11.2 Gravity
I can:
-calculate gravitational force (weight) from mass.
-recognize that the stronger the field the greater the force exerted by that
field.
-recognize that the field of an object is always present even if the object is
not interacting with anything else.
-determine that the weight of an object is proportional to its mass.
-calculate weight if given the formula Fg=mg. (with g being acceleration due to gravity)
Law of Universal Gravitation

Why do objects fall toward Earth?

What keeps the planets in motion in the sky?

All objects are attracted to each other through gravitational force.

Gravitational force increases as mass increases (elephant v. cat)

Gravitational force decreases as distance increases.

It never completely disappears! (gravity even pulls blood)
Gravitational Force decreases as
distance increases

Doubled= ¼

Tripled= 1/9

Ten x’s further= 1/100

It holds planets, stars, and galaxies together! Think about what would
happen if any one object started to slip away.
Free Fall and Weight

When gravity is the only force acting on an object, the object is said to be in
FREE FALL.

Are skydivers in freefall?

Why do astronauts in a spaceship float?

Free fall is abbreviated “g”.

All objects fall to Earth @ 9.8m/s/s

Feather, coin, paper


Heavier object experiences greater gravity, however it is also harder for a heavier
object to accerate.
https://www.youtube.com/watch?v=cgIOknFz9L4
Weight is equal to mass times free-fall
acceleration

Force on an object due to gravity is called WEIGHT.

Weight and mass are not the same!

Weight= mass x free-fall acceleration

Weight is a force, so its measured in N.

Example: A book has a mass of 1kg, so 1kg x 9.8m/s/s= 9.8N
Weight is different from mass

Mass= measure of the amount of matter in an object.

Weight= the gravitational force an object experiences because of its
mass.

It is impossible for an object to be weightless… even in space.

On Earth, a 66 kg astronaut weights 66kg X 9.8m/s/s= 650 N (150lb)

On the moon, a 66 kg astronaut weights 66kg X 1.6m/s/s= 110N (24lb)

How do we measure mass of our body then?
FYI

Planets in our solar system have different masses and different diameters.
Therefore, each has its own unique value for g. Find the weight of a 58 kg
person on the following planets:

Earth where g=9.8m/s/s:

Venus where g=8.9m/s/s:

Neptune where g=11.0m/s/s:
Something to think about

What type of skeleton to land
organism have?

What about Jellyfish?

What happens to a Jellyfish when it
washes up on the beach?
Velocity is constant when air
resistance balances weight

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.

Mass does effect terminal velocity.
Projectile Motion and Gravity

Projectile Motion: the curved path that an object follows when thrown,
launched, or otherwise projected near the surface of Earth; the motion of
objects that are moving in two dimensions under the influence of gravity.

Forward motion v. downward force of gravity.


No horizontal force=straight down motion.
Why is the path of a planet curved?