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gravity

OLogy Series
astronomy
card
066

gravity

OLogy Series
astronomy

Gravity is the force of attraction between all objects in the Universe. Objects with more mass have greater gravitational pull than objects with less mass. Gravity keeps Earth and the planets orbiting around the Sun instead of floating off into space. What would the Universe be like without gravity?

What Goes Up...
As the story goes, Sir Isaac Newton
(1642-1727) was sitting under an apple tree and admiring the Moon when an apple fell on his head. (Most historians agree that Newton did see an apple fall -- but not that it actually bopped him.) At that moment Newton had an inspiration: the force pulling the apple down and the force keeping the Moon in its orbit around the Earth were one and the same. The force was gravity, and Newton explained that gravity pulls toward the center, not just down to the ground. In other words, the apple fell towards the center of the Earth, and his head just happened to get in the way.

Stars are shaped like spheres because:

gravity pulled them into that shape as they formed

they eroded into round shapes over millions of years

round shapes have less wind resistance

Are you right?

### Correct!

Stars are round because gravity pulls their mass towards their centers. That's why you won't see any square or triangular stars.

An object's gravitational pull is affected by:

distance and mass

distance and brightness

mass and speed of light

Are you right?

### Correct!

A more massive object has a stronger pull than a less massive one. This strength is also affected by distance -- the closer the objects, the stronger the pull.

The Moon has no gravity.

or
?

### Fiction

The Moon is less massive than the Earth, so its gravity pulls less than on Earth. Astronauts can jump high, but they don't float away.

Original theory: In 1687, Sir Isaac Newton published Principia Mathematica, which explained
the theory of gravitational pull.
Characteristics: the force of attraction between all objects exists throughout the Universe.
Significance: one of four "fundamental forces" of the Universe; keeps our feet on the ground and the planets in orbit

Image credits: Jim Paillot.