STD 9
- Universal law of Gravitation
- Free fall
- To find out the value of g (acceleration due to gravity)
- Motion of the object under the influence of gravity
- Mass
- Weight
- Thrust and pressure
- Archimedes’ principle
- Relative density
Gravitation
- It is the force of attraction between two objects in the Universe.
- The force of attraction is directly proportional to their masses.
- It is inversely proportional to the square of the distance between them.
- It is the weakest force in nature.
Gravity
- Gravitational force due to Earth is called gravity.
F = force between the two bodies
m1, m2 = masses of the two bodies
G = Gravitational constant = 6.67 x 10^-11 Nm^2/kg^2
Three Equations of Motion for free fall
- The acceleration due to gravity (g) is independent of the mass of freely falling body.
- It depends on mass of Earth and its radius.
- Case-1 – When an object is falling freely towards the earth with an initial velocity (u) and after some time (t), its final velocity becomes v after travelling a height, h.
Mass and Weight
Mass (m)
- Mass of a body is the quantity of matter contained in it.
- Scalar quantity
- Has only magnitude and no direction.
- Remains constant.
- Does not change from place to place.
- Mass of the body is not equal to zero.
- Measured by physical and chemical balance only
- SI Unit: kg
Weight (w = mg)
- Weight is the force of an object.
- It describes how the object is attracted towards the centre of the Earth.
- F = ma = mg = w
- SI Unit is N (newton)
- It changes from place to place, because g changes from place to place.
- At the centre of the earth, g = 0.
- Mass of the body at the centre of the earth = 0
Relationship between 1 kg weight and newton (N)
Variation of value of g
Show that the weight of an object on the moon is about 1/6 th of the weight on Earth.
Send your solution.
[Q] A force 20 N acts upon a body whose weight is 9.8 N. Find out the mass of the body and its acceleration?
