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 F g =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 9.8m/s/s  Feather, coin, paper  Heavier object experiences greater gravity, however it is also harder for a heavier object to accerate. 

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?