Two mechanical advantage questions.

[new user]

Hey all,

I just had a question on the marine gouge. Now, can someone explain to me, the answer for question 6 in the mechanical comprehension. Maybe the stress of taking it is getting to me and I can't seem to figure it out.

http://marinegouge.com/mediawiki-1.6.6/images/ASTBgouge.pdf

It asks, "In order to balance the scale above (and it has a lever with circle A (on the left) being larger than circle B (on the right), the fulcrum should be moved...

a. towards A
b. towards B
c. in either direction.

I answered towards A because I thought the lighter of the two needed a longer arm to balance it so you move it to the left. But the correct answer is B. So can someone explain it to me?

My second question is in Practice Exercise 9, page 208, question 17 of http://www.petersons.com/pdf/free/OfficerCandidate.pdf

The answer is c, but I thought the mechanical advantage was 4 so wouldn't it be 24, instead of 18?

Any help would be appreciated.

Thanks.

 

[jus2mch]

I looked at the question and noticed B is heavier than A, because B is lower and closer to the ground. So if you moved the triangle away from B the board is just going to keep getting closer to the ground, because of the weight of B. Therefore you need to move the triangle toward B to get A lower to the ground.

 

[WVUBetaHornet]

My second question is in Practice Exercise 9, page 208, question 17 of http://www.petersons.com/pdf/free/OfficerCandidate.pdf

The answer is c, but I thought the mechanical advantage was 4 so wouldn't it be 24, instead of 18?

Mech Ad is 3, the answer is right.

 

[scoober78]

The pulley closest to the truck provides no mechanical advantage...merely a change in direction....therefore MA=3

 

[JIMC5499]

I don't know about your first question, because I agree with you.

Your second question is pretty easy. The answer is 12 feet. Mechanical advantage comes into play on the amount of force it will take to lift the weight.

 

[new user]

I don't know about your first question, because I agree with you.

Your second question is pretty easy. The answer is 12 feet. Mechanical advantage comes into play on the amount of force it will take to lift the weight.

Thanks. I thought I was going crazy for a second there.

 

[new user]

I looked at the question and noticed B is heavier than A, because B is lower and closer to the ground. So if you moved the triangle away from B the board is just going to keep getting closer to the ground, because of the weight of B. Therefore you need to move the triangle toward B to get A lower to the ground.

I thought it was just a picture showing that it is imbalanced and wanted to get it balanced. But maybe that's the answer.

Thanks.

 

[jus2mch]

Yeah it is just a picture that is imbalanced and wants to get balanced. I weigh 195 say you weigh 160. I am B and you are A. If we move the triangle away from me I am just going to get closer to the ground. If we move the triangle toward me it will start to balance us.

I to thought that the mechanical advantage would be four. However the only thing I can think of is that the fourth pulley is moving the rope in the X direction (like what Scoober said). I think de/dr is considering the y. So if it is considering y it would be 3, because the last pulley really doesn't help with the weight. So 6x3=18

 

[WVUBetaHornet]

It asks, "In order to balance the scale above (and it has a lever with circle A (on the left) being larger than circle B (on the right), the fulcrum should be moved...

a. towards A
b. towards B
c. in either direction.

I answered towards A because I thought the lighter of the two needed a longer arm to balance it so you move it to the left. But the correct answer is B. So can someone explain it to me?

The correct answer is to move the fulcrum towards B; the reason being that, yes...you are right in that circle A is the lightest of the two, which is why moving the fulcrum towards B gives the B circle less weight to shoulder, respectively allowing A to balance out with B.

So going back to what you said above...moving the fulcrum in the B vice A direction would give A the longer arm of the scale...allowing it to balance with B...you had the logic all figured out, you just didn't think it through.
Take your time and answer what the question asks...not what you think it should be asking.

 

[Schnugg]

Ignore the relative sizes of the circles and try it again. Pretend they are the same size. Then answer it as it stands.

It will be fairly obvious.

 

[new user]

I sure hope I don't fail this thing miserably. At least I still have two weeks to study for this thing.

Thanks everyone for your help.

 

[jus2mch]

Well it seems like you are putting in the effort. So I think you will be ok.

 

[bradler]

The easiest way to figure MA for pulley's is start at the pulley that connects to the object and count the ropes need in the pulley. So i this problem you have 3, MA= 3. Do not count where the last rope is attached at the pulleu providing the force.

As for the fulcrum (6)... it a problem that messes with your mind.. because you think B is smaller it should weigh less. You have to think density and the B is heavier you need to move the fulcrum towards B so that the lighter circle (A) can exert a greater MA and lift B. So the answer is toward B.