Net Torque


by spidey12
Tags: torque
spidey12
spidey12 is offline
#1
Jul23-08, 04:04 PM
P: 37
1. The problem statement, all variables and given/known data

The drawing shows a person (weight W = 581 N, L1 = 0.829 m, L2 = 0.399 m) doing push-ups. Find the normal force exerted by the floor on each hand and each foot, assuming that the person holds this position.






2. Relevant equations

I know this has something to do with net torque. Wd=wd

3. The attempt at a solution

Ok i tried figuring this out in sveral ways. first, i tried multiplying L1 with 581, the persons weight, and then i divided by 2. This did not work. so tried multplying times l1+l2 and that did not work either. What am I doing wrong? i was sure i had it right the first time around.
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LowlyPion
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#2
Jul23-08, 04:44 PM
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P: 5,346
Quote Quote by spidey12 View Post
1. The problem statement, all variables and given/known data

The drawing shows a person (weight W = 581 N, L1 = 0.829 m, L2 = 0.399 m) doing push-ups. Find the normal force exerted by the floor on each hand and each foot, assuming that the person holds this position.

2. Relevant equations

I know this has something to do with net torque. Wd=wd

3. The attempt at a solution

Ok i tried figuring this out in sveral ways. first, i tried multiplying L1 with 581, the persons weight, and then i divided by 2. This did not work. so tried multplying times l1+l2 and that did not work either. What am I doing wrong? i was sure i had it right the first time around.
First off you should note that he is "holding his position". This suggests that he is not in motion. If he is not in motion in any direction then he is at balance about his center of mass isn't he? That is if you jammed a fulcrum into his stomach he would have no weight on hands or feet and still not be in motion (though maybe in a great deal of pain).

You could treat it as a torque problem then with there being no torque about what convenient point? You look like you have enough knowns to give you an answer.
spidey12
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#3
Jul23-08, 05:20 PM
P: 37
Quote Quote by LowlyPion View Post
First off you should note that he is "holding his position". This suggests that he is not in motion. If he is not in motion in any direction then he is at balance about his center of mass isn't he? That is if you jammed a fulcrum into his stomach he would have no weight on hands or feet and still not be in motion (though maybe in a great deal of pain).

You could treat it as a torque problem then with there being no torque about what convenient point? You look like you have enough knowns to give you an answer.
I tried using the center of gravity as the fulcrum but it did not work. I used L1xW/2 for the feet and this did not work.

also i tried adding the two lengths together and just getting the midpoint and multiplying that times the Weight but I do not think this is correct.

LowlyPion
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#4
Jul23-08, 05:41 PM
HW Helper
P: 5,346

Net Torque


Quote Quote by spidey12 View Post
I tried using the center of gravity as the fulcrum but it did not work. I used L1xW/2 for the feet and this did not work.

also i tried adding the two lengths together and just getting the midpoint and multiplying that times the Weight but I do not think this is correct.
Why are you dividing the weight by 2?

Let [tex]W_T_O_T = W_H_A_N_D_S + W_F_E_E_T[/tex]

If it is balanced then: [tex]W_H_A_N_D_S * L_2 = W_F_E_E_T * L_1[/tex]

All you have to do is solve. (Remember they want the answer for EACH hand and foot. Just divide those answers by 2.)
spidey12
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#5
Jul23-08, 06:46 PM
P: 37
Quote Quote by LowlyPion View Post
Why are you dividing the weight by 2?

Let [tex]W_T_O_T = W_H_A_N_D_S + W_F_E_E_T[/tex]

If it is balanced then: [tex]W_H_A_N_D_S * L_2 = W_F_E_E_T * L_1[/tex]

All you have to do is solve. (Remember they want the answer for EACH hand and foot. Just divide those answers by 2.)

Ok i already had that equation but i dumped it because there is not unknown...we know l1 and l2, we know the weight of the hands and feet (both are 581)..so what am i solving for?
LowlyPion
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#6
Jul23-08, 08:19 PM
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Quote Quote by spidey12 View Post
Ok i already had that equation but i dumped it because there is not unknown...we know l1 and l2, we know the weight of the hands and feet (both are 581)..so what am i solving for?
The question is what is the Force on EACH hand and EACH foot.?

They all add to the TOTAL weight, yes that is given, but what is the weight on each? How is that weight DISTRIBUTED. That is what you were asked.
spidey12
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#7
Jul23-08, 09:08 PM
P: 37
Quote Quote by LowlyPion View Post
The question is what is the Force on EACH hand and EACH foot.?

They all add to the TOTAL weight, yes that is given, but what is the weight on each? How is that weight DISTRIBUTED. That is what you were asked.
This is frustrating me. i know that whatever answer i get for the hands must be divided by two..i get that.

But right now i have an equation in which i have four variables and i already know ALL FOUR variables. What am i solving for?
LowlyPion
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#8
Jul23-08, 09:55 PM
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Quote Quote by spidey12 View Post
This is frustrating me. i know that whatever answer i get for the hands must be divided by two..i get that.

But right now i have an equation in which i have four variables and i already know ALL FOUR variables. What am i solving for?
[tex]W_H_A_N_D_S = ?[/tex]
[tex]W_F_E_E_T = ?[/tex]

Because trust me [tex]W_H_A_N_D_S[/tex] is NOT equal to [tex]W_F_E_E_T[/tex]

All you know is that they add to 581N.
How that weight is distributed by solving for them using the 2 equations is what the problem wants.

And yes [tex]W_H_A_N_D = 1/2 *W_H_A_N_D_S[/tex]
and [tex]W_F_O_O_T = 1/2 * W_F_E_E_T[/tex] but they are not all equal to each other.


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