1. Gravity, Mass and Weight
Newton Does It Again!!!

2. Gravity Check Is there such thing as zero gravity?
No, there is always a force of gravity between any two objects with mass, no matter the distances apart.
What determines the amount of gravitational force acting between two objects?
The mass of the two objects and the distance between them.

3. All things with mass are affected by gravity.
Two objects, whether large or small, always have a gravitational force between them.
Gravitational force increases as mass increases.
Gravitational force decreases as distance increases.

4. Freefall acceleration near Earth’s surface is constant.
FREE FALL: the motion of a body when only the force of gravity is acting on the body.
Freefall acceleration near Earth’s surface is constant.
If we disregard air resistance, all objects in Earth’s gravitational field accelerate down (fall) at 9.81 m/s2, abbreviated g.
Because acceleration is proportional to Force/mass, sometimes we say,
g= 9.81 N/kg
When there is no air resistance (only gravity), a mouse and an elephant will fall at the same rate, 9.81m/s2

5. Free Fall Check Is a skydiver in free fall? Why?
No, free fall is when only gravity is acting upon the object. Skydivers have air resistance acting upon them.
If a feather and a billy goat are dropped from the same height near Earth’s surface, which will reach the ground first if we disregard air resistance? Why?
When there is no air resistance, they will reach the ground at the same time because the acceleration due to gravity remains constant, 9.81 m/s2.

6. Again, weight is different from mass.
MASS: a measure of the amount of matter in an object.
Measured in kilograms (kg)
WEIGHT: the gravitational force an object experiences because of its mass.
Measured in newtons (N) or pounds (lb)
Equal to mass times free-fall acceleration ( or gravitational field strength).
weight = mass x free-fall acceleration
(Fg = mg)
force = mass x acceleration
(F=ma)
Mass = .25 kg
g = 9.81m/s2
weight = mg
= (.25 kg) x (9.81 m/s2)
= 2.45 N

7. What happens when things fall?
An object’s falling velocity is constant when air resistance balances weight.
Balanced forces means constant velocity (no acceleration)…
TERMINAL VELOCITY: The constant velocity of a falling object when the force of air resistance is equal in magnitude and opposite in direction to the force of gravity.
This is the point of parachutes!
Increase air resistance

8. Weight Check Do you have the same mass on the moon? Why?
Yes, the amount of matter you have does not change no matter where you are.
Do you have the same weight on the moon? Why?
No, the moon has 1/6 the mass of the Earth so the force of gravity between you and the moon is less. (Everything seems “lighter” on the moon)

9. Orbiting objects are in free fall.
The moon stays in orbit around Earth because Earth’s gravitational force provides a pull on the moon.
Two motions combine to cause orbiting.
Forward motion and downward motion to Earth
CENTRIPETAL FORCE: Force that causes objects to travel in a curved or circular path

12. What’s your weight/mass/Force
1 pound = kilograms
1 pound = Newtons
Take your weight in pounds = (I’ll use 100…. You can pick any number to try this with)
First convert your weight to kilograms (mass)
…….. x = 100 lbs x = kg
Then take your mass in kg x g (9.81 m/s2) to get Fg (N)
……… x 9.81 m/s2 = x 9.81 = N
Last, take your force and convert back to lbs to check…
/ = / = 100 pounds!

13. Exit Ticket: 41-44

14. 41. What is another name for weight?
mass
matter
Newtons
D. Gravitational force

15. 42. Free-fall acceleration near Earth’s surface:
Is the same for all objects (9.81 m/s2)
B. Depends on an object’s weight
Depends on an object’s mass
D. None of the above

16. 43. Which statement about weight is NOT correct?
An object weighs more on the moon than it weighs on earth.
A change in an object’s location can change the object’s weight
The weight of an object depends on gravity.
An object weighs more on earth than it would on the moon.

17. 44. When an object is in free-fall, the only force acting on it is
Friction
Gravity
Inertia
D. Terminal velocity