By Moira Whitehouse PhD

GRAVITY AND

WEIGHT

•BEFORE UNDERSTANDING WEIGHT, WE MUST FIRST UNDERSTAND

GRAVITY.

•WHAT IS THIS THING CALLED GRAVITY?

THE AMOUNT OF MATTER OR “STUFF” IN AN OBJECT.

•SINCE GRAVITY IS A PRODUCT OF MASS, WE NEED TO UNDERSTAND THAT MASS IS:

•THE MORE MASS AN OBJECT HAS, THE MORE GRAVITY IT HAS AND THE MORE IT PULLS.

•FOR EXAMPLE, BOTH BLUE AND GREEN PLANETS, MADE OF THE SAME STUFF, ARE PULLING EACH OTHER.

•SO NOW WHICH IS PULLING HARDER, GREEN OR BLUE?

•WHICH PLANET NOW IS PULLING HARDER, GREEN OR BLUE?

•THIS PAST DEMONSTRATION HELPS US SEE THAT THE MORE MASS AN OBJECT HAS,

THE STRONGER ITS GRAVITY.

•CONSIDERING ONLY MASS AND GRAVITY, IF YOU DUG A HOLE RIGHT THROUGH THE EARTH AND FELL IN, HOW FAR WOULD YOU FALL?

YOU’D FALL INTO THE HOLE AND SHOOT RIGHT PAST THE CENTER BECAUSE YOU WOULD BE GOING SO FAST.

AS SOON AS YOU PAST IT YOU WOULD START TO BE PULLED BACK TOWARD THE CENTER, BOUNCING BACK AND FORTH LIKE A BUNGIE JUMPER UNTILL YOU FINALLY STOPPED AT THE CENTER.

STUCK IN THE MIDDLE WITH THE SAME AMOUNT OF MASS PULLING ON ALL SIDES.

•SO WHAT DOES GRAVITY HAVE TO DO WITH WEIGHT?

•THE MORE MASS AN OBJECT HAS THE MORE GRAVITY IT HAS AND THE STRONGER IT PULLS ON OTHER OBJECTS.

•THE WEIGHT OF AN OBJECT IS A MEASURE OF HOW HARD GRAVITY PULLS ON IT.

•THE HARDER GRAVITY PULLS ON AN OBJECT, THE MORE THE OBJECT WEIGHS.

HOWEVER, THE AMOUNT OF GRAVITY ON EACH PLANET DIFFERS. THE MOON WHICH HAS ONLY ONE SIXTH THE MASS OF EARTH ALSO HAS ONLY ONE SIXTH OF THE GRAVITY. IF YOU WERE THERE YOU WOULD WEIGH ONLY ONE SIXTH AS MUCH AS YOU DO NOW.

•SO THE SAME APPLE WOULD WEIGH JUST A TINY BIT MORE IN SOME LOW PLACE ON EARTH, LIKE DEATH VALLEY, THAN IT WOULD ON THE TOP OF A VERY HIGH MOUNTAIN.

EARTH RADIUS EARTH RADIUS EARTH RADIUS EARTH RADIUS EARTH RADIUS

•IN A NEAR EARTH VEHICLE LIKE THE SHUTTLE, YOU WOULD BE WEIGHTLESS. HOWEVER, THIS IS NOT BECAUSE YOU ARE SO FAR FROM EARTH THAT THERE IS NO GRAVITY.

INSTEAD, IT IS BECAUSE THE SHUTTLE, STILL BEING PULLED BY EARTH’S GRAVITY, CONTINUALLY FALLS BENEATH YOU.

•BECAUSE AN OBJECT’S WEIGHT CHANGES WHEN GRAVITY CHANGES, IT IS NOT USEFUL FOR SCIENTISTS TO DESCRIBE PROPERTIES OF OBJECTS IN SPACE.•SCIENTISTS NEED A MEASURE THAT DOESN’T CHANGE BECAUSE OF A CHANGE IN THE FORCE OF GRAVITY.

•THEY DISCOVERED JUST SUCH A PROPERTY OF MATTER AND THEY CALL IT MASS.