Applied force

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Okay, I just want to confirm...When we take an object to some height, let's say a bag taken upstairs, Do we have to apply the upward force equal to the weight of object or more than the weight of the object...???
I know that its more than weight..But just want to confirm...
 

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  • #2
A.T.
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.When we take an object to some height, let's say a bag taken upstairs, Do we have to apply the upward force equal to the weight of object or more than the weight of the object...???
What does Newtons 2nd Law say?
 
  • #3
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What does Newtons 2nd Law say?
An applied net force causes a body to accelerate...There should only be a net force of the object if the upward force is greater than weight...Right???!!!
 
  • #4
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Okay, I just want to confirm...When we take an object to some height, let's say a bag taken upstairs, Do we have to apply the upward force equal to the weight of object or more than the weight of the object...???
I know that its more than weight..But just want to confirm...
Do you talk about the forces when de bag falls? or the forces when you take the object upstairs?
because in the first situation your netto will be downward because it is accelerating(until air restsistance is equal to the mass*g)
in the second situation, it depends is you accelerate or not, if not your force must be equal to the mass of the bag times 9,81.
 
  • #5
jbriggs444
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An applied net force causes a body to accelerate...There should only be a net force of the object if the upward force is greater than weight...Right???!!!
Yes, that is right. So we can safely conclude that if the body starts at rest on the basement floor and transitions to a state where it is moving up the stairs, an average upward force exceeding gravity is required during that transition.

That's only one of three pieces of the problem taken care of. What about the period while the object moves up the stairs at constant speed? And what about the transition at the top of the stairs when the object is placed back on the floor?
 
  • #6
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Okay, the problem was like this
We had to calculate work done while taking a bag upstairs...The amount of force was taken to be equal to weight...From what I understand the force needed to move the bag upstairs SHOULD be greater than its weight...right???
 
  • #7
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What about the period while the object moves up the stairs at constant speed?
Okay,I think i was approaching the problem the wrong way..Can you please elaborate as to what is the relation between the application of force and constant speed???
 
  • #8
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it depends is you accelerate or not, if not your force must be equal to the mass of the bag times 9,81.
Can you please elaborate???
 
  • #9
A.T.
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Okay, the problem was like this: We had to calculate work done while taking a bag upstairs...
For that you either have to integrate or take the average force.

From what I understand the force needed to move the bag upstairs SHOULD be greater than its weight...right???
What is the average force, if the bag ends up with the same vertical velocity that is started with?
 
  • #10
jbriggs444
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Okay,I think i was approaching the problem the wrong way..Can you please elaborate as to what is the relation between the application of force and constant speed???
Newton's second law is good. If velocity is constant then acceleration is... what?

Okay, the problem was like this
We had to calculate work done while taking a bag upstairs...
The easy way to calculate work done is by energy conservation. Then one need not determine the required force pattern.
 
  • #11
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For that you either have to integrate or take the average force.

What is the average force, if the bag ends up with the same vertical velocity that is started with?
zer
For that you either have to integrate or take the average force.

What is the average force, if the bag ends up with the same vertical velocity that is started with?
It should be zero...
 
  • #12
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Newton's second law is good. If velocity is constant then acceleration is... what?


The easy way to calculate work done is by energy conservation. Then one need not determine the required force pattern.
okay, so by my understanding...you mean to say that the only acceleration will be of picking up of bag and the first step taken...then the velocity will remain constant...right???
 
  • #13
jbriggs444
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okay, so by my understanding...you mean to say that the only acceleration will be of picking up of bag and the first step taken...then the velocity will remain constant...right???
Right. At least up until the point where the last step is taken and the bag is set back down. That also involves an acceleration.
 
  • #14
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Right. At least up until the point where the last step is taken and the bag is set back down. That also involves an acceleration.
If we are to see it like this...The moment when the person lifts the bag up from floor, he has to apply an upward force greater than weight of bag..There is the acceleration...The first step he take, he has to apply a force greater than his and bag's weight combined...There is an acceleration...Now lets assume that he somehow ''floats'' up to the last step with constant velocity while holding the bag...So is it true that he will not have to apply a force greater than the weight of bag (while being in constant velocity) but equal to weight, As long as the bag has same velocity as him???
 
  • #15
jbriggs444
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For every step you start where you are applying an upward force that is momentarily greater than the bag's weight there is a step you stop where you are applying a force that is momentarily less.

No matter what you do, if the bag ends up at rest, the average force will turn out to have been equal to the weight of the bag.
 
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  • #16
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zer

It should be zero...
If the average net force is zero, what is the average applied force?
 
  • #17
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Do you talk about the forces when de bag falls? or the forces when you take the object upstairs?
because in the first situation your netto will be downward because it is accelerating(until air restsistance is equal to the mass*g)
in the second situation, it depends is you accelerate or not, if not your force must be equal to the mass of the bag times 9,81.
You picking up bag taking it upstairs, is'nt that acceleration???
 
  • #18
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For every step you start where you are applying an upward force that is momentarily greater than the bag's weight there is a step you stop where you are applying a force that is momentarily less.

No matter what you do, if the bag ends up at rest, the average force will turn out to have been equal to the weight of the bag.
How can you be at constant velocity while taking the bag upstairs???
 
  • #19
jbriggs444
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How can you be at constant velocity while taking the bag upstairs???
Why would you think that you could not be?
 
  • #20
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Why would you think that you could not be?
For every step you take, you apply an upward force, as a result of which you accelerate upwards...So how can you be at constant velocity???
 
  • #21
jbriggs444
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For every step you take, you apply an upward force, as a result of which you accelerate upwards...So how can you be at constant velocity???
You have two feet. You can walk at a steady pace if you work at it. And you can carry a laundry basket so as to give it an even velocity even if your belly button is bobbing all over the place.
 
  • #22
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You have two feet. You can walk at a steady pace if you work at it. And you can carry a laundry basket so as to give it an even velocity even if your belly button is bobbing all over the place.
So as long as the bag is taken upstairs at constant velocity, no net force will be acting on it...Unless you throw it upstairs during that transition, then there will a net force...Right??/
 
  • #23
jbriggs444
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So as long as the bag is taken upstairs at constant velocity, no net force will be acting on it...Unless you throw it upstairs during that transition, then there will a net force...Right??/
If you throw it upstairs, there will be a net force when you throw it, sure. Will it be hurtling upwards at a constant velocity? WIll the net force be zero while that is happening?
 
  • #24
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If you throw it upstairs, there will be a net force when you throw it, sure. Will it be hurtling upwards at a constant velocity? WIll the net force be zero while that is happening?
Oh i get your point...No it will not have constant velocity due to its downward-acting weight...So the conclusion is that as long the person carries the bag at a steady rate, no net force will be acting on him and he will just be ''carrying'' (applying reaction force to weight of bag) the bag...
 

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