Class 10 Gravitation Notes (PDF) Science 1 for SSC Board Exam

Chapter 1 Gravitation notes Science 1 class 10th's Top Board Questions

gravitation

 Gravitation Newton's laws of motion: 

"An object continues to remain at rest or in a state of uniform motion along a straight line unless an unbalanced force acts on it".
"The rate of change of momentum is directly proportional to the applied force and the change of momentum occurs in the direction of the force".
"Every action force has an equal and opposite reaction force which acts simultaneously".
v = Final velocity, a = acceleration, t = time, u = initial velocity, s = displacement 
v=u+at
s=ut+21​at^2
v^2=u^2+2as

Speed=Distance/Time(m/s)
​Velocity=Displacement​/Time (m/s)
Acceleration=Velocity/Time​ (m/s²) The rate of change of velocity is called acceleration.
Acceleration due to Gravity on the earth is = 9.8 m/s².
SI unit of mass is kg / Second / Minute, in CGS System: cm / gm / Seconds.

An object falls vertically downwards. Why?

  • Gravitational force: 
The earth attracts every object near it towards itself Because of the gravitational force. 
  • Directed towards the center of the earth: 
The earth attracts an objects towards the center of the earth. This Attraction force must be directed towards the center of the earth. 
  • Vertically downward direction: 
The direction from an object to the center of the earth is all the vertically downward direction at the position of the object. There for an object falls vertically downward.
  • Centripetal force: 
A force acts on any objects moving along a circle and it is directed towards the center of the circle. This is called the centripetal force.

The moon and artificial Satellites do not collide on the earth. why?

  •   Centripetal force. 
A force acts on any object moving along a circle and it is directed towards the center of the circle is called the centripetal force.
  • Velocity:
The moon and the artificial Satellites have velocity which try to pull away from the Earth. The centripetal force try to pull it towards the centre of the earth and velocity try to pull away from the earth both balanced to each other. Therefore, the moon and artificial Satellite do not collide on the earth.
  • The work is zero:
The earth constantly attracts the moon and the moon has velocity and it is always at the position of 90°. work=Fs cosθ and cos 90∘=0. Therefore, The work is 0 and the moon and the artificial Satellite do not fall on the earth.

 High and Low tides occur at different times at different places. Why?

  • The Moon's Gravitational force: 
The level of water in the Sea changes because of the gravitational force exerted by the moon.
  • High tides: 
Water directly under the moon gets pulled towards the moon and the level of water there goes up causing high tide at that place.
  • Low tides:
The level of water is minimum at 90° from the place of High tide on the earth and low tides occurs.

The value of 'g' is not same everywhere on the surface of the earth. Why? 

  • The shape of the Earth:
The shape of the earth is not exactly Spherical and the distance of a point on the surface of the earth from its centre differs somewhat from place to place.
  • Rotation of the Earth: 
The earth bulges at the equator and is flatter at the poles. Due to its rotation, its radius is largest at the equator and smallest at the poles.
  • The value of 'g': 
The value of g is thus highest [ g=9.832 m/s^2] at the poles and decreases slowly with decreasing latitude. It is lowest [ g= 9.78 m/s^2] at the equator.
Therefore , the value of g is not same everywhere.

The value of g is zero at the centre of the Earth. Why?

  • Contributing part of the earth decreases: 
The value of r in equation g = GM / r^2 decreases, but the part of the earth which contributes towards the gravitational force felt by the object also decreases.
  • The value of M (mass) decreases: 
The value of M also decreases, and as the combined result of change in r and M, the value of g is zero as we go deep inside the Earth at the center of the Earth. 

The value of g decreases as we go deep inside the earth. Why?
  • Contributing part of the earth decreases: 
The value of 'r' in equation g= GM/r^2 decreases , but the part of the earth which contributes towards the gravitation force felt by the object also decreases.
  • Value of m (mass) decreases:
The value of m also decreases , and as the combined result of change in r and m , the value of g decreases as we go deep inside the Earth.

As we go above the earth surface , the value of g decreases. Why?

  • 'r' (radius) increases : 
As we go above the earth's surface , the value of 'r' in equation g=GM/r^2 increases and the value of g decreases.
  • Small heights: 
The decrease g is rather small for small heights in comparison to the Earth's radius. The change in the value of g is negligible.
  • Larger heights: 
The change in the value of g due to the large distance is typical , as the height of communication satellite is 35700 and the value of g is 0.225 m/s^2.

When we drop a feather and a heavy stone at the same time from a height , they reach the earth at the same time. Why?


  • Buoyant force and Frictional force of air: 
When we drop a feather and heavy stone at the same time from a height , the feather experiences a Buoyant force and a frictional force of air. Therefore, feather floats and reaches the ground slowly.
  • Weight of the stone: 
The Buoyant force and fictional forces on the stone are much less than the weight of the stone and does not affect the speed of the stone much. 
  • Vacuum: 
In vacuum, there is no Buoyant force and fictional force due to the absence of air. Therefore , the feather and the stone reach at the same time from a height in vacuum only.

Space travelers as well as objects in the spacecraft appear to be floating. Why?


  • 'g' is 11% less:  
Though the spacecraft is at a height from the surface of the Earth, the value of g there is not zero. In the space station, the value of g is only 11% less than its value of the surface of the earth.
  • Velocity: 
Though the spacecraft is not falling on the earth because of its velocity along the orbit, the only force acting on it is the gravitational force of the earth.
  • Free fall: 
The height of spacecraft is not the reason of their weightlessness, their weightless is caused by their being in the state of the free fall.
Therefore , space travelers as well as object in the spacecraft appear to be floating.

Kepler's laws of planetary motion.

First Law: 
The orbit of an planet is an ellipse with the sun at one of the foci.
Second law: 
The line joining the planet and the sun sweeps equal area in equal intervals of time.
Third law: 
The square of its period of revolution around the sun is directly proportional to the cube of the mean distance of a planet from the Sun.

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