Question 12 AAMC FL (TORQUE)

[Neplina94]

Taken from AAMC FL (Practise exam 2)

"A person is sitting on a chair as shown...
(insert picture of person sitting straight up on a chair)
...Why must the person either lean forward or slide their feet under the chair in order to stand up?"

A. To increase the force required to stand up
B. To use the friction with the ground
C. To reduce the energy required to stand up
D.To keep the body in equilibrium while rising (Correct Answer)

I understand that Torque is equal to F* lever distance, and I also understand the concept of base of support, and that when the center of gravity goes outside of base of support then stability is lost. However, Im a bit confused as to how leaning forward or sliding foot under the chair maintains the equilibrium.

To me, it seems that the person is in equilibrium while sitting because COG (around the belly button area) is within base of support. Im not sure how the torque equation relates to the concept of stability here, nor do I understand how the chair contributes to stability from a physics perspective. Would someone please be able to help me out?

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[Matthew9Thirtyfive]

It's talking about rotational equilibrium, not translational. When you go to stand up, if you don't lean forward or slide your feet under the chair, there will be a torque that is extremely difficult to overcome. So we lean forward or put your feet under you to move your COG over the fulcrum, eliminating the torque. That's why it's D.

It's not A because that's the opposite and makes no sense.
It's not B because friction is always there.
It's not C because energy is not a path function and doesn't depend on how you get from A to B.

 

[Neplina94]

It's talking about rotational equilibrium, not translational. When you go to stand up, if you don't lean forward or slide your feet under the chair, there will be a torque that is extremely difficult to overcome. So we lean forward or put your feet under you to move your COG over the fulcrum, eliminating the torque. That's why it's D.

It's not A because that's the opposite and makes no sense.
It's not B because friction is always there.
It's not C because energy is not a path function and doesn't depend on how you get from A to B.

Thank you for your response. Would you mind explaining exactly how the torque is created (like what elements play a part in the torque here) and how exactly putting feet back etc balances that torque? (maybe from an eqn perspective?). (Im a little weak on torques, Im sorry).

 

[Matthew9Thirtyfive]

Sure. I did a crappy little drawing. Hopefully it helps. Sorry if it's huge.

Basically, your center of gravity is the point at which you can assume the force of gravity works on the whole object. So the center of gravity when you are sitting is above your ass, which is some distance r from the fulcrum, which is your knee. Now, when you go to stand up, if you don't lean forward, all of that force is directed down at that distance r at some non-zero angle theta (which is around 90 degrees give or take depending on the chair), which gives you a large torque.

When you lean forward (or put your feet under you), you move your center of gravity over the fulcrum. So now all the force is directed directly into the fulcrum. That gives you a torque of zero. So now there's no net torque to either direction, keeping you in rotational equilibrium.

 

[Neplina94]

Sure. I did a crappy little drawing. Hopefully it helps. Sorry if it's huge.

View attachment 238659

Basically, your center of gravity is the point at which you can assume the force of gravity works on the whole object. So the center of gravity when you are sitting is above your ass, which is some distance r from the fulcrum, which is your knee. Now, when you go to stand up, if you don't lean forward, all of that force is directed down at that distance r at some non-zero angle theta (which is around 90 degrees give or take depending on the chair), which gives you a large torque.

When you lean forward (or put your feet under you), you move your center of gravity over the fulcrum. So now all the force is directed directly into the fulcrum. That gives you a torque of zero. So now there's no net torque to either direction, keeping you in rotational equilibrium.

perfect, bless you.

 

[MRKHAN06]

Sure. I did a crappy little drawing. Hopefully it helps. Sorry if it's huge.

View attachment 238659

Basically, your center of gravity is the point at which you can assume the force of gravity works on the whole object. So the center of gravity when you are sitting is above your ass, which is some distance r from the fulcrum, which is your knee. Now, when you go to stand up, if you don't lean forward, all of that force is directed down at that distance r at some non-zero angle theta (which is around 90 degrees give or take depending on the chair), which gives you a large torque.

When you lean forward (or put your feet under you), you move your center of gravity over the fulcrum. So now all the force is directed directly into the fulcrum. That gives you a torque of zero. So now there's no net torque to either direction, keeping you in rotational equilibrium.

Just so I have a good understanding of this, the picture you drew where the COG is above the fulcrum would make the rotational torque 0 because the angle would be at sin0 right?