1. Unit 7 Lesson 2. Gravity: A Force of Attraction A
Unit 7 Lesson 2. Gravity: A Force of Attraction A. The effects of gravity on matter
All matter has mass & gravity is a result of mass therefore all matter is affected by gravity
All matter is pulled toward (has an attraction to) all other objects around them
Earth’s gravitational force is very large
Force must be applied to overcome
Reason why it takes energy to move objects

2. II. Newton and the Study of Gravity A. The Idea
Legend: Newton saw an apple fall from a tree & concluded that unbalanced force caused it to move
Concluded that unbalanced force on the moon kept it moving circularly around the Earth

3. B. The Law https://youtu.be/Jk5E-CrE1zg
Law of universal gravitation: law describes the relationships between gravitational forces, mass & distance
Universal= applies to all objects in the universe

4. III. The Law of Universal Gravitation https://youtu. be/W4Oq3SiVSR4
III. The Law of Universal Gravitation A. Gravitational Force Increases as Mass Increases
The more mass something has the more the Earth’s gravity pulls on it- the harder it is to move it
Example elephant vs cat to pick up
Astronaut “bounces” when walking on the moon b/c the moon is smaller than the Earth so it has less pull on the man

5. B. Gravitational Force Decreases as Distance Increases
Gravitational force between us and Earth is large
Sun’s gravitational force on us is very small because of the distance we are away from the sun
Sun’s gravitational force is enough to keep the planets in orbit

6. IV. Weight as a Measure of Gravitational Force A
IV. Weight as a Measure of Gravitational Force A. Difference between weight and mass
Weight changes with gravitation force
Mass: amount of matter in an object-does not change
Mass is measured by a balance
Weight measure by spring scale

7. B. Units for weight & mass
Gravity is a force so the SI unit would be Newtons (N)
Mass measured in kg or g

8. Objectives Section 2
Explain the effect of gravity and air resistance on falling objects
Explain why objects in orbit are in free fall and appear to be weightless
Describe how projectile motion is affected by gravity

9. I. Gravity and Motion A. Gravity and falling objects
Aristotle thought that the rate at which an object falls is dependent on its mass
Galileo proved this wrong by dropping 2 cannon balls of different masses from the Leaning Tower of Pisa & they landed at the same time

10. Gravity and Acceleration
Objects fall to the ground at the same rate because the acceleration due to gravity is the same for all objects
Acceleration depends on mass and force
Heavier object has more gravitational force (greater mass)
Heavier object is also harder to accelerate because it has more mass (inertia)

11. Acceleration due to gravity
Acceleration: that rate at which velocity changes over time
All objects accelerate towards Earth at 9.8m/s2
Every second the object fall the downward velocity increases by 9.8m/s2

12. Velocity of Falling Objects
To calculate change in velocity (Δ v= g x t)
g= gravity to Earth ( 9.8m/s2)
t= time the object takes to fall to Earth
Change in velocity is the difference between the final velocity and the starting velocity
PROBLEM: a stone at rest is dropped from a cliff and the stone hits the ground after a time of 3 s. What is the stone’s velocity when it hits the ground?
Δv= g x t g= 9.8m/s2 t= 3 s
Δv= 9.8 m/s x 3s = 29.4 m/s s
To find time rearrange the equation: t= Δv g

14. B. Air Resistance and Falling Objects
The force of gravity is pulling down on the apple
The force of air resistance is pushing up on the apple
Net force on the apple is equal to the force of the air resistance subtracted from the force of gravity
Apple will not accelerate as fast as it would without air resistance
More surface area = more air resistance

15. Air resistance increases as the speed of the falling objects increases until it is equal to the downward force of gravity
Terminal velocity= the constant velocity of a falling object when the force of air resistance is equal in magnitude & opposite in direction to the force of gravity

16. Free falling occurs when there is no air resistance
Object is in free fall ONLY if gravity is pulling it down and no other forces are acting on it
Air resistance is a force so free fall can only happen where there is no air
Space
Vacuum

17. C. Orbiting Objects are in Free Fall
Free fall= motion of a body when only the force of gravity is acting on it
Astronauts are not “weightless” they just have less gravitational pull from Earth because of increased distance
Two motions combine to cause orbiting: movement forward & free fall to Earth
Space shuttle would continue to move forward if there were no gravitational pull form Earth
Space shuttle orbits b/c the gravity from Earth is still pulls it downward
Path of the shuttle around Earth is called orbiting

18. Centripetal force “towards the center”: unbalanced force that causes objects to move in a circular path
Velocity is constantly changing because of the changing direction
Force is needed to cause the change in direction= gravity

19. D. Projectile Motion
Projectile motion= the curved path that an object follows when thrown, launched, or otherwise projected near the surface of the Earth
Two components to projectile motion that are INDEPENDENT of each other (no effect on each other) but when they are combined they form a curved path
Horizontal motion
Vertical motion

20. Horizontal motion: motion parallel to the ground
Ball thrown from hand
Vertical motion: motion perpendicular to the ground
gravity pulling downward on objects
Ignoring air resistance, gravity pulls objects in projectile motion to the ground at a rate of 9.8m/s2
Objects in projectile motion accelerate downward
To hit a “bulls eye” you have to aim above it