## Gravitation – Questionbank with answer for CBSE Class 9 SA1

The following questions are for a quick revision of the chapter “Gravitation” for CBSE Class 9. There are many other questions which can be asked from the portion allotted for SA1, but will mainly based on these concepts. The questions are based on the bare essential concepts; the minimum level of learning expected from the children

**State the universal law of gravitation.**

Universal law of gravitation states that every object in the universe attracts every other object with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them**Define G. Write its value with proper units of SI**

G is the universal constant of gravitation. It is defined as the gravitational force of attraction between two objects of mass 1 kg each separated by a distance of 1m. Its value is 6.67 x 10^{-11}Nm^{2}kg^{-2}- What is the difference between gravity and gravitation?

The force of gravitation exerted by earth (or any other planet) on the objects on its surface is called gravity. Every object attracts every other object due to the force of gravitation. So, Gravity is a special case of Gravitation - Define acceleration due to gravity.
- Why the moon is not “falling” to earth?
- Derive an expression for the acceleration due to gravity.
- Write the differences between g and G.
- calculate the value of g on the surface of earth.
- Discuss the variation of g with height, depth and latitude.
- Distinguish mass and weight.

## Electrostatic induction – Where does the charge go on earthing?

Rajkumar asks:”in electrostatic induction in step 3 when earthed wire is removed then where the free negative charge go.does they go to earth then what happen to them.”

Waiting for responses from visitors

## Why gravitational force of attraction is necessary for a satellite to revolve around earth?

The question was posted by Raj Kumar.

The satellite has a velocity. To revolve around, the direction of velocity must change. In the absence of a force it can move only in a straight line. It is the gravitational force which helps the satellite top change direction and revolve around the earth.

In other words, for a **satellite** to revolve in a circular path, a **centripetal force** is required. This centripetal force is provided by the force of **gravitation** only.

## Variation of acceleration due to gravity with depth. A doubt.

Rajkumar asks: “**accelaration due to gravity** decrease on depth of earth with the relation of g’=g(1-2h/R). but with relation of g=GM/(R*R) g increase.

**Answer**:

When the object is on the surface or above it, the entire mass of the earth attracts it. But when it is inside, the net attraction towards CENTRE is provided only by the mass of earth below it.

ie; if it is at a depth d below the surface, then it is attracted to the centre by a sphere of radius R-d only. The forces due to the rest of the mass of earth cancel out. (On one side there is a little mass and on the other side greater amount of mass but distributed faraway and both contribute equally towards force and cancel out.)