1. 1. T/F: Gravity is a downward force. Explain your answer!
March 17, 2009
1. T/F: Gravity is a downward force. Explain your answer!

2. Goals To explain the Universal Law of Gravity
To interpret situations using the Universal Law

3. Q: Who discovered gravity?
Issac Newton played a big part (remember his apple falling from a tree?)
He probably thought: the apple is speeding up as it moves toward the ground, so there must be a force acting on it
(Remember his 2nd law? F = ma)
Let’s call the force gravity, and the associated acceleration “acceleration due to gravity”

4. Q: Who discovered gravity?
This force reaches all the way to the moon, keeping the moon in orbit!

5. Q: Who discovered gravity?
Universal Law of Gravitation
All objects in universe experience gravitational attraction (hence UNIVERSAL!)
Newton is famous for observing the universality of gravity

6. Q: So what does the Universal Law of Gravitation tell us?
Force of gravity is DIRECTLY proportional to the product of the masses of the 2 objects.
If we increase both masses, the force will increase.

7. Q: So what does the Universal Law of Gravitation tell us?
If we double mass of object 1 and double mass of object 2, the force of gravity between them will……
QUADRUPLE! (2 X 2 = 4)
If we triple the mass of object 1 and double the mass of object 2, the force of gravity between them will…….
increase by a factor of 6 (2 x 3 = 6)

8. Q: So what does the Universal Law of Gravitation tell us?
If we double the mass of one object and cut the mass of the other in half, the force of gravity between them will ………
remain the same (2 x 1/2 = 1)

9. Q: So what does the Universal Law of Gravitation tell us?
At the same time, the force of gravity between 2 objects is INVERSELY proportional to the distance squared
As we increase the distance, the force will decrease.

10. Q: So what does the Universal Law of Gravitation tell us?
If we double the distance between two objects (increase by a factor of 2), then the force of gravity between them is……
decreased by a factor of 4 (22 = 4).
-(The force is ¼ of the original!)

11. Q: So what does the Universal Law of Gravitation tell us?
If we triple the distance between two objects (increase by a factor of 3), then the force of gravity between them is……
decreased by a factor of 9 (32 = 9).
-(The force is 1/9 of the original!)

12. Q: Why does this happen?
The relationship between gravity and distance is that of the inverse square law.
Force is inversely related to the square of the distance

14. Q: Why does this happen?
So the further something gets from the center of the earth, the less gravitational force.
When you go all the way to the moon, you don’t have a lot of gravitational force acting on you, so your weight decreases (because weight is a direct measure of the gravitational force!)

16. Q: So how can you find out how much gravitational force there is between two objects?
We must use the universal gravitational constant – G.
It is termed a "universal constant" because it is thought to be the same at all places and all times.

17. What would the units be? Lets look…
Q: So how can you find out how much gravitational force there is between two objects?
G = 6.67 x N-m2/kg2
What would the units be? Lets look… x =

18. Homework Complete WS (Front and Back) Inverse Square Law
Force and Weight