9th Class (CBSE) Science: Gravitation

Question: The volume of a 500 g sealed packet is 350 cm3. Will the packet float or sink in water if the density of water is lg cmr3? What will be the mass of the water displaced by this packet?

Answer:

Question: With the help of an activity prove that the force acting on a smaller area exerts a larger pressure?

Answer: Consider a block of wood kept on a table top. The mass of the wooden block is 5 kg. Its dimension is 40 cm x 20 cm x 10 cm.

Now, we have to find the pressure exerted by the wooden block on the table top by keeping it vertically and horizontally.

∴ The pressure exerted by the box in case (a) is more as compared to the pressure exerted in case (b).

The area is reduced and the pressure exerted is more.

This shows that pressure ∝ 1/area.

Pressure will be larger if the area is reduced.

Application:

1. Nails have pointed tips.
2. Knives have sharp edges.
3. Needles have pointed tips.

Question: State the universal law of gravitation.

Answer: The universal law of gravitation states that every object in the universe attracts every other object with a force called the gravitational force. The force acting between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres.

Masses of two objects of = m₁and m₂
Distance between the two objects = d
Force of attraction acting between them = F
It will be given by the universal law of gravitation.
F = Gm₁m₂/d²
where, G is the universal constant.
G = 6.67×10^-11 Nm²kg^-2

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

Answer:

Mass of the Earth = M_E
Mass of an object on its surface = m.
Radius of the Earth = R
According to the universal law of gravitation, the gravitational force (F) acting between the Earth and the object is given by the relation:
F = Gm₁m₂/r²
Thus, the formula will be, F = GM_Em/R²

Question: What do you mean by free fall?

Answer: Gravity of earth attracts every object towards its centre. When an object is dropped from a certain height , it begins to fall towards Earth’s surface under the influence of gravitational force. Such a motion of object is called free fall.