Solution of Gravitation (Page No - 124) - Physics by Lakhmir Singh, Class 9 | Extra Documents & Tests for Class 9 PDF Download

Question 27:
(a) What is the difference between the density and relative density of a substance ?
(b) If the relative density of a substance is 7.1, what will be its density in SI units ?
Solution :
(a) The density of a substance is defined as mass of the substance per unit volume. 

SI unit of density is kg/m³

The relative density of a substance is the ration of its density to that of water.

It has no unit.

Question 28:
Define thrust. What is its unit ?
Solution :
The force acting on a body perpendicular to its surface is called thrust. The SI unit of thrust is newton (N).

Question 29:
A mug full of water appears light as long as it is under water in the bucket than when it is outside water. Why ?
Solution :
A mug full of water appears light as long as it is under water because buoyant force acts on it which reduces its effective weight and makes it appear lighter.

Question 30:
What happens to the buoyant force as more and more volume of a solid object is immersed in a liquid ? When does the buoyant force become maximum ?
Solution :
As more and more volume of the solid object is immersed in the liquid, the upward ‘buoyant force’ also keeps on increasing. When the object is completely immersed in the liquid, the buoyant force acting on the solid becomes maximum and remains constant thereafter.

Question 31:
Why do we feel light on our feet when standing in a swimming pool with water up to our armpits ?
Solution :
As more and more volume of our body is immersed in water, the apparent weight of the body goes on decreasing and the body seems to become lighter. This is due to the increase in upward buoyant force acting on the body.

Question 32:
Explain why, big boulders can be moved easily by flood.
Solution :
Big boulders weig much less while in water and as such are easily moved by the flood.

Question 33:
An iron nail sinks in water but it floats in mercury. Why ?
Solution :
An iron nail sinks in water but it floats in mercury because density of iron is more than that of water but less than that of mercury.

Question 34:
Explain why, a piece of glass sinks in water but it floats in mercury.
Solution :
A piece of glass sinks in water but it floats in mercury because density of glass is more than that of water but less than that of mercury.

Question 35:
Steel sinks in water but a steel boat floats. Why ?
Solution :
A piece of steel sinks in water because steel is denser than water. However, a steel ship is a hollow object made of steel and contains a lot of air in it. Due to presence of a lot of air in it, the average density of the ship becomes less than the density of water. Hence a ship floats in water.

Question 36:
Explain why, school bags are provided with wide straps to carry them.
Solution :
School bags have wide straps so that their weight may spread over a large area of shoulder producing less pressure on the shoulder.

Question 37:
Why does a sharp knife cut objects more effectively than a blunt knife ?
Solution :
A sharp knife cuts objects easily because due to its very thin edge, the force of our hand falls on a very small area of the object producing large pressure.

Question 38:
Explain why, wooden (or concrete) sleepers are kept below the railway line.
Solution :
Concrete or wooden
sleepers are kept below the railway line so that the weight of passing train
is spread over a large area of ground and the track may not sink into the
ground.

Question 39:
Explain why, a wide steel belt is provided over the wheels of an army tank.

The density of iron or steel is much higher than that of water, so an object made of iron or steel (like this car) sinks in water.
Solution :
A wide steel belt is provided over the wheels of an army tank so that they exert less pressure on the ground and do not sink into it.

Question 40:
Explain why, the tip of a sewing needle is sharp.

Solution :
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.

Question 41:
When is the pressure on the ground more—when a man is walking or when a man is standing ? Explain.
Solution :
When a man is walking, then at one time only one foot is on the ground. Due to this, the force of weight of 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, then both his feet are on the ground. Due to this, the weight of the man falls on a larger area of the ground and produces lesser pressure on the ground.

Question 42:
Explain why, snow shoes stop you from sinking into soft snow.
Solution :
Snow shoes stop us from sinking into soft snow because due to large area of snow shoes, our weight is spread over a large area of the snow producing small pressure.

Question 43:
Explain why, when a person stands on a cushion, the depression is much more than when he lies down on it.
Solution :
When a person stands on a cushion then only his two feet (having small area) are in contact with the cushion. Due to this the weight of man falls on a small area of the cushion producing a large pressure causing a big depression in the cushion. On the other hand, when the same person lies down on the cushion, then his whole body (having large area) is in contact with the cushion. Here, his weight falls on a much larger area of the cushion producing much smaller pressure and very little depression in the cushion.

Question 44:
Use your ideas about pressure to explain why it is easier to walk on soft sand if you have flat shoes rather than shoes with sharp heels.
Solution :
Flat shoes have greater area in contact with the soft sand as compared to heels. Due to this, there is less pressure on soft sand because of which they do not sink much in the sand and it is easy to walk on it.

Question 45:
Explain why, a nail has a pointed tip.

Solution :
A nail has a pointed tip, so that when it is hammered, the force of hammer is transferred to a very small area of wood creating a large pressure which pushes the nail into the wood.