Isaac Newton was born the year Galileo died. In Newton's time, everything known about planetary motion could be summarized by Kepler's three laws of planetary motion. The first law states that planets move around the sun in elliptical orbits. The second states
that a planet moves out at equal areas in equal times. According to the third law, the square of the orbital period is proportional to
the cube of the distance to the sun. The laws describe what happens without explaining why. Newton believed that the bodies in
space behaved like bodies on Earth: they followed the same laws of motion. He hypothesized that some force acted on the planets
and moons, keeping them in orbit so they did not move in a straight line. Newton proposed that was the force that kept
the planets in their elliptical orbits.
A)
mass
B)
gravity
C)
electrostaics
D)
centripetal force