how long does it take for object to rotate once?

[csx]

it is rotating at 33 rpm.

I thought it would just be (33rev/min)(1min/60 sec) = ~.5

why is that not the answer??

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[t5Nitro]

Is the answer slightly less than 2 seconds?

You just need to flip your fraction. You made it to 33 rev/60 seconds. So it takes 60 seconds/33 rev (rotation). To go one revolution, it should be a bit under 2 seconds.

Think of the answer you put. You came up with roughly 0.5 revolutions per second. But you're being asked how long it takes for one full rotation (1 revolution). If it takes 1 second to go 0.5 revolutions, then it's roughly 2 seconds to go a full revolution.

These questions are nice traps.

 

[csx]

Is the answer slightly less than 2 seconds?

You just need to flip your fraction. You made it to 33 rev/60 seconds. So it takes 60 seconds/33 rev (rotation). To go one revolution, it should be a bit under 2 seconds.

Think of the answer you put. You came up with roughly 0.5 revolutions per second. But you're being asked how long it takes for one full rotation (1 revolution). If it takes 1 second to go 0.5 revolutions, then it's roughly 2 seconds to go a full revolution.

These questions are nice traps.

you just mind ****ed me.

Conceptually, now I get that .5 rev/sec is only half of one revolution and doubling that will give me just under two which is the answer. But how did you know to flip the fraction?

 

[t5Nitro]

you just mind ****ed me.

Conceptually, now I get that .5 rev/sec is only half of one revolution and doubling that will give me just under two which is the answer. But how did you know to flip the fraction?

Well, with your original set up, you have 33 revolutions per 60 seconds. Just look at the units. You are finding the number of revolutions per second. What you are being asked is the number of seconds (time, how long) that are in 1 revolution. So you want seconds per revolution (sec/rev) as your fraction.

 

[NextStepTutor_1]

One thing I want to point out is that units are a very important part of physics and thus the MCAT. Often, using just the units you can come up with the correct answer to some questions. For example, if there is a questions asking you to find the equation for some variable, say flow speed, and gives you the answers as such:

A) f = r*v/d
B) f = r^2*v^2/d
C) f = r^2*v^2/s
D) f = r^2*v^2*s

Then, just based on the units alone, we can get the correct answer, because the resulting answer would have to be in units of m^3/s--which in this case, is only answer choice D!

 

[Lunasly]

Alternatively, you can remember that period (the time it takes for one full revolution) is inversely proportional to frequency (the number of revolution per second). The 0.5 seconds per revolution represents the frequency (according to its definition and the units you proposed). Therefore, to find out the time it takes for one revolution (I.e., units are time over revolution), you just flip the fraction (I.e. 1/0.5 = 2).