## शनिवार, 29 जुलाई 2017

### GRAVITATION PART-I

(Page No. 134)
1. State the universal law of gravitation.

Ans. According to Newton’s universal law of gravitation:

Every mass in this universe attracts every other mass with a force which is directly proportional to the product of two masses and inversely proportional to the square of the distance between them.

2. Write the formula to find the magnitude of the gravitational force between the earth and an object on the surface of the earth.

Ans. The formula to find the magnitude of the gravitational force between the earth and an object on the surface of the earth is given below:

F = magnitude of gravitational force

G = Universal gravitation constant

M = mass of earth

m = mass of object

d = distance of object from the centre of earth

(Page No. 136)

1. What do you mean by free fall?

Ans. It is the object falling towards earth under the influence of attraction force of earth or gravity.

2. What do you mean by acceleration due to gravity?

Ans. During free fall any object that has mass experiences force towards centre of earth and hence an acceleration works as well. “acceleration experienced by an object in its free fall is called acceleration due to gravity.” It is denoted by g.

(Page No. 138)

1. What are the differences between the mass of an object and its weight?

Ans.

2. Why is the weight of an object on the moon 1/6th its weight on the earth?

Ans. since we know

Mass of object remains the same whether on earth or moon but the value of acceleration on moon is 1/6th of the value of acceleration on earth. Because of this weight of an object on moon is 1/6th its weight on the earth.

(Page No. 141)

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

Ans. It is difficult to hold a school bag having a strap made of a thin and strong string because a bag of that kind will make its weight fall over a small area of the shoulder and produce a greater pressure that makes holding the bag difficult and painful.

2. What do you mean by buoyancy?

Ans. It is the upward force experienced by an object when it is immersed into a fluid.

3. Why does an object float or sink when placed on the surface of water?

Ans. As an object comes in contact with the surface of a fluid it experiences two types of forces: gravitational force or gravity that pulls the object in downward direction and the second force is the force of buoyancy that pushes the object in upward direction.

It is these two forces that are responsible for an object to float or sink

i.e. if gravity >buoyancy then object sinks

if gravity

(Page No. 142)

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

Ans. Mass will be slightly more than 42 kg.

2. 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. The bag of cotton is heavier since volume of cotton bag is greater than iron bar, so the upthrust is larger in case of cotton hence real mass of cotton bag is more and it is heavier.