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David, Tutor
Category: Homework
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Experience:  PhD in Theoretical Physics
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A worker stands still on a roof sloped at an angle of 36 degrees

Customer Question

A worker stands still on a roof sloped at an angle of 36 degrees above the horizontal. He is prevented from slipping by a static frictional force of 390 N. Find the mass of the worker. {that is all the info in the question. If theyre was a coeficient of friction I would have understood how to do this but without that i am stuck)
Submitted: 4 years ago.
Category: Homework
Expert:  David replied 4 years ago.

David :

note that the projection of the mg on the inclined roof is mg*sin(36)

David :

and this must be equal to the friction force os 390

David :

so we have the following:

David :

mg*sin(36)=390 and solving for mass m we get

David :

m=390/(9.8*sin(36)) = 67.70 kg

David :

Answer: 67.70 kg

David :

hope this helps

David :

please do not hesitate to request me in the future just begin your post with "For David" and I'll revert immediately if I'm online


thanks.. i had the same answer but was told the answer was 32.

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Expert:  David replied 4 years ago.
32 is not correct simply because 390/32 is more that 9.8 which means that friction coefficient must be larger than 1. The correct answer is 67.70, or perhaps some information is missing.

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