Overview

The large-scale structure of the universe, which appears like a "sponge" with its many clusters of galaxies and voids, is shaped by the attraction of masses according to Newton's Law of Gravitation.

Explore Gravity in the Universe in the following short video lessons:

The Basics of Gravity

Advanced Concepts of Gravity

How Gravity Governs the Structure of the Universe

Why This Science Matters

In the seventeenth century, the story goes, Sir Isaac Newton was relaxing under an apple tree when he observed a piece of fruit fall nearby and immediately "discovered gravity." While this myth is entertaining, the facts are actually much more interesting. Newton, and his predecessor Galileo, understood that motion was not capricious: objects were either stationary, or moved in a straight line at constant speed, unless they were acted upon by an external force. This is the idea of inertia, Newton's First Law. Newton was perplexed because he knew that the moon was not moving in a straight line. It followed a circular path around the earth. An external force must be at work. An attractive force at that, as the moon was falling toward the earth while its inertia attempted to move it in a straight line.

Back to the apple...while no one can confirm it, nor deny it for that matter, it may have been the falling apple that caused Newton to recognize that the moon and the apple were attracted by the same mysterious force - gravity.

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Explore This Topic

The following questions accompany this lesson. The answers are given below each question. To reveal an answer, place the cursor over "THE ANSWER IS:".

Situation A: A physicist weighs 640 N on Earth. She visits a new planet which has nine times the mass and five times the radius of Earth.

1. Find her mass.

a) 640 N
b) 64 kg
c) 20 slugs
d) 140.8 lbs.
e) all of the above

2. Find g on this new planet.

a) 450
b) 18
c) 2250
d) 3.6
e) 6.67 x 10-11 m/s

Situation B: The radius of the Earth is 6400 km. A satellite orbits the Earth at altitude 3200 km.

3. Compared to the force of gravity which the satellite felt on the surface of the Earth, in its present orbit, the satellite feels a force of gravity which is only:

a) .5
b) .444
c) .333
d) .25
e) .111 times as big.

4. Occupants on this satellite feel weightless because:

a) they are experiencing free-fall as they simultaneously move forward.
b) the gravitational field is an inverse square law.
c) of their inertia.
d) space is a nearly perfect vacuum.
e) their path is elliptical.

5. The satellite fires its rockets and moves to a higher orbit at altitude 5000 km. When settled into its new orbit, which choice correctly describes the changes in the energies of the satellite, compared to its lower orbit.

	Kinetic	Potential	Total Energy
a)	increased	increased	increased
b)	increased	decreased	stayed the same
c)	decreased	increased	decreased
d)	decreased	increased	increased
e)	decreased	increased	stayed the same

6. Which of the following is not true for a geostationary satellite?

a) it orbits the Earth every 86400 seconds.
b) its altitude is 22300 miles.
c) stays above one point on the equator.
d) is delivered door-to-door directly to its orbit by the Space Shuttle.
e) is used for communications.

7. Question/Answer coming soon

8. The purpose of a Cavendish balance is to measure:

a) the mass of the Earth
b) the weight of the Earth
c) g = 10 m/s
d) G = 6.67 x 10-11 Nm2/kg2
e) the inverse square law

9. The tides are mainly caused by:

a) the tilt of the Earth.
b) the orbital motion of the moon.
c) the decrease in the moon's force of gravity as distance from the moon increases.
d) the ocean-atmosphere interaction.
e) ocean currents.

10. A comet orbits our sun at an average distance of 64 A.U. Find the period of its orbit.

a) 64
b) 4096
c) 262144
d) 512
e) 16 years.

Situation C: A planet orbits a star in another solar system. At its furthest point from the star, the planet is at distance 540 and its speed is 153. At its closest point to the star, the planet is at distance 60.

11. Find the speed of the planet when it is closest to the star.

a) 17
b) 51
c) 459
d) 1377
e) 12393

12. As the planet moves from the furthest distance to the closest distance to the star, the energies of the planet behave in which fashion?

	Kinetic	Potential	Total Energy
a)	decrease	increase	increase
b)	decrease	increase	stay the same
c)	increase	decrease	decrease
d)	increase	decrease	stay the same
e)	increase	decrease	increase

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Links & Resources

The Physics Classroom
Science teacher Tom Henderson from Glenbrook South High School in Glenview, Illinois, wrote The Physics Classroom tutorial pages. The pages cover a variety of physics topics and are wonderfully written and illustrated. The gravity page covers a short history of NewtonÕs work in gravity, CavendishÕs experiment, several example problems with solutions, and several quiz-type practice problems with solutions. An excellent resource at a level appropriate for high school students.

Gravity from the C. R. Nave at the Physics and Astronomy Department at Georgia State University
An interesting site produced by C. R. Nave at the Physics and Astronomy Department at Georgia State University. A number of hyperlinks allow the student to quickly investigate the relationships between gravity and other inverse square laws, tides, trajectories, and centripetal acceleration.

Newton's theory of "Universal Gravitation" from NASA
Part of a terrific NASA educational site on astronomy, mechanics, and space. The site is actually designed for teachers to serve as a curriculum for teaching astronomy and mechanics. It includes an index and a math refresher as well as such interesting topics as latitude and longitude, the development of calendars, and ion rockets! This stop, number 20 out of more than 100, includes discussions on how Newton related the motion of the moon to the gravitational acceleration g, how Cavendish calculated the universal gravitation constant G, and even a brief discussion of the famous "falling apple" legend.

University of California San Diego Science and Engineering Web Site
This page on the University of California San Diego science and engineering website reports the latest research results of Professor Michael Norman and his team of astrophysicists. Professor Norman and his associates use the UCSD supercomputer to simulate the formation and development of the earliest stars in the universe. The site includes computer-generated representations of these early stars.

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Lesson Developed by Dave Fleischman and William Harvie, physics teachers at Torrey Pines High School.