2. Describe the gravitational force Define weight and relate to the gravitational force Distinguish between weight and mass Determine the weight of objects.

3. Attractive force between two objects. 1. If the masses of the objects increase, the magnitude of the force increases. 2. If the distance between the objects increase, the magnitude of the force decreases.

4. Thus you cannot feel the gravitational force between you and the book you are reading because your mass and the mass of the book are not significantly big. You cannot feel the gravitational force between you and the sun because the sun is too far ( distance is too big) But You can feel the gravitational force between you and earth ( mass of earth is big and distance is relatively not big)

5. This force is the reason you feel yourself pulled towards the Earth. The gravitational force that you experience depends on the mass of the Earth, and on the separation between you and the mass of the Earth.

6. Do all objects experience the same gravitational force? IN FACT, No, the gravitational force depend on the mass of the object of the earth’s surface

7. - Gravitation is responsible for keeping the Earth and the other planets in their orbits around the Sun. - Earth's gravity pulls other objects twords it's center without touching them. An objects weight is the amount of gravitatinal force.

8. According to the second law of motion, whenever a force is acting on an object, it produces acceleration in it. So: If we consider the example of the stone (Rf: the figure at the right), the gravitational force produces an acceleration on both the earth and the stone.

9. This acceleration usually called g is equal to 9.8m/s² and is equal on all objects on the earth’s surface. g= 9.8m/s² This acceleration changes from one planet to another,. On the moon, the gravitational acceleration is much less.

10. Weight and gravitational force Even if you are not moving, the earth is pulling you downward. Whether you are moving, falling or standing, Earth exerts a gravitational force on you. This force is called: weight.

11. According to Newton’s second Law, Force (newtons)= mass (kg) x acceleration (m/s²) Where: Force is the weight (gravitational force) Acceleration = gravitational acceleration = 9.8m/s²

12. Thus, we can write:

13. 1. Calculate the weight of a 5 kg object (g= 9.8m/s²) 2. Find the mass of an object if its weight is 4N. 3. Calculate the weight of a 400 g object on the moon if the gravitational acceleration on the moon is : 1.67 m/s². + Solve questions: p: 82 no.6, 7 & 8

14. In everyday life people often confuse mass with weight. In science we should distinguish between them. The weight of an object is the gravitational force acting on it by the Earth. Like other force, weight is measured in newtons. If the object falls freely, its weight gives it an acceleration of g

15. Mass is the measure of amount of matter in a certain substance. Mass is a physical quantity with no direction. Mass doesn't from a place to another Mass is measured using a balance (scale): its basic unit is: Kg Weight is the gravitational force between the earth and the object. Weight is a force which is always downward. Weight changes from a place to another. Weight is measured using a spring balance: its basic unit is Newton.

16. The picture at the right shows a spring scale used to measure the weight of a substance and a balance beam used to measure the mass of an object

17. Look at the following diagram and conclude:

18. Gravitational force is the force of attraction between two masses. Weight is the gravitational force that pulls objects downward towards the center of the earth. Mass is different than weight: it is a constant quantity which only depends on the amount of the substance

19. Solve section review p: 82 no. 1-5 Download PowerPoint presentation Revise section : 3.1 & 3.2 System wide quiz 2: - Chapter 1: section 1.3 - Chapter 2: sections 2.1, 2.2 & 2.3 Good Luck!!!