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 Oct 19th 2008, 07:09 PM #1 Member   Join Date: Sep 2008 Posts: 39 Newton's 2nd law Hello everyone, Could someone please help me with this or give me a hint? If the mass of the object is m, what is the minimum force F needed to raise the object? Use Newton’s 2nd law. My attempt: F=ma Thank you very much
 Oct 19th 2008, 07:15 PM #2 Senior Member     Join Date: Apr 2008 Posts: 815 I give you a big hint. An object has a mass $\displaystyle m$, so it has a weight $\displaystyle m\cdot g$. To raise it, the total force in the direction of the weight force must be in the upward direction. Do you catch what I'm saying? __________________ Isaac If the problem is too hard just let the Universe solve it.
 Oct 19th 2008, 08:08 PM #3 Member   Join Date: Sep 2008 Posts: 39 Thank you very much So, since the force of the weight=mg and when you raise the total force it is in the upward direction, when you raise the minimum force it would be in the negative direction, right? Thank you Last edited by chocolatelover; Oct 19th 2008 at 08:11 PM.
Oct 19th 2008, 08:23 PM   #4
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 when you raise the minimum force it would be just off the ground, right?
Yes. You will get something like : The minimum force must be greater than ...$\displaystyle N$.
In fact, if you maintain the minimal force required to rise the body in the air applied on the body, it will rise up in the air faster and faster.

 So, since the force of the weight=mg and when you raise the total force it is in the upward direction, when you raise the minimum force it would be in the negative direction, right?
The answer is no.
Chose a referential system in which the positive direction is upward and the negative direction is downward. The weight (which is a force) has its direction downward while the minimum force required to raise the object must be upward, otherwise the sum of the forces will point downward (that is toward the center of the Earth) and the object would not move at all.

EDIT : If you don't mind, once you get the answer post it here so that we can check it out.
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Isaac
If the problem is too hard just let the Universe solve it.

 Oct 20th 2008, 05:41 PM #5 Member   Join Date: Sep 2008 Posts: 39 Is it "The minimum force must be F=mg"? Thank you very much or is it "the maximum force-minimum force=0" or "the minimum force=0-maximum force" Last edited by chocolatelover; Oct 20th 2008 at 05:45 PM.
Oct 20th 2008, 06:07 PM   #6
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Join Date: Apr 2008
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 Is it "The minimum force must be F=mg"?
Almost. If you apply a force equal to the weight but in the opposite direction, the object will not move (it will move at a constant velocity in fact. But as it was with an initial velocity equal to $\displaystyle \frac{0m}{s}$, it will conserve this velocity). So the answer should be "the minimal force to raise an object of mass m must be greater than $\displaystyle mg$".
Anyway I think you got the idea, so congratulations!
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Isaac
If the problem is too hard just let the Universe solve it.

 Oct 20th 2008, 07:17 PM #7 Member   Join Date: Sep 2008 Posts: 39 Thank you very much Regards

 Tags 2nd, law, newton

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