Class 10 Science Notes of Force

Gravitational force:

The force of attraction between any two heavenly bodies present in this universe is called gravitational force.

Some effects of Gravitational force:

1. The existence of the solar system is due to gravitational force.
2. Occurrence of sea tides due to the gravitational force of the sun and the moon on the seawater.
3. The satellite orbiting around the planet.

Theory related to Universe

Heliocentric Theory

The sun lies at the centre of the Universe and all other planets and satellites revolve around the sun in their own elliptical orbit is called heliocentric theory.

Geocentric theory

Before 16th century, astrologists it was believed that at the center of the universe there is the earth and other heavenly bodies like the sun, moon and other planets revolve around the earth is called geocentric theory.

Newton's Universal Law of Gravitation

Newton's Universal law of gravitation state that, "The gravitational force of attraction between any two heavenly bodies is directly proportional to the product of their masses and inversely proportional to the square distance between them from their centre."

Universal gravitational constant (G) G can be defined as the gravitational force of attraction between two bodies of unit masses (1kg each) separated by a unit distance (1m).

Properties of universal gravitational constant(G)

At what condition, gravitation and Universal gravitational constant are equal?

Ans. If the mass of both objects is 1kg and the distance between there is1m then the gravitation and gravitational constant are equal.

 State the factors that affect the gravitational force.

Ans: The factors that affect the gravitational force are:

1. Mass of two objects
2.  Distance between two objects from their centres.

Write down applications of Newton's law of gravitation.

Ans: The applications of Newton's law of gravitation are:

i. It helps us to determine the mass of the earth and other planets.

ii. This law is useful to determine the mass of the sun, star, moon etc.

iii. This law is useful to calculate the di5tance between any heavenly bodies.

Define gravity or weight. With its SI unit.

Ans. The force of attraction exerted by the earth on a body towards its centre is called gravity or the weight of the body.

The SI unit of gravity or weight is Newton (N).

Define mass with its SI unit.

Ans. The total amount of matter contained in a body is called mass.

The SI unit of mass is the kilogram(kg).

Acceleration due to gravity (g):

The acceleration developed in an object falling freely (without any resistance) towards the surface of the earth as a result of the gravity is called acceleration due to gravity (g).

The SI unit of acceleration due to gravity is m/s^2.

Note: As we know that our earth is not perfectly spherical so, the value of 'g' on the earth surface is not constant.

Here,

The value of g at the poles of the earth is 9.83m/s^2 and that in the equatorial region is 9.78 m/s 2.

Hence, the average value of g on the earth's surface is 9.8 m/s^2.

Prove that  g= GM/r^2 

Solution,

Let 'M' be the mass of the earth with its radius 'R'. If a body of mass 'm' is on the surface of the earth. Then,

According to Newton's law of gravitation,

F = GMm/r^2

According to Newton second law of motion,

F=m x g (ii)

Now,

By replacing the value of 'F' from equation (ii) to (I), we get,

m x g = GMm/r^2

or, g= GM/r^2  [ Where G and M are constant terms.]

 

Prove that acceleration due to gravity is independent to the mass of the object lying on the earth's surface.

Solution,

Let 'M' be the mass of the earth with its radius 'R'. If a body of mass 'm' is

on the surface of the earth. Then,

According to Newton's law of gravitation,

F=GMm/r^2(i)

According to Newton second law of motion,

F=m x g (ii)

Now,

By replacing the value of 'F' from equation (i) to (ii), we get,

m x g = GMm/r^2

or, g= GM/r^2  [Here G and M are constant terms]

or, g ∝ r^2

Hence, in equation iii, the mass of an object lying on the earth surface can not be observed. So, the value of acceleration due to gravity is independent of the mass of an object lying on the earth's surface.

The mass of Jupiter is 319 times greater than that of the earth but its gravity is just 2.5 times greater than that of earth. Why?

Ans. The mass of Jupiter is 319 times greater than that of the earth but its gravity is just 2.5 times greater than that of the earth because the radius of Jupiter is much greater (i.e. about 11 times than that of the earth and F is inversely proportional to the square of the radius of the earth)

The paper made into the shape of the ball drops faster than the unfolded paper, why?

 The paper made into the shape of balls drops faster than the unfolded paper because the effect of air resistance on the folded paper is less while on the unfolded paper is greater. Due to this, unfolded paper sways in the air due to air resistance but the unfolded paper drops faster.

Free Fall

When a body falls freely under the effect of gravity only neglecting the air resistance, then the body is said to be free fall.

Some conditions for free fall are:

1. When a body falls freely.
2. When a body is in the satellite orbiting around the earth or a heavenly body.
3. When a body is taken to the centre of the earth.

Weightlessness

Weightlessness is the condition of a body when its weight seems to be zero during free fall.

Differentiate between:

Gravity and gravitational force

Gravity	Gravitational force
It is the force by which a body is attracted towards the centre of a planet or satellite.  It depends on the mass and radius of the planet or satellite.	It is the mutual force of attraction between any two objects.
Its formula is; g=GM/r^2	Its formula is; F= GMm/r^2

 

b Gravity and acceleration due to gravity.

Gravity	Acceleration due to gravity
It is the force by which a body is attracted towards the centre of a planet or satellite.	It is the acceleration produced in a freely falling body due to the effect of gravity.
Its SI unit is newton (N)	Its SI unit is m/s^2
It is the cause for acceleration due to gravity.	It is the effect of gravity.

c. Freefall and weightlessness

Freefall	Weightlessness
When a body falls freely under the effect of gravity only neglecting the air resistance, then the body is said to be free fall.	Weightlessness is the condition of a body when its weight seems to be zero during free fall.
In every free fall, there is weightlessness.	In every weightlessness, there may not be a free fall.