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A computer disk drive is turned on starting from rest and has constant angular acceleration.

(a) How long did it take to make the first complete rotation?

(b) What is its angular acceleration, given that the disk took 0.750 sec for the drive to make its second complete revolution?

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Ans: 1.81 sec, 3.83 rad/sec²


Sagot :

Let's solve this problem step-by-step.

### Given:
- The disk took 0.750 seconds to make the second complete revolution.
- The disk started from rest with a constant angular acceleration.

### Assumptions and Variables:
- Let's denote the angular acceleration as [tex]\( \alpha \)[/tex].
- Let the time taken to complete the first revolution be [tex]\( t_1 \)[/tex].
- Let the total time taken to complete the second revolution be [tex]\( t_2 = 0.750 \)[/tex] seconds.
- For a rotating object with constant angular acceleration starting from rest, the angle [tex]\( \theta \)[/tex] covered is given by:
[tex]\[ \theta = \frac{1}{2} \alpha t^2 \][/tex]
- One complete revolution corresponds to [tex]\( 2\pi \)[/tex] radians.
- Therefore, after the first and second revolutions, the total angles covered are [tex]\( 2\pi \)[/tex] and [tex]\( 4\pi \)[/tex] radians, respectively.

### Steps to Solution

#### Part (b): Determine the angular acceleration [tex]\( \alpha \)[/tex]
1. Equation for the second revolution:
[tex]\[ 4\pi = \frac{1}{2} \alpha t_2^2 \][/tex]
2. Plug in the given value for [tex]\( t_2 \)[/tex]:
[tex]\[ 4\pi = \frac{1}{2} \alpha (0.750)^2 \][/tex]
3. Solve for [tex]\( \alpha \)[/tex]:
[tex]\[ \alpha = \frac{4\pi}{0.5 \times (0.750)^2} \][/tex]
[tex]\[ \alpha \approx 44.68 \text{ rad/sec}^2 \][/tex]

#### Part (a): Determine the time to complete the first rotation
1. Equation for the first revolution:
[tex]\[ 2\pi = \frac{1}{2} \alpha t_1^2 \][/tex]
2. Substitute the value of [tex]\( \alpha \)[/tex] found in part (b):
[tex]\[ 2\pi = \frac{1}{2} \times 44.68 \times t_1^2 \][/tex]
3. Solve for [tex]\( t_1 \)[/tex]:
[tex]\[ t_1^2 = \frac{2\pi}{0.5 \times 44.68} \][/tex]
[tex]\[ t_1 \approx 0.530 \text{ sec} \][/tex]

### Summary of Results:
- The time to complete the first revolution ([tex]\( t_1 \)[/tex]): [tex]\( 0.530 \)[/tex] seconds.
- The angular acceleration ([tex]\( \alpha \)[/tex]): [tex]\( 44.68 \)[/tex] rad/sec².

Therefore, the detailed step-by-step solution concludes that the time to complete the first rotation is approximately 0.530 seconds, and the angular acceleration is approximately 44.68 rad/sec².
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