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When you say "pushing upwards on the roof"

I meant: When you say "the net force pushing upwards on the roof" does that mean the lift force?

Hi hb808, thanks for the question.

Well, the net force is the upward force minus the downward force. The is not the "lift force", since lift force normally refers only to the force pushing upwards. In this solution, "lift force" would be the force due to the stationary air inside the building which is at atmospheric pressure. This is a good question since the answer isn't obvious: it just depends on what people agree is meant by the term "lift force" (see http://en.wikipedia.org/wiki/Lift_%28force%29).

I understand. There is no way to calculate just F1 then? I have an exam question asking for a lift force with a given wind velocity and area. This question had a similar approach but asking for different answers. Might be a trick question. Thank you for your reply.

Hmmm... after thinking about this more, I would say "Lift Force" is the resultant of the upward and downward forces due to air pressure. This wouldn't be the global Newton's Second Law "Net Force". You would have to include gravity to calculate the Newton's Second Law Net Force. I get into this idea at 3:58 in the video. I think "Lift force" is a kind of "subtotal", which makes the calculation for it seem like a net force, but it has no relation to acceleration since not all forces are considered (since it ignores gravity).