1. Gravity & Acceleration

2. Learning Objectives Describe Newton’s Universal Law of Gravity
Explain Einstein’s description of Gravity
Relate gravity to acceleration

3. What is a force?
A force is the amount of push or pull on matter.
Forces can cause a change in motion.

4. What is acceleration?
Acceleration is a change in speed and/or direction in some amount of time.

6. Mass is the amount of material contained in an object.
What is mass?
Mass is the amount of material contained in an object.
Empty 747 Jet
160,000 kg
Average Man
73 kg
5¢ coin kg

7. Practice
Mass is sometimes defined as “the amount of space an object takes up”. Why is this not correct?

8. 1 kg of mass weighs 2.2 pounds
Weight
Weight is a force.
Weight is the gravitational force acting on a mass.
On Earth
1 kg of mass weighs 2.2 pounds

9. Practice 1) Does weight depend on volume?   
2) If you were instantly transported to our Moon, which would change?
Mass
Weight
Both

10. Mass = 120 kg
Weight = 270 lbs
Mass = 120 kg
Weight = 45 lbs

11. What is Newton’s description of gravity?
Newton’s Universal Law of Gravitation
Every object with mass attracts every other object with mass, with a force that depends directly on the masses of the two objects and decreases with the distance squared.
F = GMm d2

12. Gravity Mutual force of attraction
All masses pull the same on each other!
Causes acceleration due to gravity
9.81 m/s2

13. Mutual Force = Same Forced

14. Practice
1) Would the acceleration due to gravity (9.81 m/s2) be different for an object dropped from a high mountain top than it is at sea level? 2) If Earth had twice as much mass, would this change your weight? Would it change your mass? 3) Is the gravitational force zero in space?

16. What is Einstein’s description of Gravity?
Every object with mass creates a curvature of space-time.

17. Space-time tells matter how to move, and matter tells space-time how to curve.

18. More Mass = More Curvature
According to Einstein, mass does not create a force, but rather a warping of space which other objects follow.
More Mass = More Curvature

19. Objects fall independent of their mass
Objects fall independent of their mass. Heavy and light objects can follow the same curve.
Heavy
Light

20. Practice Do you create a curvature of space-time?
Consider the curvature of space-time caused by Earth. Is the Moon being sucked into the Earth? Explain your answer.

21. Force & Acceleration
Objects are accelerated downward because of the force of gravity.
Free Falling (No Air) m/s of speed every second mph of speed

22. Practice
1) What do you notice about the changes in distances as the ball is falling? 2) What does this show?

23. Light versus Heavy
The gravitational pull from the Earth is the same on all objects, so the downward acceleration is the same on all objects.
Any difference in the net acceleration is due to air resistance.

25. Practice
Assume the ball is in free-fall. Discuss the rate of acceleration in each situation.

26. More Practice
Which will land on the ground first if they are in free fall?
Which will land on the ground first if there is air?