# Velocity in a Vacuum (1 Viewer)

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#### ksinclair13

Here is a question on a quiz I took the other day:

An object is in free fall in a vacuum. Which of the following is true?

A. Velocity is increasing
B. Acceleration is increasing
C. A and B
D. None are correct

I might be thinking about this wrong, but I assumed it was a vacuum in space, which means there would be no gravity, which would mean the answer is D. However, if it was a man-made vacuum on Earth, gravity would increase it's velocity, which would mean the answer is A.

Can someone clear this up for me, please?

#### DaveC426913

Gold Member
"...a vacuum in space, which means there would be no gravity..."

Bzzzt!

Go over that logic again?

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#### DaveC426913

Gold Member
Vacuum and zero gravity are unrelated. One doesn't mean the other.

What do you think keeps the Moon and all the satellites from flying off into space? What do you think the astronauts are experiencing on the Moon?

#### AussieDave

What would the answer be then? I'd also have assumed D but you seem to be suggesting otherwise?

#### jtbell

Mentor
ksinclair13 said:
I assumed it was a vacuum in space
I think they simply want you to ignore the effects of air resistance. Imagine the object falling down a long evacuated chamber at or near the earth's surface.

#### AussieDave

If that is the case then wouldn't it be A) velocity is increasing and the acceleration would be 9.8 m/s/s

#### semc

Yea i thought that it was trying to tell you to ignore air resistance when reading this.so i think the ans might be A?so it is correct to say that the object will not reach terminal velocity when falling thru the vacuum.....rite?

#### AussieDave

Yeah that's correct. A quick definition for terminal velocity tells you that. "The terminal velocity of an object falling towards the ground, in NON-VACUUM, is the speed at which the gravitational force pulling it downwards is equal and opposite to the atmospheric drag (also called air resistance) pushing it upwards."

#### DaveC426913

Gold Member
So, we have an object that is falling freely. Since it is falling, we know it is under the influence of gravity. Since it is in a vacuum, we can discount terminal velocity. The acceleration will be g.

ksinclair13:
You know its velocity is increasing.
Is its acceleration increasing, or is it constant?

#### ksinclair13

It's acceleration is constant. The answer is A. I made a foolish assumption.

Thank you for the help!

Edit:
jtbell said:
I think they simply want you to ignore the effects of air resistance. Imagine the object falling down a long evacuated chamber at or near the earth's surface.
If I knew the question meant that, I would have said A without a doubt (re-read my first post).

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