Muscle tension required to lift a child at a constant speed

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Homework Help Overview

The discussion revolves around a physics problem involving muscle tension required to lift a child at a constant speed, specifically focusing on the forces and torques involved in the scenario of lifting with a foot. The subject area includes concepts of mechanics, particularly levers and torque.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants explore the use of work and force equations, questioning whether to sum forces and distances. There is a suggestion to convert forces to torques and sum them instead. Some participants express uncertainty about the units of force and torque, and whether the calculations align with the problem's requirements.

Discussion Status

Participants are actively engaging with the problem, with some providing calculations and others offering guidance on the correct approach. There is a recognition that torque is relevant to the solution, and some participants are attempting to derive the necessary force from the torque values calculated. Multiple interpretations of the problem are being explored.

Contextual Notes

There is a discussion about the appropriate units for force and torque, with some participants clarifying that force should be expressed in Newtons rather than Newton-meters. The problem's setup and the assumptions regarding the forces acting on the system are also under consideration.

annanause
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Homework Statement


When a mother lifts he child with her foot (as shown in figure) what force should the upper muscle exert to lift her child at a constant speed?
24e163d.jpg


Homework Equations


W = F x D
f = mg

I think these might be the right equations to use, correct me if I'm wrong.

The Attempt at a Solution


I'm not sure if I need to add up the forces and distances shown in the figure? If I did W would be:
W = (0.2m + 0.38m)(39N + 98N)
W = 0.58 x 137 = 79.46Nm

Do I need to convert kg into N?

f = mg
f = 137N x 9.80 m/s
f = 1342.6

Is this at all in the right tracks?
 
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Work is not needed here. This is a levers problem. You need to convert the forces to torques and sum them.
 
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russ_watters said:
Work is not needed here. This is a levers problem. You need to convert the forces to torques and sum them.

I gave it a shot and got an answer that is 45Nm, can the force of the muscle be written in Nm?
 
annanause said:
I gave it a shot and got an answer that is 45Nm, can the force of the muscle be written in Nm?
No, the force has to be in N (Newtons).
I think your answer of 45 Nm is the correct torque that the muscle has to provide, but it is not the force. You have to calculate the force from that torque.
 
annanause said:
I gave it a shot and got an answer that is 45Nm, can the force of the muscle be written in Nm?
No, force is in Newtons. Newton-meters is probably a torque... By my math, you calculated the torque applied to lift the leg and the baby. So you aren't done yet: the question asked for the force applied by the muscle above the knee.

You can take the problem in steps that way, but generally the physicists here (and probably your professor too) would prefer you write the torque-sum equation and then solve for the force you are looking for. Then plug in the numbers and compute in one step.
 
russ_watters said:
No, force is in Newtons. Newton-meters is probably a torque... By my math, you calculated the torque applied to lift the leg and the baby. So you aren't done yet: the question asked for the force applied by the muscle above the knee.

You can take the problem in steps that way, but generally the physicists here (and probably your professor too) would prefer you write the torque-sum equation and then solve for the force you are looking for. Then plug in the numbers and compute in one step.
So I can do F = t/D ?
I tried to do 45Nm / 0.02m = 2250 N

Is there any more work than that needed?
 
I think that's right.
 
annanause said:
So I can do F = t/D ?
I tried to do 45Nm / 0.02m = 2250 N

Is there any more work than that needed?
That looks like the right answer to me!
 
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TomHart said:
I think that's right.

russ_watters said:
That looks like the right answer to me!

Thanks for the help, really appreciate it.
 
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