1. SPH3U – Unit 2 Gravitational Force Near the Earth

2. Some Definitions Force of Gravity  The force of attraction between any two masses in the universe. Mass  the amount of material in an object. The more mass an object has, the harder it is to change its motion. The SI unit for mass is the kg. Mass does not change unless some matter is added to it.

3. Some Definitions Weight  The term used to describe the force of gravity that a celestial body like the earth exerts on a mass.

4. Acceleration due to gravity Without friction, all objects near the surface of our earth will fall with the same acceleration: 9.8 m/s 2 [down].

5. Acceleration due to gravity We say 9.8 m/s 2 [down] = the acceleration due to gravity = = gravitational field intensity of the earth

6. Acceleration due to gravity ** Note **  This is NOT the same as the force of gravity,.  The force of gravity, is a force. It causes an acceleration downward of 9.8m/s 2 =.

7. Force of Gravity The force of gravity is the Force of attraction between two objects. Note that because of Newton’s 2nd Law  = m  = m = m is an important equation!!!

8. Example 1 A falling object experiences a force of gravity downward of 49.05 N [down]. What is the object’s mass?

9. Law of Universal Gravitation Newton showed that the force of gravity between any two masses in the universe is directly proportional to the product of the masses and inversely proportional to the square of the distance between their centers. This statement is called Newton’s Law of Universal Gravitation.

10. Law of Universal Gravitation We can write it as an equation if we introduce a constant. In this case, we will introduce the constant G  the universal gravitation constant. F G = magnitude of the force of gravity (in Newtons) m 1 = mass of object 1 m 2 = mass of object 2 Δd = the distance between the centers of the two objects in meters G = universal gravitation constant = 6.67 x 10-11 N∙m2 / kg2

11. Example 2 Earth has a mass of 5.98 x 10 24 kg. The mass of the moon is 7.34 x 10 22 kg, and the mean radius of its orbit is 3.8 x 10 8 m. Calculate the force of gravity between Earth and the moon.

12. Air Resistance Air resistance is the force of the air against any object moving through the air. Air resistance increases with the cross-sectional area and the speed of the object. It always acts in the opposite direction to the velocity of the object.

13. Free Fall Since air resistance will increase the faster you are going, whenever a falling object is experiencing acceleration (velocity is increasing), the object must be experiencing greater and greater air resistance as well.

14. Free Fall As the air resistance increases, the net force downward decreases. Eventually, these two will balance and a terminal velocity for the falling object will be reached.

15. Free Fall Forces for a person in Free fall. (See pg. 163 for something similar)

16. Terminal Speed Terminal speed is the maximum constant speed of a falling object.

17. Homework Read pages 162-166, paying special attention to the sample problems. Complete Questions: Pg. 167 # 1,2,5,6,9