Quick Physics Help Please
Hey guys, I have a quick physics question I need help on:
The question reads:
In the figure, a rigid beam is attached to two posts that are fastened to a floor. A small but heavy safe is placed at the six positions indicated, in turn. Assume that the mass of the beam is negligible compared to that of the safe.
In the following questions, you will need to rank the various positions. If multiple positions rank equally, use the same rank for each, then exclude the intermediate ranking (i.e. if objects A, B, and C must be ranked, and A and B must both be ranked first, the ranking would be A:1, B:1, C:3). If all positions rank equally, rank each as '1'.
Figure: http://imgur.com/WMmLQkV
1) Rank the positions according to the force on post A due to the safe, greatest compression first, greatest tension last.
Position 1: ?
Position 2: ?
Position 3: ?
Position 4: ?
Position 5: ?
Position 6: ?
My answer for part one is:
1
2
3
4
5
6
(In same order for positions stated above.) Is this correct?
1. Greatest compression (least tension)
2. Second greatest compression
3. Third greatest compression
4. Fourth greatest compression
5. Fifth greatest compression
6. Sixth greatest compression (most tension)
2) Indicate where, if anywhere, the force is zero. (Several choices may be correct.)
Position 1: ?
Position 2: ?
Position 3: ?
Position 4: ?
Position 5: ?
Position 6: ?
I'm unsure on this part.
Lastly,
3) Rank them according to the force on post B.
Position 1: ? I put 3 here
Position 2: ?
Position 3: ?
Position 4: ?
Position 5: ?
Position 6: ? I put 6 here
Then I'm unsure about the rest.
1. Greatest compression
2. Second greatest compression
3. Third greatest compression
4. Fourth greatest compression
5. Fifth greatest compression
6. Sixth greatest compression
Sorry if its so much, if anyone can help that would be great, thank you!
And if you can fill them out according to the same format above, that would be great as this is like an online thing and the rankings can be confusing given the directions above.
Thanks again.
6 Comments • Newest first
[quote=Allison118]I'm still confused on this. I know that 3 has the greatest compression and 6 has the least compression
for part c), however idk the other positions and I'm just confused.[/quote]
You have 6 separate cases, positions 1 - 6.
Imagine in each case they are evenly spaced a length l.
In any position if you draw a free body diagram, you know that A and B can both have an upwards force, and the safe exerts a downwards force.
We don't want this floor to move. For that to happen we need sum of the forces and sum of the moments to be 0.
If Fs is safe force, Fa is support A force and Fb is support b force, we know
Fs = Fa + Fb in every case so this rigid floor will not accelerate up or down. To solve the relative forces in A and B compared to F, we need another relation. We make sure the sum of moments is 0 so the beam does not rotate. I chose to sum moments around point A.
In position 2 the sum of moments for static equilibrium would be 0 = 2l*Fb - l*F, reduced to 2Fb = F, you can relate magnitides of Fb to Fa now. If you draw diagrams and do the forces and moments you will get the relative numbers for all the forces including +/- signs.
I'm still confused on this. I know that 3 has the greatest compression and 6 has the least compression
for part c), however idk the other positions and I'm just confused.
I just got everything I need to answer this question in about 3 minutes by drawing 6 free body diagrams with a "F" for the safe in each position and doing a really easy sum of forces and moments. To be more specific I balanced moments from point A to get a relationship for Fb then solved for force on A through the force balance.
If you can't figure something out in physics, always resort to free body diagrams so that you can visualize the problem you're solving with assumptions, not the real scenario.
[quote=Harlequinist]Hmm.. well, from what I learned in my years of math:
1 + 1 = a window
0 + 0 = a stereo
80085 = boobs.[/quote]
if jim has 8 apples and gives 4 to meg, how many pencils does sharkeisha have?
Did you know that Siemens released a plugin for Sketchup, Rhino, AutoCAD etc that does exactly this?
Part 1 is right.
Part 2, no idea what it wants. If it's asking where he force on the beam would be zero based on the position... the safe is going to put a force on it no matter what so... yeah.
Part 3. My guess would be 3, 4=2, 5=1, 6.