## Q1. Why is it difficult to hold a school bag having a strap made of a thin and strong string?

If a school bag has a strap made of thin and strong string, the whole weight of school bag will fall of over a very small area (equal to that string) on the shoulder. So pressure on shoulder P =\(\frac{Weight of the bag}{Area of string}\) of child will be very large and hence painful for the child. Hence, it is difficult to hold a school bag having a strap made of thin string.

## Q2. What do you mean by buoyancy?

Ans. When an object is placed in a liquid, the liquid exerts an upward force on it. The tendency of liquid to exert such an upward force on the object is called the buoyancy and the upward force exerted by liquid on the object is called the buoyant force (or the force of buoyancy).

## Q3. Why does an object float or sink when placed over the surface of water?

Ans. When an object is immersed in water, the water exerts an upward force on the object. This upward force equal to the weight of water displaced by object is called the buoyant Force.

If on completely immersing the object, the buoyant force is more than the weight of object will float on water. In other words, if the density of object is less than that of water, then the object will float on water.

On the other hand if buoyant force, on completely immersing the object in water, is less than the weight of the object, the object will sink in water. In other words, if the density of object is greater than water, then the object will sink in water.

In brief, if p _{object }< p _{water} , the object will float and if p _{object} > p _{water}, the object will sink.

## Q4. You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?

Ans. When we stand on a weighing machine, we replace air equal to volume of our body. So measured weight, *W = *actual weight — upthrust

= mg — V p _{air }g

Therefore, Measured mass, m’ = \(\frac { W }{ g }\) = (m-V _{air })

That is the measured mass will be less than the actual mass. Therefore, our mass is more than 42 kg.

## Q5. You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality, one is heavier than other. Can you say which one is heavier and why?

Ans. Actually mass of cotton is more than the mass of iron bar. This is because

Weight measured = Actual weight — buoyant force

Therefore, Actual weight = weight measured + buoyant force

The volume of cotton is much more than the volume of iron bar, so buoyant force (B = volume of object x density of air x g) of air on cotton is much more than that of air on iron bar, so mass of cotton will be more than the mass of iron bar.

What happen when upthrust increases