All questions of Gravitation for Class 9 Exam

C. 10 kg

Conversion of 1 kg wt to Newtons

Definition of kilogram weight (kg wt)

Kilogram weight (kg wt) is a unit of force used in the metric system. It is defined as the weight of a kilogram mass under standard gravity.

Definition of Newton (N)

Newton (N) is the SI unit of force. It is defined as the force required to accelerate a mass of one kilogram at a rate of one meter per second per second.

Formula for conversion of kg wt to N

The formula for converting kg wt to N is:

1 kg wt = 9.8 N

Explanation of the formula

The conversion factor for kg wt to N is the acceleration due to gravity (g). In SI units, the value of g is approximately 9.8 m/s2. Therefore, the weight of a mass of 1 kg under standard gravity is:

Weight = Mass x Acceleration due to gravity
= 1 kg x 9.8 m/s2
= 9.8 N

Conclusion

Therefore, 1 kg wt is equal to 9.8 N. This conversion factor is used to convert between units of force in the metric system.

Weight on moon=1/6of weight on earth =1/6×48 =8N

Option b

B is the key to success

To find the mass of a body, we can use the formula:

Weight = mass × acceleration due to gravity (g)

Given that the weight of the body is 59 N and the acceleration due to gravity is 9.8 m/s^2, we can rearrange the formula to solve for mass:

mass = weight / acceleration due to gravity

Substituting the given values:

mass = 59 N / 9.8 m/s^2

Calculating this equation gives us:

mass ≈ 6 kg

Therefore, the mass of the body is approximately 6 kg.

Explanation:
- Weight is the force exerted by a body due to gravity, and it is measured in Newtons (N).
- The acceleration due to gravity, denoted by 'g', is the acceleration an object experiences due to the gravitational force. On Earth, the average value of g is approximately 9.8 m/s^2.
- The formula weight = mass × acceleration due to gravity relates weight, mass, and acceleration due to gravity.
- To find the mass, we rearrange the formula and divide both sides by acceleration due to gravity.
- By substituting the given values of weight (59 N) and acceleration due to gravity (9.8 m/s^2) into the formula, we can calculate the mass.
- The final result is approximately 6 kg.

Mass of body on surface of Earth vs. Moon

Explanation:

On the surface of the Earth, the gravitational force acting on a body is given by:

F = mg

where F is the force of attraction due to gravity, m is the mass of the body and g is the acceleration due to gravity.

On the surface of the Moon, the acceleration due to gravity is much lower than on Earth. The value of g on the surface of the Moon is approximately 1/6th of its value on Earth.

Therefore, the force of attraction due to gravity on the surface of the Moon is given by:

F' = mg/6

where m is the same as the mass of the body on Earth.

The mass of the body, however, remains the same on the surface of the Moon as it was on the surface of the Earth, since mass is an intrinsic property of an object and does not change with location.

Hence, the correct answer is option B, which states that the mass of the body on the surface of the Moon is the same as its mass on the surface of the Earth.

Speed can be constant but velocity may not be and thus acceleration
force of gravity pulls the satellite towards earth
and a centripetal force produces an acceleration to keep the satellite in their orbits
hence option c)

Gravity of earth is 9.8 N acceleration due to gravity is also 9.8 N gravitational force is nothing but the force applied by earth Therefore, f = m*a f,= 10*9.8 f= 98 N

A truss is said to be perfect or rigid if
M+3= 2J where M= no of members and N=no of joints.
in the given truss M=3 and J= 3 so 6=6. this is a perfect frme

A geostationary satellite revolves around the earth with the same angular velocity and in the same sense as done by the earth about its own axis, i.e. west-east direction. A polar satellite revolves around the earth's pole in north-south direction.

Force of gravitation is inversely proportional to square of distance 1/r^2

Earth is not a perfect sphere. Its radius at equator is greater than poles. Acceleration due to gravity is inversely proportional to the square of its radius. So, the acceleration due to gravity is greatest at poles. Hence, from relation, W = mg, it is clear that weight is highest at the poles.

Yes, by coulombs law
where force is inversely proportional to distance between two bodies or masses.

In combination, the equatorial bulge and the effects of the surface centrifugal force due to rotation mean that sea-level effective gravity increases from about 9.780 m/s2 at the Equator to about 9.832 m/s2 at the poles, so an object will weigh about 0.5% more at the poles than at the Equator.

To covert kg to N , we have to multiply the kg value by 9.8 . So 1x9.8=9.8. hence the answer A is correct

Becuse weight is just a force which may vari due the less mass of moon but mass is a quantiy of matter contained my any object ..thus it is

Understanding Free Fall on the Moon
When two objects of different masses fall freely near the surface of the moon, it's essential to understand how gravity and motion work in this context.
Key Principles of Free Fall
- Uniform Acceleration: On the moon, the acceleration due to gravity is approximately 1.6 m/s², which is much weaker than Earth's 9.8 m/s². However, this gravitational force acts equally on all objects, regardless of mass.
- Newton's Second Law: According to this law, the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass (F=ma). However, when considering free fall, the force of gravity is the only force acting on the objects.
Why They Have the Same Velocities
- Acceleration is Constant: Both objects, regardless of their masses, experience the same constant acceleration due to the moon's gravity. This means that they will gain speed at the same rate.
- Initial Conditions: If both objects are dropped from the same height at the same time, they will have the same initial velocities (which is zero if dropped). As they fall, they will continue to have the same velocity at any given moment because they accelerate uniformly.
Conclusion
Thus, the correct answer is option 'B': They have the same velocities at any instant. This phenomenon illustrates the fundamental principle that in a vacuum (or near a celestial body like the moon), the mass of an object does not influence its rate of fall. All objects fall at the same rate under gravity when air resistance is negligible.

Understanding Pressure
Pressure is defined as the force applied per unit area. It is given by the formula:
Pressure = Force / Area
Comparison Between a Nail and a Flat Surface
When comparing a nail to a flat surface, it is essential to consider how force is distributed over an area.
Surface Area
- A nail has a small surface area at its pointed end.
- A flat surface has a larger surface area.
Force Concentration
- When a nail is driven into a surface, it exerts a force over its small area.
- This small area leads to a higher concentration of force, resulting in increased pressure.
Effect of Pressure
- The increased pressure from the nail allows it to penetrate materials like wood or drywall more effectively than a flat object, which would distribute the same force over a larger area.
- The higher pressure is what makes nails efficient for fastening objects together.
Conclusion
- Therefore, when comparing the pressure exerted by a nail to that of a flat surface, the pressure exerted by the nail increases due to its smaller area.
- This is why nails are more effective for penetrating surfaces compared to flat objects.
In summary, the correct answer is option 'C': It increases.

SI unit of g is m/s2

Explanation:
- Definition of g: g is the acceleration due to gravity, which is the force that pulls objects towards the center of the earth.
- SI unit of acceleration is m/s2, which represents the change in velocity per unit time.
- Therefore, the SI unit of g is also m/s2.
- This unit is commonly used in physics and engineering to describe the acceleration of objects under the influence of gravity.
- Other units of acceleration include km/h2, cm/s2, and ft/s2, but these are not part of the SI system.

Weight is a fundamental concept in physics that refers to the force exerted on an object due to gravity. Here is a detailed explanation of the given statement:

Weight is the force experienced by an object due to the gravitational pull of the Earth or any other celestial body. It is a vector quantity because it has both magnitude and direction. The direction of the weight vector is always towards the center of the celestial body.

-
- Weight is a vector quantity because it has both magnitude and direction. The magnitude of weight is equal to the product of the mass of the object and the acceleration due to gravity. The direction of weight is always towards the center of the celestial body.
-
- Weight is directly proportional to the mass of the object. In interplanetary space, where the gravitational field is weak or negligible, the weight of a body would be much less compared to its weight on Earth. Therefore, the weight of a body in interplanetary space is not maximum.
-
- As bodies go up in the Earth's atmosphere, they move farther away from the center of the Earth. The distance between the body and the center of the Earth increases, which leads to a decrease in the acceleration due to gravity. Consequently, the weight of the body decreases as it goes up, rather than increasing.
-
- The correct answer is not "D: None of these" because option A, which states that weight is a vector quantity, is correct.

Explanation:

Gravitational Acceleration:
- The value of gravitational acceleration, denoted as 'g', near the Earth's surface is approximately 9.8 m/s^2.
- This value represents the acceleration experienced by an object due to the Earth's gravitational pull.

Acceleration Due to Gravity:
- The acceleration due to gravity is the acceleration that an object experiences when it is in free fall near the Earth's surface.
- It is approximately 9.8 m/s^2, which means that every second an object in free fall near the Earth's surface will increase its velocity by 9.8 m/s.

Variation in g:
- The value of g can vary slightly depending on the location on Earth, altitude, and other factors.
- However, for most practical purposes, a value of 9.8 m/s^2 is commonly used as the standard value of g near the Earth's surface.

Conclusion:
- Therefore, the correct value of g near the Earth's surface is 9.8 m/s^2, making option 'C' the correct answer.

Towards the centre of Earth means acts in the opposite direction of motion.

Explanation:

Introduction:

The value of G is the universal gravitational constant. It is a fundamental constant of nature that appears in the law of gravitation along with the masses of the two interacting bodies and the distance between them. It is a measure of the strength of the gravitational force between two objects.

Nature of the interacting bodies:

The value of G is independent of the nature of the interacting bodies. This means that the gravitational force between two objects, no matter what they are made of, follows the same law of gravitation and is determined by the same value of G. This is because the force of gravity is a fundamental force of nature that acts between all objects with mass, regardless of their composition or properties.

Size of the interacting bodies:

The value of G is also independent of the size of the interacting bodies. This means that the gravitational force between two objects, no matter how large or small they are, follows the same law of gravitation and is determined by the same value of G. This is because the gravitational force depends on the masses of the objects, not on their size or volume.

Mass of the interacting bodies:

The value of G is determined by the masses of the interacting bodies. This means that the gravitational force between two objects increases as their masses increase, and it decreases as the distance between them increases. However, the value of G itself is independent of the masses of the objects. This is because G is a constant of nature that relates the masses of the objects to the strength of the gravitational force between them.

Conclusion:

In summary, the value of G is independent of the nature and size of the interacting bodies, but it is determined by their masses. This makes G a fundamental constant of nature that has a universal value and applies to all objects with mass, no matter what their composition or properties.

The value of 'g' that is gravity is greater at the poles because the gravitational pull is maximum at the poles and decreases as it comes down toward the equator.

Understanding Acceleration Due to Gravity
Acceleration due to gravity (g) varies across the surface of the Earth. The value of g is influenced by several factors, including the Earth's shape and rotation.
1. Earth's Shape and Rotation
- The Earth is not a perfect sphere; it is an oblate spheroid, meaning it is slightly flattened at the poles and bulging at the equator.
- Due to this shape, the distance from the Earth's center to the surface is greater at the equator than at the poles.
2. Gravitational Force Variation
- Gravitational force decreases with increasing distance from the center of the Earth.
- Therefore, because you are farther from the center at the equator, the force of gravity (and thus g) is slightly less there compared to the poles.
3. Centrifugal Force Influence
- The Earth's rotation causes a centrifugal force that counteracts gravity. This force is strongest at the equator due to the higher rotational speed.
- As a result, the effective acceleration due to gravity is less at the equator than at the poles.
4. Conclusion
- The value of acceleration due to gravity is indeed least at the equator and greatest at the poles.
- This leads to the conclusion that the value of g increases from the equator to the poles, making option 'C' the correct answer.
Understanding these concepts can help clarify why gravity behaves differently across the globe, providing insight into the dynamics of our planet.

W = mg
mg = W
m=W÷g
m = 59÷9.8
m = 6.02 or 6 ( Approx )

A closed and empty plastic bottle floats in water because it displaces a volume of water whose weight is greater than the weight of the bottle. The buoyant force on the bottle, which is equal to the weight of the displaced water, is sufficient to counteract the weight of the bottle, causing it to float.

Understanding Archimedes’ Principle
Archimedes' Principle is a fundamental concept in fluid mechanics that explains the behavior of objects when submerged in a fluid. It specifically addresses the buoyant force acting on an immersed object.
What is Buoyant Force?
- Buoyant force is the upward force exerted by a fluid on an object that is submerged or floating in it.
Key Statement of Archimedes’ Principle
- The principle states that *the buoyant force on an immersed object is equal to the weight of the fluid displaced by the object*.
Why Option C is Correct
- When an object is placed in a fluid, it pushes some of that fluid out of the way, or displaces it. The amount of fluid displaced depends on the volume of the object submerged in the fluid.
- The weight of the displaced fluid can be calculated using the density of the fluid and the volume of the fluid displaced. This weight is what determines the buoyant force acting on the object.
Examples of Archimedes’ Principle
- Floating Objects: When a ship floats, it displaces a volume of water equal to its weight, allowing it to stay afloat.
- Sinking Objects: If an object is denser than the fluid, it will displace some fluid but may not displace enough to counteract its weight, causing it to sink.
Conclusion
- In summary, Archimedes' Principle highlights that the buoyant force is directly linked to the weight of the fluid displaced, making option C the correct choice. Understanding this principle is crucial in various applications, including shipbuilding, swimming, and designing underwater vehicles.

Pascal’s Law states that pressure applied to a confined fluid is transmitted undiminished in all directions. This principle is crucial in hydraulic systems, where an applied force can be magnified through the fluid, demonstrating how pressure changes are uniformly distributed throughout the fluid.

Archimedes’ Principle explains why a ship made of iron floats on water while a sheet of the same material sinks. According to this principle, an object will float if the buoyant force (equal to the weight of the displaced fluid) is greater than or equal to the weight of the object. A ship has a large volume, which displaces a substantial amount of water, generating a buoyant force that supports its weight. Conversely, a thin sheet of iron displaces less water, resulting in insufficient buoyant force to counteract its weight, causing it to sink.

Explanation:

Weight is the force with which an object is attracted towards the center of the earth due to gravity. It is measured in newtons (N) or in pounds (lb). The correct option is A, which says that weight is measured by a spring balance.

What is a spring balance?

A spring balance is a device that measures weight or force by the extension or compression of a spring. When an object is hung from the spring balance, the spring stretches, and the extension of the spring is proportional to the weight of the object. The spring balance has a calibrated scale that indicates the weight of the object in newtons or pounds.

Why is a spring balance used to measure weight?

A spring balance is used to measure weight because it is simple, accurate, and easy to use. The spring balance is portable and can be carried to different locations, making it useful for measuring weight in the field. Also, the spring balance does not require a level surface to measure weight, unlike a beam balance, which requires a level surface.

Can weight be measured in kg?

Weight is not measured in kg. Kg is a unit of mass, whereas weight is a force. Weight is measured in newtons (N) or pounds (lb). However, the mass of an object can be measured in kg using a balance.

Is weight a scalar or vector quantity?

Weight is a vector quantity because it has both magnitude and direction. The direction of weight is always towards the center of the earth. However, in most cases, weight is treated as a scalar quantity because its direction is always downwards and does not change.

The pressure exerted by the block of wood is greatest when it is on a side with the smallest area. This is because pressure increases as the contact area decreases, assuming the force (thrust) remains constant. A smaller area means that the same force is concentrated in a smaller space, leading to higher pressure.

Wide foundations are used in buildings to distribute the weight of the building over a larger area. This distribution reduces the pressure exerted on the ground, preventing the building from sinking or causing excessive stress on the foundation soil. By spreading the load, the building's stability is enhanced.

Explanation:
The force of gravitation between two bodies is determined by the following factors:
1. Their separation:
The force of gravitation between two bodies is inversely proportional to the square of the distance between them. As the separation between the bodies increases, the gravitational force decreases. Similarly, as the separation decreases, the gravitational force increases.
2. Gravitational constant:
The force of gravitation also depends on the gravitational constant. The gravitational constant, denoted by G, is a fundamental constant in physics that determines the strength of the gravitational force. It is a universal constant and has the same value throughout the universe.
3. Product of their masses:
The force of gravitation is directly proportional to the product of the masses of the two bodies. If the masses of the bodies increase, the gravitational force between them increases. Similarly, if the masses decrease, the gravitational force decreases.
Therefore, the force of gravitation between two bodies depends on their separation, the gravitational constant, and the product of their masses. All of these factors contribute to the overall strength of the gravitational force.
Answer: D. All of these.