Dynamics

Weight

Gravity

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Do Now warm-up
Do Now
Force diagram

Which force represents the weight force?

Weight – net gravitational force on an object

Doctor weighing a girl on a scale

w  =  Fg  =  mg

(N)  =  (kg)(m/s2)

g = acceleration due to gravity

Weight on Earth and the Moon

Earth Earth

g = 9.8 m/s2

Moon Moon

g = 1.6 m/s2

Apollo 11 astronaut on the Moon
Apollo 11 astronaut Edwin Aldrin stands near the leg of the lunar module on the Moon. Despite the fact that the backpacks and astronauts only weighed 1/6 on their 350 pound Earth weight, their center of gravity was shifted so they had to lean slightly forward to balance. Footprints are clearly visible in the foreground. (Apollo 11, AS1140-5902)

Graphing Weight vs. Mass

Weight vs. mass graph

slope = g

g  =  Fg ÷ m

(m/s2)  =  (N) ÷ (kg)

Mass

Mass – amount of matter an object possesses

Astronaut waving arms

Weightless
Not Massless!

Question

Did you know that an astronaut can be up to 2 inches taller after returning from space? Why?

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The cartilage disks in their spine expand in the absence of gravity

Sciensational

Mass vs. distance graph

What is the relationship between mass and distance?

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Choice 1

Mass is not affected by distance
(only weight)

Mass vs. distance graph

Converting Units

1 kg ≈ 2.2 lbs

Pound and kilogram comparison
University of Michigan

1 lbs ≈ 5 N

Average Senior Weighs

120 lbs ≈ 600 N

Finding Acceleration Due to Gravity Using Weight

Satellite

Ex) A satellite having a mass of 25. kg experiences a gravitational force of 200 N. Find the acceleration due to gravity at this location.

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g  =  Fg ÷ m

(m/s2)  =  (N) ÷ (kg)

g = 200 N / 25 kg

a  = 8 m/s2

or 8 N/kg

Weight and Distance

Polar bear on the earth
When an object is on the earth, it is one earth radius away.

Ex) A rocket weighs 18,000 N at the earth's surface. If the rocket rises to a height where its distance from the earth is 3 earth radii, its weight will be ...

Newton's Law of Universal Gravitation

F  =  Gm1m2 ÷ r2

Distance from earth to rocket triples so, ....

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....weight is 1/9 as great

w = 2000 N

(original weight, 18,000 N)

Explore: Move Away From the Planet

Drag the test mass away from the planet and watch the force drop.

Drag the yellow test mass. Watch g and the force on the mass change.

Try This

Drag the test mass from 1R to 3R. What fraction of its surface weight does it have? Does that match the rocket example?

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At 3R, g is about 1.09 N/kg — 1/9 of the surface value, just like the rocket’s 18,000 N → 2,000 N.

More: Gravitational Fields lessons

Wringing out Water on the ISS – for Science!

Canadian Space Agency

Bad Science Humor

Omg

Ranking Task

Try it

Rank the gravitational forces from greatest to smallest.

Ranking task diagram

Enrichment

Cartoon owl sitting on a book
www.wpclipart.com

Einstein's Conception of Gravity
The Elegant Universe - PBS

Mythbusters Moon Landing Hoax 4 HQ

Guest Lecturer

Floating Cannon Ball

School blocks YouTube?
Click on the link below.

| Onsite |

Question

"Why can mercury make a cannon ball float?"

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Answer

"Mercury must be more dense than the steel cannonball"

Check Your Understanding

5 quick questions — pick an answer to see if you’re right.

1. What is weight?

2. An astronaut's gear has a mass of 10 kg. What does it weigh on the Moon, where g = 1.6 m/s²?

3. An astronaut floats "weightless" far out in space. Compared with on Earth, the astronaut's mass is…

4. A 25 kg satellite experiences a gravitational force of 200 N. What is the acceleration due to gravity at its location?

5. On a graph of weight (y-axis) versus mass (x-axis), the slope equals…

Friction →

Also see: Gravitational Fields — field strength, energy in fields, and gravity vs. electric fields