Help me solve this rotational motion problem

[bonoz]

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Problem: "How long does it take for a 33-rpm record to rotate once, at constant speed? A) 0.033 seconds B) 0.29 s C) 2.66 s D) 3.33 s.

I first coveted 33 rpm into rps. I divided 33 rpm by 60 seconds and got 0.55 rps.

After this I can only think of dividing 0.55 by 1 to get the time it takes to complete 1 revolution. I get 1.81 seconds.

What am I doing wrong? is there a blatant equation I should be using?

 

[thatscorrect7]

that is weird lol
i get what you get

33 times in 60 seconds.
so 1 time would take 60/33 = 1.81 seconds.

work backwards:
1.81 seconds per turn. 33 turns = 1 minute.

WTF

 

[TheBoondocks]

that is weird lol
i get what you get

33 times in 60 seconds.
so 1 time would take 60/33 = 1.81 seconds.

work backwards:
1.81 seconds per turn. 33 turns = 1 minute.

WTF

It's a mistake. The actual answer is 1.82 which should be c

 

[csau]

If you get flustered with these types of questions, an alternate solution can be to focus on the units.


33 rotations per minute
...is the same as...
33 rot/min (rot = rotations, min = minutes) (1)

Convert to seconds

33 rot/min * 1 min / 60 sec
=33/60 rot/sec (2)

You want the amount of time it takes to rotate once
...which is the same as...
sec/rot (seconds per rotation)

to obtain that, take the inverse of (2)

(33/60 rot/sec)^-1
=60/33 sec/rot
A quick glance gives you a value that is a bit less than 2 (66/33 being 2).

EDIT: Looks like you were right all along.

 

[bjb305]

you are all wrong

you forgot to factor in the DJ that was using this record to scratch thereby slowing the rotations by 0.8 seconds.. therefore the correct answer is C at 2.6 seconds

 

[bonoz]

you are all wrong

you forgot to factor in the DJ that was using this record to scratch thereby slowing the rotations by 0.8 seconds.. therefore the correct answer is C at 2.6 seconds

If you get flustered with these types of questions, an alternate solution can be to focus on the units.


33 rotations per minute
...is the same as...
33 rot/min (rot = rotations, min = minutes) (1)

Convert to seconds

33 rot/min * 1 min / 60 sec
=33/60 rot/sec (2)

You want the amount of time it takes to rotate once
...which is the same as...
sec/rot (seconds per rotation)

to obtain that, take the inverse of (2)

(33/60 rot/sec)^-1
=60/33 sec/rot
A quick glance gives you a value that is a bit less than 2 (66/33 being 2).

EDIT: Looks like you were right all along.

Nicely explained.