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Rotational Motion Unit



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Jacob Murad • 10 months agologin to reply

The problems with Sally and John in the slides (Questions 37 and 38) specify that the rock is being lifted by the pole that is angled at 45degrees and that the rock is 1 m away from the tree stump. If I understand this geometry correctly the lever arm on that side of the pole is 1.41 m not 1 m as used in the published answer. Can you please adjust the problem statement or the published answer to reflet this geometry or am I misunderstanding the problem?

John Ennis • 9 months, 4 weeks agologin to reply

Jacob, you have the geometry correct - but since the angle for both the pole applying a torque in the counter clockwise direction and the mass of the rock applying a torque in the clockwise direction is the same, the angle cancels out of the sum of the torque equation. Please check out the answer tab which shows how sin 45 deg cancels. John

Jacob Murad • 9 months, 4 weeks agologin to reply

Thank you. I am not questioning the sin45 in the torque calculations. I am questioning the use of 1 m for Rr. The problem specifies that the distance between the rock and the fulcrum is 1 m. I interpret that to mean that the horizontal distance (along the ground) is 1m. A lever arm at a 45 degree angle would have to be 1.41 m long in order to reach the rock if the rock is 1 m away from the fulcrum.

John Ennis • 9 months, 3 weeks agologin to reply

Jacob, I'm sorry - I meant the distance along the lever from the rock to the fulcrum. Since I confused you - what description would make it clearer? Then, I'll fix it. thanks, John

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