gravitation ninth (class IX) .pptx

Ninth Street gravitation chapter

1. gravitation

2. Introduction to gravitation We have learnt that a force is needed to change the speed or direction of motion of an object. We know that an object dropped from a height falls towards the Earth, all the planets go around the Sun, the Moon goes around the Earth, in all these cases there must be some force acting on the objects, the planets and the moon. Issac Newton grasp that the some force is responsible for all these .This force is called as gravitational force. This Photo by Unknown Author is licensed under CC BY-SA

3. CENTRIPETAL FORCE Centripetal force is the force acting on an object in circular motion directed towards the axis of rotation or the centre. Centripetal force of an object moving in a circular path always acts towards the centre of the circle.

4. newton’s law of gravitation Every object in the universe attracts every other object which is proportional to the product of their masses and inversely proportional to the square of the distance between them . The force is along the line joining the centres of two objects. The square of the distance in the denominator is crucial in the equation because it reflects the nature of how gravitational force weakens with distance.

5. T he value of G was found out by Henry Cavendish

6. Importance of universal law of gravitation 1. The force that binds us to the Earth. 2.The motion of the Moon around the Earth. 3.The motion of the planets around the Sun. 4.The tides due to the Moon and the Sun.

7. When a body falls towards the earth under the effect of gravitational force then it is called free fall. While falling there is no change in the direction of motion of the object, but due to the earth’s attraction there will be a change in the magnitude of velocity ,any change in velocity involves acceleration . This acceleration is due to the earth’s gravitational force ,so it is called gravitational acceleration which is denoted by ‘g’ . SI unit ‘g’ is same as acceleration, i.e , ms-2. FREE FALL , TO CALCULATE THE VALUE OF ‘ g’

8. According to 2 nd law of motion, F = m x a m = mass of stone F= mg g = acceleration due to gravity Let the stone be on or near the surface of the earth.

9. Equations of motion ( for constant gravitational acceleration ) = + = + ½ = + g is taken as positive when it is in the direction of motion (downward direction ) and it is taken as negative when it is opposite to the direction of motion ( upward direction )  

10. Mass and weight Mass : The quantity of matter contained in the body is called its mass. Its SI unit is kilogram . The mass of an object is constant everywhere. Weight : The force with which a body is attracted by the earth is known as the weight of the body . Its SI unit is newton ( N ). It varies place to place . Its value becomes zero at the centre of earth . We know that F = ma = mg this force is known as the weight of the body denoted by W so W = mg , it acts vertically downwards, it is a vector quantity

11. Weight of an object on the moon The force with which moon attracts the object is called its weight on the moon . By applying the universal law of gravitation W = G M m / Where M is the mass of the moon m is the mass of the object and R is radius of the moon From calculation it is clear that weight of the object on the moon = 1/6 its weight on the earth  

12. Thrust and pressure Thrust : The perpendicular force acting on the surface of a body is called thrust . The SI unit of thrust is N . Pressure : The thrust acting per unit area of a body is called pressure acting on it . Pressure = thrust / area. Its SI unit is pascal . The same force acting on a smaller area exerts a larger pressure and smaller pressure on a larger area . This Photo by Unown Author is licensed under CC BY

13. Pressure in fluids All liquids and gases are fluids. A solid exerts pressure on a surface due to its weight . Similarly fluids have weight and they also exert pressure on the base and walls of the container in which they are closed . Pressure exerted in any contained mass of fluid is transmitted undiminished in all directions .

14. buoyancy When a body is immersed in a fluid it experiences an up thrust force ( buoyant force ) , this phenomenon of fluid is called as buoyancy . This buoyant force tending to counter balance the weight of this body acting vertically downward .

15. Why objects float or sink when placed on a surface of water ? If the buoyant force is greater then weight then a body floats and vice versa, like a rubber cork floats whereas a iron nail sinks . This Photo by Unknown Author is licensed under CC BY-SA This Photo by Unknown Author is licensed under CC BY-NC-ND

16. Archimede’s principle When a body is immersed fully or partially in a fluid it experiences an upward force that is equal to the weight of the fluid displaced by it . Up thrust force or buoyant force = weight of fluid displaced by a body = (weight of body in air - weight of body in fluid)

17. Relative density Relative density of a substance is defined as the ratio of the density of the substance to the density of water . Relative density = density of substance / density of water . It has no unit . Relative density > 1 : A body sinks. R D < 1 : A body floats . R D = 1 : A body just floats.