1. Newton didn’t discover gravity; he discovered that gravity is universal. Everything pulls on everything else in a simple way that involves only mass and distance. Newton’s Law of Universal Gravitation states that every object attracts every other object with a force that for any two objects is directly proportional to the mass of each object.

2. So, the greater the masses, the greater the force of attraction between the two. The greater the distance between the two objects, the less force of attraction. The law can be expressed as an equation: F~m 1 m 2 /d 2

3. When G is introduced, we can write an exact equation for the law. F = G m1m2 d 2 In words, the force of gravity between two objects is found by multiplying their masses, dividing by the square of the distance between their centers, and than multiplying by the constant G.

4. The magnitude of G is given by the magnitude of the force between two masses of 1kg each, 1 meter apart. G=0.000 000 000 0667N Expressed in scientific notation G= 6.67 X 10 -11 N*m 2 /kg 2

5. Gravity is a very weak force. We sense gravitation only when masses like that of the earth are involved. The mass of the earth is 6x10 24 kg