Thanks again for taking the time to submit your Stump the Scientist questions! We had some great questions submitted this week, hope you enjoy this one!
Question from fan Natalie Armstrong:
“Why can gravity hold a human down, but not completely?”
Response from Chief Scientist Jim Bray:
Gravity holds everything down on earth, not just humans. When Natalie asks “but not completely”, I suppose she is referring to the fact that we can force humans and other things up for a while, but they usually fall back to the ground. The faster we force or throw things up, the longer it takes for gravity to slow them down and pull them back to earth; you can check that out just by throwing a ball up with various speeds. Another fact is that gravity get weaker as an object gets higher.
When you combine those 2 facts and think about it, it becomes reasonable that, if we throw something (including a human) up fast enough, gravity might not be strong enough to ever pull it back to earth. That is exactly what happens, and we call that speed “escape velocity”. On earth, escape velocity is about 25,000 miles/hour, which is why we don’t see it happen much. However, our rockets can achieve this, which is why we can put humans into space or put satellites into space such that they never return to earth.
Escape velocity is determined by the strength of gravity on a celestial body, so it would be a much smaller number on our moon but much larger on planet Jupiter.