universal Gravitation Key Terms

Tycho Brahe

teacher of Kepler; mapped the night sky over 20 years of observations.

epicycle

smaller circular path followed along a larger circular path.

Johannes Kepler

astronomer who used geometry and mathematics with a sun-centered solar system to calculate the motion of the planets; developed three laws that describe the behavior of every planet and satellite.

Kepler’s First Law of Planetary Motion

the paths of the planets are ellipses with the center of the sun at one focus; went against the belief that the planets follow a circular orbit.

ellipse

oval shape drawn about two foci; in the equation of an ellipse, a is the radius along the x-axis and b is the radius along the y-axis.

Kepler’s Second Law of Planetary Motion

an imaginary line from the sun to a planet sweeps out equal areas in equal time intervals; planets move fastest when closest to the sun and slowest when farthest from the sun; went against the belief that the Earth is stationary and proposed the idea that planets travel at varying speeds throughout their orbits.

period

time to complete one full cycle of motion.

Kepler’s Third Law of Planetary Motion

the ratio of the squares of the periods of any two planets revolving about the sun is equal to the ratio of the cubes of their average distances from the sun, where T is period, r is average distance from the sun, and a and b represent two planets; provided accurate data for the distance the planets are from the sun.

Inverse Square law

variance of physical quantities inversely as another quantity squared.

Law of Universal Gravitation

for any pair of objects, each object attracts the other with a force directly proportional to the product of the masses of the objects, and inversely proportional to the square of the distance between their centers.

universal gravitational constant

constant used to provide the proper units and proportion when using Newton’s Law of Universal Gravitation.

Henry Cavendish

Measured the value of G.

support force

force that completely balances the weight of an object at rest; normal force.

apparent weightlessness

experiencing the effects of zero gravity due to the lack of a support force; g = 0.

gravity

the way in which masses communicate with one another.