Lesson 5 Video 3 Error

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amkelly
Posts: 5
Joined: Fri Dec 07, 2018 9:45 am

Lesson 5 Video 3 Error

Post by amkelly » Mon Feb 04, 2019 5:15 pm

I have a question about the ankle example given in lesson 5 (3rd video)

We learn that the ankle is an example of a class 2 lever so the weight/load is in the middle of the force and the fulcrum. In this case the fulcrum is where the foot touches the ground, the weight/load is the body, and the force is applied to the heel through the Achilles tendon. I understand that the force exerted by the ground onto the foot is a counterclockwise force and the force exerted by the Achilles tendon is a clock wise force, so the two must equal each other in order for them to cancel out and have equilibrium.

We are asked to find the force in the Achilles tendon if the dancer stands on one foot. The ankle joint itself is 0.12 m away from the fulcrum so the force exerted by the weight of the person (75 kg x 10 m/s^2) is 750N x 0.12 m which gives you the torque from the ground (90N). At this point we just need to solve for force of the Achilles tendon. In order to solve this, we did 90N/0.04m to get 2250N. Correct me if I'm wrong but I thought the fulcrum was the pivot point of the lever, so wouldn't you divide 90N by (0.04m +0.12m)? I'm confused why the pivot point seems to have changed during the calculations.

Thanks for your help!
NS_Tutor_Will
Posts: 271
Joined: Fri May 25, 2018 9:15 am

Re: Lesson 5 Video 3 Error

Post by NS_Tutor_Will » Tue Feb 05, 2019 7:58 am

Thanks for the question!

Remember, with torque, the distance is the magnitude of the distance from the pivot point (fulcrum). So, the achilles is 0.04 m from that point (in one direction -- the left, say). This would be the clockwise force. On the other side, we take the distance from the fulcrum as 0.12 m since that's the distance from the pivot point. This is the counterclockwise force.

We aren't interested in the distance that the two are from each other, only in how far the two forces are applied from the pivot point.

I hope this helps!
amkelly
Posts: 5
Joined: Fri Dec 07, 2018 9:45 am

Re: Lesson 5 Video 3 Error

Post by amkelly » Tue Feb 05, 2019 8:01 pm

I'm still not getting it. We were told the fulcrum is where the foot hits the ground. So why is the pivot point only 0.04 m away from the Achilles tendon (the force applied) when we were told the fulcrum is at the toes (which is really 0.04 + 0.12 m) away from the Achilles tendon. I guess I'm confused because the calculations suggest that this is a class 1 lever (where the fulcrum is between the force and the load) instead of a class 2 lever. It was my understanding that the load/body was 0.04 m away from the Achilles tendon and not the fulcrum.

Maybe I am understanding it incorrectly but this is the set up I have for the lever:

Achilles tendon----(0.04m)----body------------------(0.12m)--------------------fulcrum

To get the distance of the force from the fulcrum wouldn't you have to add (0.04m+0.12m). The calculations suggest that the body is the fulcrum, when in reality that is the load/weight.
NS_Tutor_Will
Posts: 271
Joined: Fri May 25, 2018 9:15 am

Re: Lesson 5 Video 3 Error

Post by NS_Tutor_Will » Wed Feb 06, 2019 7:53 am

Oh, I see! In this particular question, we're asked about the pivot point of the ankle, despite the fact that we're told the pivot point is the toes.

The Achilles tendon is 4 cm from the pivot point of the ankle, but you are correct that is would be 16 cm from the pivot point of the toes (which seems to be what is indicated for the ankle, since it's a class 2 lever).

Does that make sense?

I'll forward this to our content team so they can clarify it one way or the other! Thanks for sharing and keep up the good work!
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