Isaac Newton's law of universal gravitation says that every object in the universe attracts every other object. For example, the Earth's gravitational force on your body pulls you toward the ground. That s why you don't float in the air.
The gravitational force between two objects depends on the mass of each object and on the distance between them. If the mass of both objects is very small, the gravitational force between them is extremely small. For example, the gravitational force between two buildings is too small to measure. However, the gravitational force between the Earth and the Moon is easily measured, because both objects have a large amount of mass.
Gravitational force is also affected by distance. The gravitational force between a house and Neptune is too small to measure because Neptune is very far away. However, because the distance between a vase on a table and the Earth is short, the gravitational force is large and can be measured.
Newton realized the gravitational force that applies to objects on the Earth also applies to the Moon. He proposed that the Moon travels around the Earth because the Earth pulls on the Moon.
Newton also predicted that objects on the Moon would weigh less than they do on the Earth. Because the mass of the Moon is one sixth the mass of the Earth, Newton reasoned that the force of gravity on the Moon would be less. The astronauts who visited the Moon proved Newton s theory correct. If you weigh 150 pounds on the Earth, you weigh about one sixth of 150 pounds, or 25 pounds, on the Moon.
The Earth and the Moon are affected by each other's gravitational force. As the Moon orbits the Earth, the Moon pulls on the surface waters of the Earth. The gravitational pull of the Moon causes the ocean waters nearest the Moon to bulge. You know these bulges as tides.
