Gravity in the Solar System

“I have not as yet been able to discover the reason for these properties of gravity from phenomena, and I do not feign hypotheses.” – Isaac Newton, in Philosophiae Naturalis Principia Mathematica , 1687.

The Role of Gravity

Isaac Newton first described gravity as the force that causes objects to fall to the ground and also the force that keeps the Moon circling Earth instead of flying off into space in a straight line. Newton defined the Universal Law of Gravitation, which states that a force of attraction, calledgravity , exists between all objects in the universe ( Figure below ). The strength of the gravitational force depends on how much mass the objects have and how far apart they are from each other. The greater the objects’ mass, the greater the force of attraction; in addition, the greater the distance between objects, the smaller the force of attraction.

Drawing of gravity between two planets

The force of gravity exists between all objects in the universe; the strength of the force depends on the mass of the objects and the distance between them.

The distance between the Sun and each of its planets is very large, but the Sun and each of the planets are also very large. Gravity keeps each planet orbiting the Sun because the star and its planets are very large objects. The force of gravity also holds moons in orbit around planets.

BigThink video: Who was the greatest physicist in history? According to Neal deGrasse Tyson, it was Sir Isaac Newton: .


  • Newton developed the Universal Law of Gravitation, which recognizes the gravitational attraction between objects.
  • All objects have a force of attraction between them that is proportional to their mass anddistance from each other.
  • Gravity keeps the planets orbiting the Sun because they are very large, just as gravity keeps satellites orbiting the planets.


Use this resource to answer the questions that follow.

  1. How long have the Earth and Moon existed?
  2. What evidence shows that the Moon’s gravity affects the Earth?
  3. What does Newton’s law of gravitation state?
  4. What happens as mass increases?
  5. What happens as distance increases?


  1. Why is the gravitational attraction of the Moon to Earth greater than the attraction of Earth to Sun?
  2. Why doesn’t the Moon fly off into space? Why does an apple fall to the ground rather than orbiting Earth at a distance?
  3. What is the Universal Law of Gravitation?


Fluids in Space

Nothing behaves normally in space. Juggled tomatoes float. Fluids don’t flow into your mouth when you tip a cup. Researchers study the behavior of different materials and then design inventions to get around these problems.

Why It Matters

Wastes are difficult to deal with in microgravity too, as this toilet on the ISS suggests [Figure2]

  • Astronauts spend months at a time in microgravity on the International Space Station and at times in other spacecraft.
  • These astronauts need to do all the bodily functions of a human on Earth: eat, drink, and eliminate waste.
  • Without gravity, fluids do not behave normally. Capillary action causes fluids to flow in the absence of gravity.

Can You Apply It?

With the link below, learn more about fluids in microgravity. Then answer the following questions.

  1. Describe what happens to fluid in a cup as you drink it, especially in regards to gravity.
  2. How do you drink a fluid such as coffee in microgravity, according to the video?
  3. How do fluids in microgravity flow without a pump?
  4. Does capillary flow happen on Earth? Why is it not a major mover of fluids?
  5. How does the patented low gravity coffee cup work?

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