The figure below shows a bicep curl. On the figure diagram...

The figure below shows a bicep curl. On the figure diagram the torque created by the weight in the hand at each position. C. How does the moment arm to the weight change throughout the range of motion? D. Compare the relative magnitudes of the muscle moment arm and the moment arm of the weight force. What does this mean in terms of the amount of muscle force that must be generated?

Transcript

00:01 Okay, so this question, i'm going to assume i won't get in trouble for copying the picture, right? because you provided me with this picture, and i can't really draw this.

00:09 But there's two forces here, right? so i'm going to draw the force of the gravity acting on the ball, which is basically the weight of this ball.

00:22 Right.

00:22 So that is this.

00:24 I'm going to draw the weight of the ball in red.

00:27 Right.

00:28 So this is actually, you know what? i'm not going to write fg every single time.

00:32 I'm just going to say fg is a red arrow, right? so here's the weight acting in all these cases, right? and then there's also a force pointing up, and i'll call that fb for the bicep.

00:53 So here my bicep is pulling up this way and up this way.

00:59 It's see how it's this arrow that's drawn on here.

01:02 Here every single time.

01:07 Ok.

01:08 So the question says, how does the moment arm to the weight change throughout the range of motion? ok, so back to the red force.

01:15 The moment arm is how far this is from the pivot point perpendicularly.

01:23 So yes, here's my r.

01:25 In this case, this is r.

01:28 And this is f.

01:30 But i only need the component of r that's perpendicular, because torque, right? torque is a cross product of f and r, right? it's the cross product.

01:41 And to get a cross product, you need your vectors to be perpendicular to each other.

01:46 So let's say two vectors aren't perpendicular to each other.

01:50 So let me just draw that.

01:52 So here's a vector, here's a vector.

01:55 These aren't perpendicular, but this vector here has a component pointing this way that is perpendicular.

02:03 Right so if this is r this component of r is perpendicular and it has another component that's parallel that doesn't matter right so we don't care about that these are the two that caused the torque so here this are the component that really matters i'm going to erase that the component that really matters is imagine drawing this all the way down straight down this right here that's your component of r that matters...

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