12.0 - Introduction
The topic of gravity has had a starring role in some of the most famous tales in the
history of physics. Galileo Galilei was studying the acceleration due to the Earth’s
gravity when he dropped two balls from the Leaning Tower of Pisa. A theory to
explain the force of gravity came to Isaac Newton shortly after an apple fell from a
tree and knocked him in the head. Although historians doubt whether these events
actually occurred, the stories have come to symbolize how a simple experiment or a
sudden moment of insight can lead to important and lasting scientific progress.
Galileo is often said to have dropped two balls of different masses from the tower so
that he could see if they would land at the same time. Most scientists of his era
would have predicted that the heavier ball would hit the ground first. Instead, the
balls landed at the same instant, showing that the acceleration due to gravity is a
constant for differing masses. While it is doubtful that Galileo actually dropped balls
off that particular tower, his writings show that he performed many experiments
studying the acceleration due to gravity.
In another famous story, Newton formed his theory of gravity after an apple fell and
hit him on the head. At least one person (the daughter of French philosopher
Voltaire) said that Newton mentioned that watching a falling apple helped him to
comprehend gravity. Falling apple or no, his theory was not the result of a
momentary insight; Newton pondered the topic of gravity for decades, relying on the
observations of contemporary astronomers to inform his thinking. Still, the image of
a scientist deriving a powerful scientific theory from a simple physical event has
intrigued people for generations.The interactive simulations on the right will help you begin your study of gravity.
Interactive 1 permits you to experiment with the gravitational forces between
objects. In its control panel, you will see five point masses. There are three identical
red masses of mass m. The green mass is twice as massive as the red, and the
blue mass is four times as massive.You can start your experimentation by dragging two masses onto the screen. The
purple vectors represent the gravitational forces between them. You can move a
mass around the screen and see how the gravitational forces change. What is the
relationship between the magnitude of the forces and the distances between the
masses?
Experiment with different masses. Drag out a red mass and a green mass. Do you expect the force between these two masses to be smaller or
larger than it would be between two red masses situated the same distance apart? Make a prediction and test it. You can also drag three or
more masses onto the screen to see the gravitational force vectors between multiple bodies. There is yet more to do: You can also press GO
and see how the gravitational forces cause the masses to move.
The other major topic of this chapter is orbital motion. The force of gravity keeps bodies in orbit. Interactive 2 is a reproduction of the inner part
of our solar system. It shows the Sun, fixed at the center of the screen, along with the four planets closest to it: Mercury, Venus, Earth and
Mars.
Press GO to watch the planets orbit about the Sun. You can experiment with our solar system by changing the position of the planets prior to
pressing GO, or by altering the Sun’s mass as the planets orbit.
In the initial configuration of this system, the period of each planet’s orbit (the time it takes to complete one revolution around the Sun) is
proportional to its actual orbital period. Throughout this chapter, as in this simulation, we will often not draw diagrams to scale, and will speed
up time. If we drew diagrams to scale, many of the bodies would be so small you could not see them, and taking 365 days to show the Earth
completing one revolution would be asking a bit much of you.(^220) Copyright 2000-2007 Kinetic Books Co. Chapter 12