Dec 4, 2008
About the Exam 3 Solution
Professor, I noticed on the solution for the first problem on the exam that there was no force due to gravity shown acting on the bar in the Free Body Diagram. Is there supposed to be? I am unclear as to why there wouldn't be one somewhere on there. The way it's shown, it looks like the only force acting in the y direction is the Normal force at A pushing the bar upward, which I assume would imply that it's being lifted off the ground.
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3 comments:
I think that system was in the horizontal plane, so there wouldn't be any force from gravity shown when "looking down" on the system.
Sam is correct -- motion in the horizontal plane means that the gravitational force does not influence the motion since that force acts perpendicular to the plane of motion.
You are correct -- the normal force at A (NA) is the only force acting in the y-direction. Because of this, NA creates the y-component of acceleration aGy. NA results from the constraint that A must move in only the x-direction. However, NA does NOT cause the bar to lift off the surface; it simply keeps it there.
Let us know if this does not answer your question.
OK, thanks. I must not have read it correctly.
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