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June 30th, 2019, 07:20 AM   #1
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Typo?

Heya,

Looking at an introduction to vectors.

In my textbook it gives the example question:

"A particle of mass 0.1kg is held at rest on a rough plane inclined at 20° to the horizontal by a friction force, F, acting up the plane. Find the magnitude of F and the normal reaction."

The given answer is:

"F = 0.1g sin20° = 0.335 N"

Is this a typo, or is there something that I'm missing?

Thanks very much as ever

Theo

ps can anyone explain what's happening with the units here? How do you get from grams * degrees to Newtons?
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June 30th, 2019, 07:50 AM   #2
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Right.

g is gravity, not grams.

Gotcha.
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June 30th, 2019, 09:06 AM   #3
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Might I add as an engineering instructor, if any of my students turned in a report with this in it, I would likely take points off and definitely reiterate the importance of clear communication. I would have accepted, "F = (0.1 kg) g sin 20°," or, better yet, "F = mg sinΘ = (0.1 kg)(9.807 m/s²)(sin 20°) = 0.335 N."
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June 30th, 2019, 09:21 AM   #4
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Quote:
Originally Posted by theo tree View Post
Right.

g is gravity, not grams.

Gotcha.
No. g is the acceleration due to gravity, not gravity (which is a force.)

-Dan
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