What keeps the Moon in orbit around Earth instead of flying off into space or crashing into Earth?
AThere is no gravity in space, so the Moon just floats in place
BThe Moon is balanced between the pull of Earth and the push of the Sun
CThe Moon is moving sideways fast enough that as gravity pulls it toward Earth, it curves around rather than hitting it — it is continuously falling and missing
DEarth's magnetic field holds the Moon in place
An orbit is a continuous free fall. The Moon has sideways velocity that would carry it in a straight line into space if there were no gravity. Earth's gravity constantly pulls the Moon toward it. The combination of sideways motion and inward pull produces a curved path — the Moon keeps falling toward Earth but keeps missing because of its sideways speed. This is the fundamental mechanism of all orbits.
Question 2 True / False
Astronauts on the International Space Station float because there is no gravity in space.
TTrue
FFalse
Answer: False
At the altitude of the ISS (about 400 km), Earth's gravity is still about 90% as strong as at the surface. The astronauts float because they and the station are in free fall — orbiting Earth together, falling toward it at the same rate. They experience weightlessness for the same reason you feel weightless at the top of a roller coaster drop — you are in free fall, and your body and your surroundings are falling together.
Question 3 Short Answer
Why do planets farther from the Sun orbit more slowly?
Think about your answer, then reveal below.
Model answer: Gravity gets weaker with distance, so the Sun pulls less strongly on distant planets. A weaker pull means the planet needs less sideways speed to maintain its orbit. Additionally, distant planets have longer orbital paths (larger circumferences) to travel. The combination of lower speed and longer path means distant planets take much longer to complete one orbit — Neptune takes 165 Earth years while Mercury takes only 88 days.
This relationship was first described mathematically by Johannes Kepler: the square of a planet's orbital period is proportional to the cube of its distance from the Sun. It follows directly from Newton's law of gravity and explains the precise pattern of planetary orbital periods in our solar system.