1. Centripetal force Acceleration Mass Contact force momentum
**Match Word to Definition** 1. The rate at which velocity changes with time 2. A measure of mass in motion 3. the force that keeps an object moving in a circle.
Centripetal force
Acceleration
Mass
Contact force
momentum

2. Key Terms
Gravity
Weight
Orbit

3. Gravity is a Force Exerted by Masses
Before you Learned:
Every action force has an equal and opposite reaction force
Newton’s laws are used to describe the motions of objects
Mass is the amount of matter an object contains
Now, you will Learn:
How mass and distance affect gravity
What keeps objects in orbit

4. Masses Attract Each Other
The downward pull that occurs when you drop something is due to gravity.
Gravity – the force that objects exert on each other because of their MASSES
Gravity is present everywhere in the universe between large objects such as the sun and moon and small objects like dust and gas in the air.
Gravity is considered a UNIVERSAL FORCE because it is present everywhere in the universe. It actually helped form our solar system.

5. The Force of Gravity The mass of the objects
Greater mass results in greater force
The distance between the objects
Greater distance results in smaller force

6. Gravity on Earth
The acceleration due to Earth’s gravity is called g and is equal to 9.8 m/s² (squared)
You can use Newton’s Second Law (F = ma) to calculate the force of gravity on an object. However, instead of a (acceleration) you use g (gravitational acceleration = 9/8 m/s²). Therefore, the formula for gravitational force is F = mg.

7. Earth’s Gravitational Acceleration
We know that Newton’s Second Law of motion is
Force = mass x Acceleration
If we wanted to calculate the acceleration of an object due to gravity we would apply the same law with a set acceleration.
Force (N) = mass (kg) x acceleration of gravity (m/s2)
F = mg
F=mass x 9.8m/s2

8. Anticipation Guide – True or False
When two spheres that are the same size and have similar surfaces but unequal masses are dropped from the same height above the ground, the more massive sphere will hit the floor first.
When two spheres that are the same size with similar surfaces and other characteristics, but unequal masses are dropped from the same height, the more massive sphere will hit with greater force.

9. Weight and Mass Weight and mass are related, but NOT THE SAME.
Weight – the force of gravity on an object
I cannot have weight without gravity, but I can have mass without gravity

10. Gravity Keeps Objects in Orbit
Sir Isaac Newton hypothesized that the force that pulls objects to the ground – gravity – also pulls the Moon in its orbit around Earth.
Orbit – the elliptical path one body, such as the Moon, follows aro8und another body, such as Earth, due to the influence of gravity.
The centripetal force keeping one object in orbit around another object is due to the gravitational pull between the two objects.

13. Review Questions
If two objects have the same mass and fall from the same height, which one will hit the ground first?
If two objects have no friction affecting them, and they have different masses and you drop the two objects from the same height. Which object will hit the ground first?
What is the centripetal force of the Moon’s orbit?

14. Review Questions
4. What effect would increasing the mass of two objects have on the gravitational attraction between them? 5. What effect would decreasing the distance between objects have on their gravitational attraction to each other? 6. How does gravity keep the Moon in orbit around Earth?

15. Fill in the Blank
1. The sun has a great gravitational force, because of its ______.
2. If I increase the mass of 2 objects the gravitational force between them would _______.
3. Gravity acts as a __________ force which keeps the moon in orbit around Earth.