At what speed would a moving apple become free falling?

• A. 5,000 miles per hour
• B. 17,000 miles per hour
• C. 50,000 miles per hour
• D. 100 miles per hour

Answer: (B)

When gravity is the only force and an object has enough sideways velocity, it can enter orbit around Earth, effectively staying in free fall as it continually curves around the planet. The speed needed to keep a circular orbit right at Earth’s surface is about 7.9 km/s, which is roughly 17,000 miles per hour. At that speed, the apple’s downward pull toward Earth is balanced by its forward motion, so it keeps missing the ground and continues circling the globe.

Too slow, and it would simply fall to Earth. Much faster than that orbital speed would push the object toward escaping Earth’s gravity altogether, rather than remaining in a bound orbit. So the speed near 17,000 mph is the one that makes the apple stay in orbit, i.e., be in free fall around the planet.