MCQ : Gravitation (Floatation) - 1 - Class 9 MCQ

The SI unit of pressure is the Pascal, which is equivalent to one Newton per square meter. This unit is named after Blaise Pascal, a prominent French mathematician, physicist, and philosopher known for his contributions to fluid mechanics and pressure studies.

• Option A: Universal law of gravitation - Explains motion of planets around the Sun: - Explanation: This is correctly matched. The universal law of gravitation explains several phenomena including the motion of planets around the Sun.
• Option B: Centripetal force - Force acting away from the center: - Explanation: This is incorrect. The centripetal force acts towards the center, not away from it.
• Option C: Gravitational force - Does not affect the moon: - Explanation: This is incorrect. The gravitational force is responsible for the moon's orbit around the Earth.
• Option D: Acceleration due to gravity - Denoted by F: - Explanation: This is incorrect. The acceleration due to gravity is denoted by g, not F.

The gravitational force between two objects is calculated using Newton's law of gravitation:
Formula: F = G * (m₁ * m₂) / r²
Where:

G = 6.7×10⁻¹¹ N m² kg⁻²
m₁ = 5×10³ kg
m₂ = 2×10³ kg
r = 10 m

Substitute the values into the formula:
F = 6.7×10⁻¹¹ * (5×10³ * 2×10³) / 10²
F = 6.7×10⁻¹¹ * 1×10⁷ / 100
F = 6.7×10⁻¹¹ * 1×10⁵
F = 6.7×10⁻⁶ N
The correct answer is B: 6.7×10⁻⁶ N

An object weighs 98 N on the surface of the Earth. To find its weight on the surface of the Moon, follow these steps:

Weight is the force of gravity on an object.
The Moon's gravity is about 1/6th of Earth's gravity.
Weight on Moon = Weight on Earth / 6
Weight on Moon = 98 N / 6 = 16.33 N
Therefore, the correct answer is:
A: 16.33 N

To determine the time it takes for the stone to reach its maximum height, consider the following:
The initial velocity (u) = 10 m/s.
The acceleration due to gravity (g) = -9.8 m/s² (negative as it opposes the motion).
At maximum height, the final velocity (v) = 0 m/s.
Using the formula v = u + at, we solve for t:
Therefore, the correct answer is B: 1 s.

• 0 = 10 + (-9.8)t
• t = 10 / 9.8
• t ≈ 1.02 s

If a body is dropped from a height of 45 m, how long does it take to reach the ground? (Use g =10 m/s²)
To find the time (t) taken to reach the ground, use the formula:
h = (1/2) * g * t²
45 = (1/2) * 10 * t²
45 = 5 * t²
t² = 45 / 5
t² = 9
t = √9
t = 3s
Therefore, the correct answer is A: 3s.

 The correct answer is a

   Archimedes' principle, a physical law of buoyancy, states that any body completely or partially submerged in a fluid (gas or liquid) at rest is acted upon by an upward, or buoyant, force the magnitude of which is equal to the weight of the fluid displaced by the body.

Upthrust = apparent loss of weight of object = weight in air - weight in liquid.

weight in air= W
Upthrust ,U= W/4
weight in liquid = weight in air- Upthrust
= W - W/4
=3W/4

• The assertion is correct as per the universal law of gravitation, which states that every object in the universe attracts every other object with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between them.
• The reason is also correct as it correctly explains the relationship between the force of attraction, the masses of the objects, and the distance between them. Therefore, Option A is the correct choice as the reason effectively explains the assertion.