Go Back   Physics Help Forum > High School and Pre-University Physics Help > Kinematics and Dynamics

Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

Reply
 
LinkBack Thread Tools Display Modes
Old 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
chocolatelover is offline   Reply With Quote
Old Oct 19th 2008, 07:15 PM   #2
Senior Member
 
arbolis's Avatar
 
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.
arbolis is offline   Reply With Quote
Old 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.
chocolatelover is offline   Reply With Quote
Old Oct 19th 2008, 08:23 PM   #4
Senior Member
 
arbolis's Avatar
 
Join Date: Apr 2008
Posts: 815
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.

And about
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.
__________________
Isaac
If the problem is too hard just let the Universe solve it.
arbolis is offline   Reply With Quote
Old 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.
chocolatelover is offline   Reply With Quote
Old Oct 20th 2008, 06:07 PM   #6
Senior Member
 
arbolis's Avatar
 
Join Date: Apr 2008
Posts: 815
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!
__________________
Isaac
If the problem is too hard just let the Universe solve it.
arbolis is offline   Reply With Quote
Old Oct 20th 2008, 07:17 PM   #7
Member
 
Join Date: Sep 2008
Posts: 39
Thank you very much

Regards
chocolatelover is offline   Reply With Quote
Reply

  Physics Help Forum > High School and Pre-University Physics Help > Kinematics and Dynamics

Tags
2nd, law, newton



Thread Tools
Display Modes


Similar Physics Forum Discussions
Thread Thread Starter Forum Replies Last Post
newton anwaar Waves and Sound 2 Dec 22nd 2012 06:17 AM
Newton's 2nd Law help. INeedHelp Kinematics and Dynamics 2 Oct 26th 2009 04:07 PM
newton's laws indranideb Kinematics and Dynamics 1 Oct 2nd 2009 08:12 PM
Newton's 2nd law chocolatelover Kinematics and Dynamics 1 Sep 15th 2008 04:21 PM
Newton's third law werehk Kinematics and Dynamics 3 Jun 22nd 2008 06:47 PM


Facebook Twitter Google+ RSS Feed