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Two wheels are connected via a conveyor belt. The larger [#permalink]
02 Jun 2012, 21:43

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Question Stats:

67% (03:04) correct
33% (02:29) wrong based on 203 sessions

Two wheels are connected via a conveyor belt. The larger wheel has a 40cm diameter and the smaller wheel has a 32cm diameter. In order for the conveyor belt to work smoothly, each wheel must rotate the exact same number of centimetres per minute. If the larger wheel makes r revolution per minute, how many revolutions does the smaller wheel make per hour in terms of r?

Re: Two wheels are connected via a conveyor belt. The larger [#permalink]
16 Jul 2013, 16:24

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This post received KUDOS

macjas wrote:

Two wheels are connected via a conveyor belt. The larger wheel has a 40cm diameter and the smaller wheel has a 32cm diameter. In order for the conveyor belt to work smoothly, each wheel must rotate the exact same number of centimetres per minute. If the larger wheel makes r revolution per minute, how many revolutions does the smaller wheel make per hour in terms of r?

A. 1280π B. 75r C. 48r D. 24r E. (64π)/3

1. Let the number of revolutions made by the smaller wheel in 1 hr be s 2. Given: Number of centimeters rotated by both the wheels in 1 hr is the same

3. Number of centimeters rotated by the larger wheel in one revolution = its circumference=2*pi*20=40Pi 4. Number of centimeters rotated by the larger wheel in 1 hr = 40*pi*60*r

5. Number of centimeters rotated by the smaller wheel in one revolution=its circumference=2*pi*16=32*pi 6.Number of centimeters rotated by the smaller wheel in 1 hr= 32*pi*s

Re: Two wheels are connected via a conveyor belt. The larger [#permalink]
03 Jun 2012, 02:16

1

This post received KUDOS

Expert's post

macjas wrote:

Two wheels are connected via a conveyor belt. The larger wheel has a 40cm diameter and the smaller wheel has a 32cm diameter. In order for the conveyor belt to work smoothly, each wheel must rotate the exact same number of centimetres per minute. If the larger wheel makes r revolution per minute, how many revolutions does the smaller wheel make per hour in terms of r?

A. 1280π B. 75r C. 48r D. 24r E. (64π)/3

The radius of the larger wheel is 20cm so, its circumference is 2\pi{R}=40\pi. Since the larger wheel makes r revolution per minute or 60r revolutions per hour then it rotates 60r*40\pi centimeters per hour.

The radius of the smaller wheel is 16cm so, its circumference is 2\pi{R'}=32\pi. Now, in order the smaller wheel to rotate the same centimeters per time interval it should make \frac{60r*40\pi}{32\pi}=75r revolutions per hour.

Re: Two wheels are connected via a conveyor belt. The larger [#permalink]
25 Sep 2013, 22:33

1

This post received KUDOS

Visualization is the key in such problems.

To work smoothly

The belt left by big wheel should be taken up by small wheel. Implicitly,

Length left by big wheel /min = length accepted by small wheel/min

40 *pi * r = 32 * pi * x

x = (5/4) * r in minute

x= (5/4) * r * 60 in hour Hence B

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Re: Two wheels are connected via a conveyor belt. The larger [#permalink]
12 Jul 2012, 01:51

My take on it, the wheel will cover PI*40 cm in one rotation. Thus to if we know total distance in a unit time, the expression for rev. should be No of centi per minute/PI*40 = rev per minute Also as if you look at the unit at both side LHS: no of cm per minute => units are cm/min PI*40 => units are cm RHS Rev per minute => units are (min)^-1 or (1/min) Thus, [cm/min] /[cm] = [1/min] showing the equation balances and in your case you inverted the LHS of the equation. Rest is you can equate the total distance for both wheel PI*32*R = PI*40 *r R = 5/4*r in cm/m or 5/4*r*60 = 75*r cm/h.

The circumference of the larger wheel is 2\pi{R}=40\pi. Since the larger wheel makes r revolution per minute or 60r revolutions per hour, then it rotates 60r*40\pi centimeters per hour.

The radius of the smaller wheel is 16cm so, its circumference is 2\pi{R'}=32\pi. Now, in order the smaller wheel to rotate the same centimeters per time interval it should make \frac{60r*40\pi}{32\pi}=75r revolutions per hour.

Re: Two wheels are connected via a conveyor belt. The larger [#permalink]
27 Nov 2013, 03:42

There is a simple formula for this kind of problems. Given two pulleys/wheels are connected tightly with a conveyor belt, each wheels completes same distance in an hour as the other wheel unless there is slippage. only their revolutions per min or hour differ due to their difference is size.

If r1 and r2 are revolutions per hour of each wheel and R1 and R2 are radius of each wheel.

Re: Two wheels are connected via a conveyor belt. The larger [#permalink]
12 Dec 2013, 11:53

Two wheels are connected via a conveyor belt. The larger wheel has a 40cm diameter and the smaller wheel has a 32cm diameter. In order for the conveyor belt to work smoothly, each wheel must rotate the exact same number of centimetres per minute. If the larger wheel makes r revolution per minute, how many revolutions does the smaller wheel make per hour in terms of r?

Interesting to note that the larger wheel has a diameter of 40 (8*5) while the smaller one has a diameter of 32 (8*4)...

If the large wheel has a diameter of 40 and the small wheel, 32, then their circumferences are 40pi and 32pi respectively. In order for them to move the conveyor belt at the same rate, the smaller wheel would need to rotate 1.25 times as fast as the larger wheel. Lets say the large wheel makes 10 revolutions per minute, the smaller wheel would need to make 10*1.25 = 12.5 revolutions per minute. If the large wheel makes 10 revolutions per minute it makes 600 per hour. Therefore, the smaller wheel would need to make 600*1.25 = 750 revolutions per hour.

If r = 10, then the answer choice must be b.

B. 75r

gmatclubot

Re: Two wheels are connected via a conveyor belt. The larger
[#permalink]
12 Dec 2013, 11:53