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theo tree June 30th, 2019 07:20 AM


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 :)


ps can anyone explain what's happening with the units here? How do you get from grams * degrees to Newtons?

theo tree June 30th, 2019 07:50 AM


g is gravity, not grams.


DarnItJimImAnEngineer June 30th, 2019 09:06 AM

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."

topsquark June 30th, 2019 09:21 AM


Originally Posted by theo tree (Post 611227)

g is gravity, not grams.


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


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