Section 2: Gravity and motion Key concepts: What determines the strength of the force of gravity between two objects? What two factors combine to keep the moon and Earth in orbit? Key terms: force, gravity, law of universal gravitation, mass, weight, inertia, Newton’s first law of motion

Gravity Force – push or pull Gravity – the force that attracts objects toward the earth Law of universal gravitation – states that every object in the universe attracts each other The strength of the force of gravity between two objects depends on two factors: the masses of the objects and the distance between them

Gravity, mass, and weight Mass – amount of matter in an object Weight – force of gravity on an object Mass doesn’t change depending on where you are, but weight does. MEMORIZE THIS 

For example: Let’s say you fly on Virgin Galactic to the moon and set up moon-camp there and eat moon-cookies and do moon things. Will your weight change? YES. Because there is less gravity on the moon. Hence people bouncing around like goofs. Will your mass change? NO. You are still the same size. Only the force acting on you has changed.

Gravity and distance Strength of gravity is affected by the distance between two objects as well as their masses. The force of gravity decreases rapidly as their distance increases.

Inertia and Orbital Motion Inertia – tendency of an object to resist a change in motion Greater inertia makes it difficult to start or stop Newton’s first law of motion: an object at rest will stay at rest and object in motion will stay in motion

Orbital motion Newton concluded that two factors: inertia and gravity – combine to keep Earth in orbit around the moon in orbit around Earth