4.2 Gravity

Objectives Describe the gravitational force. Describe the gravitational force. Express the dependence of gravitational field on mass and distance, using proportions. Express the dependence of gravitational field on mass and distance, using proportions. Distinguish between mass and weight. Distinguish between mass and weight.

Gravity The force of attraction that exists between all matter in the universe. Key word is matter Matter is the stuff you are and everything else is made of It takes a lot of mass for the gravity to be noticeable. Like planetary mass

Law of Gravity The force of attraction between any two objects depends on their masses and the distances between them. The bigger the object, the more gravity it has The closer an object is to something the stronger the pull.

4 Basic Forces Gravity Electromagnetic Force Causes electricity and magnetism Also causes chemical interactions between atoms and molecules Strong Nuclear Force Weak Nuclear Force Along with Strong Nuclear Force – only act on particles in the nuclei of atoms

Law of Universal Gravitation Formulated from Newton’s data on motion of planets in 1687

Law of Universal Gravitation G is a constant called the universal gravitational constant d is the distance between the two masses, m1 and m2. Enables us to calculate the force of gravity between any two objects if their masses and the distance between them is known.

Range of Gravity Gravity’s force depends on how close the object is to something. As an object gets further away, the force of gravity on that object decreases Inverse square law As the distance increases the gravity decreases exponentially

Universal Gravitation Because it relates to all matter, on earth as well as the planets and stars Earth’s motion is affected by the gravitational pull of other planets 1846 – astronomers used Law of Universal Gravitation to discover Neptune due to its pull on Uranus

Gravitational Acceleration When all forces except gravity are ignored, falling object is in free fall All objects fall at same acceleration in free fall = 9.8 m/s 2 By Newton’s second law of motion, the force of Earth’s gravity on a falling object is the object’s mass times the acceleration of gravity. F=mg

Weight Gravitational force exerted on an object g = Gravitational acceleration constant 9.8 m/s 2

Weight, Mass, and Gravity Mass How much matter you are made of Does not change depending location It is the same if you are on earth or in space or on the moon Weight The relationship between mass and gravity Changes depending on location Is different on earth and the moon and space

Weight The table shows how various weights on Earth would be different on the Moon and some of the planets.

In-Class Assignment