1. Why does an object fall downwards…………….? Does the object also fall downwards on the moon or on other places………….?

2. Wee see objects falling every day……………. Do they fall with constant velocity or with an acceleration?

3. Does the boy have the same weight on the earth and on the moon….? The Earth The Moon

4. Video What do you see in the video………? Where is it…………….?

5. Without gravitation, everything will floats in the air, including us, so how can we walk on the ground? Also, have you even thought about that : without gravitation, how can we go to the toilets? What will happen to the substances in the toilets? Can you imagine that? More important, if there is no gravitation, the Earth will not rotate around the Sun at all. At that time, there will be no change in season. So that some places will have 24 hours daytime and some places will have 24 hours nighttime. Can you imagine how our life will be? What Will Happen If There Is No Gravitation On The Earth…?

6. UNDERSTANDING GRAVITY

7. Gravity Gravity is a force of attraction only between bodies that have mass.

8. All objects fall to the surface of the earth because they are pulled towards the earth by a force. Gravitational force The force that pulls everything towards the center of the earth. Gravitational Force An apple fall from a tree, it’s accelerated, since its velocity changes from zero as it is hanging on the tree and moves toward the ground.

9. At this point, the gravitational field strength (g) is defined as the gravitational force acting on a 1 kg mass place at this point. Gravitational Field on the Earth Gravitational Field is The region around the earth which an object experiences a gravitational force. g = gravitational field strength (N kg -1 ) F = Gravitational force m = mass of body Gravitational field strength at the surface of the earth is 9.8 N kg -1. An object of mass 1 kg will experience gravitational force (f) of 9.8 N. It is also said gravitational acceleration (g).

10. Acceleration Due to Gravity The gravitational force cause object to fall with increasing velocity Acceleration F g (m s -2 )

11. Situation 1: Situation 2: 1.Acceleration 2.With air resistance 1.Acceleration 2.No air resistance FREE FALL Acceleration Due to Gravity Acceleration due to gravity Falling Objects g = 9.8 ms -2

12. FREE FALL 1.No air resistance 2.Acceleration due to gravity vacuum g = 9.8 ms -2 vacuum video

13. Weight The bigger W The smaller W The bigger g The smaller g Weight (W) depends on mass (m) and gravitational acceleration (g). W = m g W = weight (N) M = mass (kg) g = gravitational acceleration (ms -2 ) The Weight is the gravitational force exerted on the object by the earth or other Massive body

14. The Earth (g = 9.8 ms -2 ) m = 15 kg The Moon (g = 1.6 ms -2 ) The weight of the baby on the earth: 15 x 9.8 = 147 N The weight of the baby on the moon: 15 x 1.6 = 24 N

15. Differences between weight and mass 1.Measurement of the gravitational force on body. 2.The weight on an object varies with the gravitational field strength of the location. Ex: The weight of an object on earth is different from its weight when on the moon. 3.A vector quantity (as it involves forces). 4.A derived quantity : Weight, W=mg. 5.SI unit : Newton (N) 1.Measurement of the quantity of matter in a body. 2.The mass of an object is constant. (everywhere). 3.A scalar quantity. 4.A base quantity : Mass = m 5.SI unit : kilogram (kg). MassWeight

16. Problem Solving 1.A spacecraft of mass 800 kg is orbiting above the Earth’s surface at a height where its gravitational field strength is 2.4 N kg -1. Calculate the gravitational force experienced by the spacecraft. F = mg F = 800 x 2.4 F = 1920 N Solution:

17. 2. A rock has a mass of 20 kg and weight of 90 N on the surface of a planet. a.What is the gravitational field strength on the surface of the planet? b.What are the mass and the weight of the rock on the surface of the Earth where its gravitational field strength is 9.8 N kg -1 ?. Solution: a. g = 4.5 N kg -1 b. Mass remains unchanged, that is 20 kg Weight = mg Weight = 20 x 9.8 Weight = 196 N

18. Quiz

19. 1. A cat has a mass of 7 kg. what is its weight if a.It is on the surface of the earth where the gravitational field strength is 9.8 N kg -1 ?. b.It is on the surface of the moon where the gravitational field strength is 1/6 of that on the surface of the earth? Answer Problem

20. Summary 1.The gravitational field strength at a point in the gravitational field is the gravitational force acting on a mass of 1 kg placed at that point. 2.The gravitational field strength on the surface of the Earth is 9.8 N kg -1. 3.An object on the surface of the Earth experiences free fall with an acceleration of 9.8 m s -2.