons to the wall by just rubbing them in her clothes. She bent the water stream from a tap without touching it. She did so by bringing a big balloon near the flowing water. All the children were very happy to see this little magic. Everybody praised Shreya.
How do balloons stick to walls?
How Shreya bent the water stream by bringing a big balloon near it and without touching it?
What values of Shreya are shown here?
Answer:
On rubbing the balloon with hair or clothes, it acquires negative charges. The negative charges are attracted to the positive charges on the wall and balloon sticks to the wall.
Negative charge on the balloon attracts positive charge on water molecules. Thus, bent the water molecules.
Shreya is dutiful, friendly with scientific aptitude.
- Explain why, snow shoes stop you from sinking into snow.
Answer: The snow shoes have large, flat soles so they exert less on the soft snow and stop the wearer from sinking into it.
- What happens to the springs fixed to the seat of a bicycle when we sit on it?
Answer: When we sit on the seat of a bicycle, the force exerted by the body weight compresses the spring and changes its shape.
- Why do balloons expand when filled with air?
Or
Why does the balloon get inflated?
Or
What makes a balloon get inflated when air is filled in it?
Answer: Air exerts pressure on the inner walls of the balloon causing it to expand and get inflated.
- Explain why, a wide steel belt is provided over the wheels of an army tank.
Answer: The wide belts increase the area and reduce the pressure. This avoids sinking of the tank into the ground.
- Why do the shape and size of balloon change when filled with air or water?
Answer: The high pressure produced by the gas or water molecules on the walls of the balloon causes it to expand and get inflated.
- Why does water come out more slowly from an upstairs tap than from a similar tap downstairs?
Answer: This is so because water pressure is less in the upstairs tap due to lesser depth.
- Why are we not crushed by atmospheric pressure?
Or
If the pressure due to the atmosphere is so great, then why are we not crushed by it?
Answer: We do not get crushed because the atmospheric pressure acting on our body from outside is balanced by the blood pressure acting from inside.
- Why does a sharp knife cut better than a blunt knife?
Ans. Lesser the area, larger the pressure for the same force applied. Therefore, it is easier to cut with sharp knives than blunt ones, with same force.
- Why do the tools meant for cutting and piercing always have sharp edges?
Answer: The cutting edges of tools like blades, knives etc., are provided with sharp edges to cut the things easily because the smaller the area, larger the pressure on a surface for the same force.
- Explain why, the tip of a sewing needle is sharp.
Answer: The tip of the sewing needle is sharp so that due to its sharp tip, the needle may put the force on a very small area of the cloth, producing a large pressure sufficient to pierce the cloth being stitched.
- Explain why, porters place a thick, round piece of cloth on their heads when they have to carry heavy loads.
Answer: By this they increase the area of contact of the load with their head. So, the pressure on their head is reduced and they find it easier to carry the load.
- Explain now why shoulder bags are provided with broad straps and not thin straps?
Or
Why are the straps of school bags made broader?
Or
Why is it more comfortable to lift a school bag with broad straps than thin straps?
Answer: Shoulder bags are provided with broad straps and not thin strap so that the weight of the bag may fall over a large area of the shoulder of the child producing less pressure on the shoulder.
- A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.
Answer: The two forces acting on the rocket are the force of gravity, which pulls the rocket towards the ground, and the force of friction due to earth’s atmosphere, which opposes its motion.
- Why a fountain of water is created at the leaking joints or holes of pipes of the main water supply line?
Answer: Many times we see a fountain of water rushing out of the leaking joints (or holes) in the pipes of main water supply line. It is due to the very high pressure exerted by water on the sides (or walls) of the pipes that such a fountain of water is formed.
- Why does the liquid rise in a syringe when its piston is pulled up?
Or
How does a liquid medicine enter a syringe?
Answer: When the nozzle of a syringe is dipped in a liquid and its piston is withdrawn, the pressure inside the syringe is lowered. The greater atmospheric pressure acting on the surface of the liquid pushes the liquid up into the syringe.
- Why does nose bleeding occur at high altitudes?
Or
Why do mountaineers suffer from nose bleeding at high altitudes?
Answer: At high altitudes, the atmospheric pressure becomes much less than our blood pressure. Since, our blood is at higher pressure than outside pressure, therefore, some of the blood vessels in our body burst and nose bleeding takes place at high altitudes.
- Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.
Answer: In this case, muscular force is acting in upward direction and the force of gravity is acting in downward direction. The two forces are equal in magnitude but opposite in direction. Therefore, the net force on the bucket is zero. Hence, there is no change in its state of motion.
- Why atmospheric pressure decreases as we go higher up above the earth’s surface?
Answer: The pressure at any level in the atmosphere may be interpreted as the total weight of the air above a unit area at any elevation. Since most of the atmosphere’s molecules are held close to the earth’s surface by the force of gravity, air pressure decreases as we go higher up above the earth’s surface.
- Why does a plastic comb rubbed with dry hair attract tiny pieces of paper?
Answer: When a plastic comb is rubbed in dry hair, the comb gets electric charges by friction. And when this electrically charged comb is brought near tiny pieces of paper, it exerts an electrostatic force of attraction on them. Due to this electrostatic force, the tiny pieces of paper move towards the comb and stick to it.
- If the area of my head were 10 cm × 10 cm, how much weight of air would I be carrying on my head? Why don’t we feel the weight of such a huge amount of air on us?
Answer: The weight of air in a column of the height of the atmosphere and area 10 cm × 10 cm is as large as 1000 kg. The reason we are not crushed under this weight is that the pressure inside our bodies is also equal to the atmospheric pressure and cancels the pressure from outside.
- If a vacuum is created between two Magdeburg hemispheres joined together, they cannot be separated easily. Give reason.
Or
Why do Magdeburg hemispheres stick together?
Answer: This is due to the fact that since there is no air inside, the unopposed atmospheric pressure acting over the whole surface of the hemispheres from outside presses them very, very hard and does not allow them to be separated.
- Explain why, when a person stands on a cushion, the depression is much more than when he lies down on it.
Answer: When a man stands on a cushion then only his two feet are in contact with the cushion. Due to this, the weight of man falls on a small area of cushion producing a large pressure. This large pressure causes a big depression in the cushion. On the other hand, when the same man is lying on the cushion, then his whole body is in contact with the cushion. In this case, the weight of man falls on a much larger area of the cushion producing much smaller pressure. And this smaller pressure produces a very little depression in the cushion.
- Some facts:
- Skis have a large surface in order to reduce the pressure on snow.
- Wooden sleepers are placed under railway tracks to reduce the pressure on the ground.
- Pressure is directly proportional to the force applied.
- If the force acting on an area is increased, pressure will increase.
- The straps and handles of travel bags are made broad to reduce the pressure on our shoulders.
- An object exerts a pressure of 2 Pa on the ground. If another object of the same weight is kept above it, then the net pressure will become 4 Pa.
- The pressure exerted by a liquid at a given depth of the liquid same is in all directions.
- If the height above the sea level increases, atmospheric pressure will decrease.
- Army tanks are provided with caterpillar tracks under their wheels in order to decrease pressure on the ground.
- School bags have broad straps to reduce the pressure on our shoulders.
- Smaller the area on which a force acts, greater is the effect of force.
- Pressure exerted on a surface depends upon thrust and area of contact.
- If the thrust acting on a surface decreases, then the pressure decreases.
- A long spanner is used to loosen a tight nut. This is because by increasing the length of the spanner, the moment arm gets increased and even a small force can produce a large turning effect.
- The hand flour grinder is provided with a handle near its rim to increase the moment arm so that even a small force applied can produce a. large turning effect.
- It is easier to open a door by applying the force at its handle than at the hinged end because the moment of force is large when the force is applied at the handle as the moment arm is larger as compared to when the force is applied at the hinged end.
- Nose bleeding may occur at a high altitude because the atmospheric pressure decreases.
