Slide 1

1) What variables affect the force of gravity between two objects? 2) How does Einsteins Theory of General Relativity Compare to Newtons Law of Gravity?

Slide 2

Suppose Elvis (we found him!) is in orbit around Earth at a distance twice as far from the planets center as the surface of Earth is. Would you expect his weight to be greater than, less than, or equal to his weight on the surface of the planet? What variables determine your weight. Answer: His weight would be less because of a lower gravitational pull on his body. Variables that determine your weight are your mass, the mass of the object pulling on you and your distance from that object. Your weight is just the measure of the force of gravity on your body. Your weight changes depending where you are. Your mass does not on your location, just on how many jelly donuts you eat.

Slide 3

Check out this website to see how your weight changes throughout the universe. Notice your weight on Moon. (Neil Armstrong, what a loser he was.) Notice your weight on a Neutron Star (the blueberry muffin is really going straight to the hips.) http://www.exploratorium.edu/ronh/weight/ http://www.exploratorium.edu/ronh/weight/ Thinking Question: Notice your weight on Jupiter. Jupiter has a mass greater than 318 times the mass of Earth. Why then is your weight not 318 times greater on Jupiter? Think-pair-share with your partner.

Slide 4

You may suspect that because Jupiter is 318 times as massive as the Earth, you should weigh 318 times what you weigh at home. This would be true if Jupiter was the same size as the Earth. But, Jupiter is 11 times the radius of the Earth, so you are 11 times further from the center. The force of gravity is measured from the center of objects, not their surface. This reduces the pull by a factor of 11 2 resulting in about 2.53 times the pull of Earth on you. Standing on a neutron star makes you unimaginably weighty. Not only is the star very massive to start with (about the same as the Sun), but it is also incredibly small (about the size of San Francisco), so you are very close to the center and r is a very small number.

Slide 5

Sir Isaac Newton (16421727) generalized his observations on gravity in a law now known as the law of universal gravitation. Universal Gravitation Equation m 1 and m 2 are the masses of the two objects d is the distance between the two objects G is the gravitational constant

Slide 6

All matter is affected by gravity. Two objects, whether large or small, always have a gravitational force between them. Gravitational force increases as mass increases. Gravitational force decreases rapidly as distance increases because distance is squared.

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Slide 8

1) A satellite orbits the Earth the Earth at 300 km above the Earths surface. If you double the satellites mass what will happen to the gravitational force between the Earth and the satellite? Answer: Gravitational force will double. When doing these type of problems where you are only worried about the change you can drop the constant G in Newtons Law of Gravity. Just work with the proportion: F (m1m2) / d 2 2) If the satellite climbs to an orbit of 600 km how will that change the gravitational force between Earth and the satellite? Answer: Gravitational force will decrease by because the distance is squared in the denominator of the Law of Gravity.

Slide 9

Newtons Law of Gravity is a mathematical model that works great in nearly all applications important to us. NASA still uses it to plot the trajectories of its spacecraft. Yet, it does not work in all situations. Among other things: Newtons Law of Gravity does not fully explain the precession of orbits for the planets, especially of planet Mercury. [precessionorbitsplanetMercury It does not correctly predict the bending of light by gravity. It does not work well with objects travelling near the speed of light

Slide 10

Einsteins Theory of General Relativity (1916) is an improvement on Newtons Law of Gravity because it is in closer agreement with the observations. Einsteins Theory of General Relativity In General Relativity objects with mass act by curving space time. (Get it. Your one of the few.) Fortunately for you the the mathematics of General Relativity will not be on the FCAT.

Slide 11

Planet A and Plant B have identical masses. Planet B three times farther from a star than Planet A. How do the gravitational forces between the star and the planets compare? A. the force on A is 1/9 the force on B. B. The force on A is 1/3 the force on B. C. The force on A is three times the force on B. D. The force on A is nine times the force on B. Answer: D

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