1. Lecture 4 Force and Motion Gravitation Chapter 2.7  2.15 Outline Laws of Motion Mass and Weight Gravitation The law of gravity – 1666, 3 laws of motion – 1686

2. First Law of Motion If no net force acts on it, then an object in motion remains in motion A force is needed to start motion or change its velocity A force is any influence that can change the speed or direction of motion of an object an object remains at rest An object continues to be accelerated as long as a net force acts upon it

3. Mass The reluctance of an object to change its state of rest or of uniform motion in a straight line is inertia Mass is a property of matter that shows itself as inertia The SI unit of mass is kilogram (kg) 1 liter (0.26 gallon) has a mass of 1 kg 1 liter = 10cm x 10cm x 10cm More about mass and inertia

4. Second Law of Motion The more mass an object has, the less its acceleration for a given force. F = net forcem = massa = acceleration F a = --- m F=ma The net force on an object equals to the product of the mass and the acceleration of the object. The direction of the force is the same as that of the acceleration.

5. The Newton The second law of motion defines the unit of force. F=ma, [m]=kg, [a]=m/s 2  [F]=kg m/s 2 = newton (N) 1 lb = 0.453 kg x 9.8 m/s 2 = 4.45 N  1 N = 0.225 lb

6. Mass and Weight The weight of an object is the force it is attracted by the earth’s gravitational pull. Mass is a measure of the amount of matter included in an object. F=maw=mgw=mg

7. Third Law of Motion When an object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first object. Action force is equal to reaction force

8. Gravitation Centripetal force is an inward force on an object moving in a curved path mv 2 F c = ---- r is the radius of a circle r The centripetal force always points toward the center of curvature of the object’s path.

9. Centripetal and Centrifugal Force Centrifugal force

10. Mass of the Earth The Earth’s mass can be found from the law of gravity. Weight of an object F = mg Gravitational force on an object G m M F = ---------- R 2 G = 6.67 10 -11 N m 2 /kg 2 R = 6378 km = 6.38 10 6 m g = 9.8 m/s 2 gR 2 M = ------ = 6 10 24 kg G

11. Orbiting the Earth In order to launch a satellite, the gravitational force has to be overcome. Centripetal force = gravitational force mv 2 ------ = mg  v 2 = rg  v =  rg r Escape speed (v e ): the speed required to leave the gravitational influence of a body. Minimal orbital speed For the Earth: v = 28,400 km/h, v e =40,000 km/h

12. Summary The three Newton’s laws of motion and the law of gravity represent the foundation of the classic or Newtonian mechanics.three Newton’s laws Mass is a measure of an object’s inertia, while weight is the gravitational force on an object. They hold everywhere in the Universe, when an object’s speed is much less than the speed of light.