Gravity - An Introduction, page 1



			Day 1 - Gravity: An Introduction (To download a pdf of this lesson click here. You will need Adobe Acrobat Reader. Get it here free.)

	Teacher information: Gravity is a force that acts at a distance, unlike a “contact" force that we might use to push a chair across the floor. In 1661 Isaac Newton formulated the Universal Law of Gravitation, which states that two objects do not need to be in contact to exert forces on one another. Newton's explanation of gravity states that two objects attract each other with a force that depends on how much mass they possess and the distance between them. The greater the masses, the greater the force of attraction. The greater the distance between them, the weaker the force. More formally, Newton's Universal Law of gravitation states that every object attracts every other object with a force that is directly proportional to the mass of each object and inversely proportional to the square of the distance between them. When an object falls to the ground, it does so because a force of attraction exists between the mass of the object and the mass of the Earth. They pull each other. The combined masses of the Earth and the object give rise to the force of attraction. We usually don't think of the object contributing to the force of gravity, because when an object (such as a ball of clay) is dropped, the ball is the only object that moves because the Earth is so much more massive than the ball of clay. Galileo's Experiment: According to books of the time (in 1588) things fell at a rate proportional to their weight. That's what Aristotle had said. The heavier things were supposed to fall faster. Galileo had other ideas. According to the story, he tested his idea by lugging an iron cannon ball and a wooden ball of an equal size (but less weight) up the Leaning Tower of Pisa. He dropped both of them at the same time. They hit smack in the middle of the town square, striking the ground at the same instant.
It proved that bodies fall at the same rate no matter what they weigh. Click on the camera to the right to see for yourself. (QuickTime 5 or later required.)

		next page

Day 1 - Gravity: An Introduction