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Lecture 4

# AST101 Lecture 4 Notes

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University of Toronto St. George

Astronomy & Astrophysics

AST101H1

Dan Dolderman

Fall

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Week 2 – Lecture 4 Thurs. Sept. 20/12.
• the only force that matters in big terms is gravity because it will never cancel out (no charges)
ex. moon and earth – strong gravitational force
• newton's law of gravity
• if you have two objects the force of gravity between them is the mass of one objects times the
mass of the other object divided by the square of the distance between them
• earth and a person exerts the same amount of gravity towards each other
• misconception: confusing gravity with acceleration
• the formula for force of gravity applies everywhere (not just only for earth)
• everything in the universe feels the force of gravity anything or anyone produces
• gravity adds up as you add more material. There is no 'anti-gravity'
• Newton's law of gravity: the force of gravity between masses M and m is equal to the product
of their masses, times the gravitational constant G, divided by the square of the distance
between masses
• F = GMm / r^2
• orbiting
◦ force of gravity attracts objects
◦ if i drop a ball, it falls to earth. Both objects have mass, so they attract each other
gravitationally (ball falls straight down)
◦ moon has mass and is near the earth (feels its gravity) does not crash into earth because the
moon is moving parallel to the earth (perpendicular to gravity)
◦ the moon also has mass and is therefore attracted by earth's gravity. (goes around earth)
◦ if i throw the ball very slowly, it will fall along a curving trajectory (curved a bit in front of
me falling towards the ground)
◦ without air resistant an object will be in orbit if thrown fast enough ex.Aball
◦ minimum speed an object needs to orbit = V orbital
◦ if i throw the ball just fast enough, it will “miss” the earth on the far side. It will be in orbit.
(going all the way around to the beginning of where you threw the ball with the orbit shaped
like an oval [unequal distances around the earth] ; where you are the orbit is further away
from the earth, at the other end its very close to earth)
◦ but the ball is still “

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