Why do we ignore upthrust on a free body diagram of a skydiver?

Why do we only show air Resistance/drag going up and weight doing down? Wouldn't there be an upthrust force too? If so, why don't we show that on the free body diagram?

Reply 1

uberteknik

21

Original post by vector12

Why do we only show air Resistance/drag going up and weight doing down? Wouldn't there be an upthrust force too? If so, why don't we show that on the free body diagram?

Yes, there is an upthrust. Whether it needs to be included depends on whether the upthrust will have any noticeable affect on the outcome.

Air has a low density. Unless the mass of air displaced by the object's volume, is a good fraction of and comparable to the mass of the falling object, then the upthrust will be negligible and can be safely ignored for practical purposes.

For instance, hot air balloons work because of upthrust. Problems with 100kg humans falling do not.

(edited 6 years ago)

Reply 2

vector12

OP

10

Original post by uberteknik

Yes, there is an upthrust. Whether it needs to be included depends on whether the upthrust will have any noticeable affect on the outcome.

Air has a low density. Unless the mass of air displaced by the object's volume, is a good fraction of and comparable to the mass of the falling object, then the upthrust will be negligible and can be safely ignored for practical purposes.

For instance, hot air balloons work because of upthrust. Problems with 100kg humans falling do not.