Q1. Why does a sharp knife cut better than a blunt knife?
Ans. A sharp knife has a very thin edge to its blade. A sharp knife cuts objects better because due to its very thin edge, the force of our hand falls over very small area of the object producing a large pressure. And this large pressure cuts the object easily. On the other hand, a blunt knife has a thicker edge. A blunt knife does not cut an object easily because due to its thicker edge, the force of our hand falls over a larger area of the object and produces lesser pressure. This
lesser pressure cuts the object with difficulty.
Q2. Why is the tip of the needle sharp?
Ans. The tip of the 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 enough to pierce the cloth being stitched.
Q3. Why is the tip of the nail pointed?
Ans. A nail has a pointed tip, so that when it is hammered, the force of hammer falls on a very small area of wood creating a large pressure which pushes the nail into wood.
Q4. Why is the pressure on ground more when a man is walking than when he is standing?
Ans. When a man is walking, then at one time only his one foot is on the ground. Due to this, the force of weight of the man falls on a smaller area of the ground and produces more pressure on the ground. On the other hand, when the man is standing both his feet are on the ground and produces lesser pressure on the ground.
Q5. Why is depression much more when a man stands on the cushion than when he lies down on it?
Ans. 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 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.
Q6. Why is it easier to walk on soft sand if we have flat shoes rather than shoes with sharp heels?
Ans. This ¡s because flat shoes have a greater area in contact with the soft sand due to which there is 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 so exerts greater pressure on the soft sand. Due to this greater pressure, the sharp heels tend to sink deep into soft sand making it difficult for the wearer to walk on sand.
Q7. Why do tractors have broad tyres?
Ans. The tractors have broad tyres so that there is less pressure on the ground and the tyres do not sink into comparatively soft ground in the fields.
Q8. Why do snow shoes stop you from sinking into snow?
Ans. The snow shoes have large, flat soles so that there is less pressure on the soft snow and stop the wearer from sinking into it.
Q9. Why does a ship made of iron and steel float in water whereas a small piece of iron sinks in it?
Ans. Ship is not a solid block of iron and steel. A ship is a hollow object made of iron and steel which Contains a lot of air in it. Air has a very low density. Due to the presence of a lot of air in it, the average density of the ship becomes less
than the density of water, therefore, a ship floats in water.
Q10. A boy gets into a floating boat.
(a) What happens to the boat?
(b) What happens to the weight of water displaced?
(c) What happens to the buoyant force on the boat?
Ans. (a) The boat floats lower in water.
(b) The weight of water displaced increases.
(c) The buoyant force on boat increases.
Q11. A 0.5 kg sheet of tin sinks in water but if the same sheet is converted into a box, it floats. Why?
Ans. The average density of hollow box made up of sheet of tins becomes less than the density of water, so it floats.
Q12. Why does the driver wear metallic cover while going into the sea?
Ans. The pressure increases with increase of depth of water. The increase in pressure is 1 atm by 10 m increase of water. If this pressure falls on the driver directly, he will not tolerate it. Therefore, if a driver wants to go to large
depth, he should wear the metallic cover. The pressure of water column falls on the metallic cover and the pressure inside the cover remains nearly equal to atmospheric pressure. Thus, by wearing metallic cover the driver remains quite safe.
Q13. Why are the walls of a dam made thick at the bottom and thin upwards?
Ans. The pressure at a point inside a liquid depends on the depth of point from the free surface, therefore, the pressure is very high at the bottom of the dam. To tolerate this pressure, the walls of dam are made thick at the bottom. As
we move upwards, the pressure goes on decreasing: so the thickness of wall is made smaller and smaller. That is why the walls of dam are made thick at the bottom and thin at the top.
Q14. Why is it easier to float in sea water than in river water?
Ans. The body of man is lighter than water but the head is heavier than water. Therefore, total weight of man happens to be greater than the weight of water displaced: therefore the man can sink in water. Hence, for floating in water, the man has to displace more water by moving his arms and legs. By doing so when the weight of water displaced by man becomes equal to the weight of man, the man begins to float. The density of sea water is greater than the fresh water of river, therefore the weight of sea water displaced becomes easily equal to the weight of man. Hence, it is easier to float in sea water than in river water.