Newton
got the idea of gravity by watching an apple drop to the ground (so the
story goes) and then, looking at the moon, wondered if it was the same gravity
making it go in circles around the earth. Kepler had already found the equal
area in equal time and period cubed per radius squared laws and Newton used
them to investigate gravity. Look at the lines radiating out of the center
of the earth. Using the formula for area of a sphere, where is the density
of the lines per square area greatest? If the density represents the relative
force of gravity, on which circle is the force of gravity the greatest?
Open the link below and after the applet has loaded click the show forces
button and then in one motion, click in the white field and drag your mouse,
let go and repeat until you get 3 or 4 planets orbiting the red sun. |