1. Force

2. Four fundamental forces

3. Gravity The gravitational force keeps you, the atmosphere, and the seas fixed to the surface of the planet. Though gravity is the weakest of all interactions, it is also the least selective – it acts between all particles. Because its range is unlimited and because it is only attractive, gravity rules the Cosmos on a grand scale. It holds the Earth in orbit around the Sun, keeps the Sun locked within our galaxy of a hundred thousand million stars, and reaches all across the thousands of millions of galaxies that constitute the known Universe. - E. Hecht, ‘Physics: Calculus’

4. Newton’s Universal Gravity The gravitational force on object 1 by object 2 is given by Object 1Object 2 G = 6.67 x 10 -11 m 3 /kg s 2 万有引力

5. Gravitational force due to the Earth, near the surface (weight) The acceleration g is approximately constant (9.8 m/s), close to the Earth’s surface. 重力

6. Remember: constant acceleration

7. What about on the surface of the moon?

8. Acceleration due to gravity g does not depend on mass Without air With air

9. R = 3.84 x 10 8 m Gravitation force between the Earth and the Moon We already calculated the centripetal acceleration of the Moon as it orbits the Earth. Does this agree with the gravitational force between them, predicted by Newton?

10. What if we swap the two objects? The gravitational force on object 2 by object 1 is given by Object 1Object 2

11. Reciprocity: Newton’s 3 rd Law When two bodies interact, the forces on the bodies from each other are: (1) equal in magnitude and (2) opposite in direction.