# Q & A: Object falling in vacuum

Q:
We would like to know if an object is falling with a constant pull of gravity would the speed of the object reach a maximum speed or would the object constantly increase to an unknown speed. This object is falling inside a vacuum.
- jimmy (age 11)
Raleigh, NC
A:
According to Newtonian mechanics, F=ma, where F is the force of gravity (no air resistance here), and a is the acceleration, and m is the mass of the object. For gravity, F=mg, where g is the gravitational acceleration, giving a=g. If the object doesn't hit something first, it will continue to accelerate.

Something falling in towards Earth will have a speed of at least the escape velocity of the Earth, about 25,000 miles per hour. The Earth's gravitational field is not uniform -- it gets weaker the farther out you go. Also, objects such as meteors can have some additional energy due to their motion before they got near the Earth.

There is a speed limit to everything, though, and that's the speed of light; nothing goes faster, not even things that have been falling in a gravitational field for a long time.

Long before that speed limit is approached, air resistance will keep falling objects from exceeding a "terminal velocity" -- that speed when the force of air resistance exactly cancels out the downward pull of gravity.

Tom

(published on 10/22/2007)