# Forces and Moments acting on an entire system.

## Homework Statement

The ceiling-mounted bicycle rack (shown in "Capture" image in attachments) with dimensions p = 50 mm, q = 1000 mm and r = 150 mm for a bicycle that weighs F = 200 N.

The rack is to be made from two parts, ABC and BD, cut from a steel tube and then welded together at B.

Determine the forces and moments acting on the whole rack, ABCD.

## Homework Equations

The only equations that I can see would be relevant here are the force and moment equations:
M = (F*d)
Ʃ M = 0
Ʃ F = o

## The Attempt at a Solution

I have found M(of B) and M(of C) by doing the following:

M(of B) = 200*0.15 = 30Nm Clockwise

Therefore, 30 = 0.05*F(at A), so F(at A) = 600N

So... M(of C) = 1.05*600 = 630Nm Clockwise.

So I have found M at points B and C, and the force at A, however I am not confident that is what's required of the question. What does it mean by "the forces and moments acting on the whole rack ABCD"??

#### Attachments

• 2.4 KB Views: 385

## Answers and Replies

Related Introductory Physics Homework Help News on Phys.org
haruspex
Science Advisor
Homework Helper
Gold Member
There seems to be something missing in the problem statement. I gather from your attempted solution that there is a a horizontal force FA at A.
I have found M(of B) and M(of C) by doing the following:

M(of B) = 200*0.15 = 30Nm Clockwise

Therefore, 30 = 0.05*F(at A), so F(at A) = 600N
If you are taking moments about B then you must consider a contribution from the force at C. It does not necessarily act vertically. Better might be to take moments about C.

There seems to be something missing in the problem statement. I gather from your attempted solution that there is a a horizontal force FA at A.

If you are taking moments about B then you must consider a contribution from the force at C. It does not necessarily act vertically. Better might be to take moments about C.
There is no acting force on A. I just calculated that because if there is a moment at B, then having the 200N force at D would be the equivalent of having a Force of 600N at A. Am I wrong?

haruspex
Science Advisor
Homework Helper
Gold Member
There is no acting force on A. I just calculated that because if there is a moment at B, then having the 200N force at D would be the equivalent of having a Force of 600N at A. Am I wrong?
Yes, that'd be wrong. If there's no force acting below B then the piece of the rack below B is irrelevant. Just take moments about C.
The question is fairly clear: what are all the forces and moments acting on the rack, assuming it stays in balance?

All that said, the question looks strange to me. It would be more reasonable if there were a horizontal force at A.