reply to post by Joey Canoli
You expect us to find your knowledge of physics credible when you first try to explain it away with an F=ma approach, see that almost your entire post
is proven incorrect, so change tact and attempt to prove your point with momentum vectors?
You've asked how the upper part could apply a net force of greater than 1.0W, when gravity cannot be greater than 1g.
Momentum is the answer.
I said gravity cannot apply a force greater than 1.0W downwards to the upper block, therefore the net force acting on the block in the downwards
direction should be less than 1.0W, as gravity is the only
downwards force acting on the block.
Your internet physics education has obviously not provided you with a sufficient understanding of momentum, as I will explain.
F=ma, therefore we both agree that for there to be an acceleration of the mass there must be an imbalance of forces resulting in some net force
downwards (the direction of the acceleration).
Net force from the descending part was 1.7W.
With 1.0W being provided by gravity and 0.7W of downwards force being provided by momentum. Only, here is (one of) your mistake(s):
Momentum is not a force!
Force is the push or pull applied to an object to change its momentum. Without a change in momentum there is no force. It is just another way of
stating what I have been saying the entire time in this thread, that if there was a deceleration (negative change in momentum), this would have meant
large loads were being applied to the lower structure. As the top section continuously accelerates for most of the collapse it is gaining momentum
(positive change in momentum), this ALL comes from the downwards gravitational force being applied to the top section.
What you are trying to tell us is that momentum is a force which is acting on the upper section to accelerate it. I'm saying that gravity is
providing the only downwards force acting on the upper section. Let's say hypothetically that the leaning top section continued to lean over and fell
off the tower, meaning the only thing between it and the ground was air. As gravity is the only downwards force acting on it, the highest acceleration
it could achieve would be near free-fall (there would be some wind resistance), ie 1.0W due to gravity acting downwards minus about 0.1W (this would
change throughout the fall as velocity changed) upwards wind resistance, resulting in a net 0.9W downwards. If momentum was indeed a downwards force
acting on the upper section helping it to accelerate as you say it is, then in this scenario we would have the 1.0W downwards from gravity plus some
downwards momentum (you have given 0.7W). This would result in a net downwards force greater than its weight and greater than free fall accelerations.
Another error which you have made (but is less relevant because your entire method of using momentum as an an accelerating force is wrong) is that you
have said that the net force is 0.7W downwards to give a 70% free fall acceleration, but then stated the momentum equation (I'm not sure what for,
you didn't use it and it's not as if I don't know how momentum is calculated) P=mv, where v is velocity. So if velocity increases momentum
obviously increases, but you have given us a momentum "force" which is constant throughout the constant acceleration. With constant acceleration the
momentum would constantly increase.
Kids understand momentum.
Maybe you should consult with some children and have them check your work before posting here then.
Try again Joey.
[edit on 8-8-2010 by Azp420]