1. Gravity and
Free Fall

2. Gravity Gravity - attraction between objects with mass Gives weight
Causes free fall

3. Weight Fw = mg Weight – force caused by gravity measured in N
depends on mass, gravity, and location
weight is less on the moon due to less gravity
Weight
Gravity
Mass

4. Mass Mass (amount of matter) measured in kg
does NOT depend on gravity or location
mass is the same on Earth and on the moon

5. Gravity on earth = 9.8 m/s2
Gravity on the moon = 1.6 m/s2
Fw = mg
The weight of 1 kg object ON EARTH
(1 kg)(9.8 m/s2) = 9.8 N
The weight of 1 kg object ON THE MOON
(1 kg)(1.6 m/s2) = 1.6 N

6. Free Fall
Free Fall – when gravity is the only force acting on the object
Mass of object does not affect free fall
Falling objects speed up 9.8 m/s2 for every second they fall
Time
Speed 1
9.8 2
19.6 3
29.4 4
39.2 5 49

7. Practice
Unless the problem says otherwise, assume you are on Earth, where g = 9.8 m/s2
What is the weight of a 100 kg person?
(100 kg) x (9.8 m/s2) = 980 N
What is the mass of a rock that weighs 196 N?
(196 N) ÷ (9.8 m/s2) = 20 kg

8. What’s your weight on Mars?
Calculate and compare the weight of a 100 kg person on Earth and on Mars.
(g = 3.7 m/s2 on Mars)
Earth = (100 kg) × (9.8 m/s2) = 980 N
Mars = (100 kg) × (3.7 m/s2) = 370 N
The MASS did NOT change!!!

9. Free Fall
Free Fall – when gravity is the only force acting on the object
Mass of object does not affect free fall
Falling objects speed up 9.8 m/s2 for every second they fall
Time
Speed 1
9.8 2
19.6 3
29.4 4
39.2 5 49

10. Practice
The force of gravity on the surface of Mercury is much less than on Earth. A rock has a weight of 100 N on Earth. What do you know about the weight of the rock on Mercury?
The rock will weigh exactly 100 N
The rock will weigh more than 100 N
The rock will weigh less than 100 N
There is not enough information

11. Practice
The force of gravity on the surface of Mercury is much less than on Earth. A rock has a weight of 100 N on Earth. What do you know about the weight of the rock on Mercury?
The rock will weigh exactly 100 N
The rock will weigh more than 100 N
The rock will weigh less than 100 N
There is not enough information

12. Practice
A chair has a mass of 10 kilograms on Earth. What is the mass of the chair on Mercury?
Exactly 10 kg
More than 10 kg
Less than 10 kg
Not enough information

13. Practice
A chair has a mass of 10 kilograms on Earth. What is the mass of the chair on Mercury?
Exactly 10 kg
More than 10 kg
Less than 10 kg
Not enough information

14. Practice
If an astronaut, with a mass of 90 kg, has a weight of N on the Moon, what is the value of the Moon’s gravitational acceleration? (145.8 N) ÷ (90 kg) = 1.62 m/s2