# Kinematics of Rotational Motion Part II

1. May 15, 2012

### freshbox

1. The problem statement, all variables and given/known data
I cannot get the answer for part c, -13.02rad/s^2

3. The attempt at a solution

52.08=260.42+α(10)
-208.34=10α

0=52.08+α(10)
-52.08=10α

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2. May 15, 2012

### Staff: Mentor

How long did it take to come to rest? (How long was it moving at constant speed?)

3. May 15, 2012

### tiny-tim

hifreshbox!

(try using the X2 button just above the Reply box )
the 10 seconds for 300 to 60 mph is correct

the 10 seconds for 60 to 0 mph is not correct …

you need to allow for the time spent cruising

4. May 15, 2012

### freshbox

How long did it take to come to rest? (How long was it moving at constant speed?)

Since it states that total is 20sec, 300km/h-60km/h is 10sec, so 60km/h-0km/h is 10sec since 20-10=10

5. May 15, 2012

### tiny-tim

but in between, "it travels for another 100 m at the constant speed of 60 km/h" !!

6. May 15, 2012

### freshbox

but it says
"and it takes a total of 20 seconds to stop at the pit from its top speed of 300km/h"

total =20 secs?

7. May 15, 2012

### Staff: Mentor

Yes, the total time is 20 seconds. But that includes three segments:
- initial slowdown to intermediate speed
- constant speed motion at intermediate speed
- final slowdown to zero speed

Do not ignore the 'constant speed' segment. How much time is spent there?

8. May 15, 2012

### freshbox

Is 2nd segment vi=16.67m/s v=16.67m/s a=0m/s^2 ?

9. May 15, 2012

### freshbox

How do i calculate the time when my acceleration is 0 for segment b?

ω=ωi+αt,v=vi+at
a times t = 0

No t in the equation anymore..

10. May 15, 2012

### Staff: Mentor

It's constant velocity motion: distance = speed * time.

11. May 15, 2012

### freshbox

my book only has this formula.. no wonder i keep getting stucked..

So S=d/t is the only way to solve?

Thanks btw Doc Al

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12. May 15, 2012

### freshbox

13. May 15, 2012

### Staff: Mentor

What formula? The attachment was blank.
That's the basic definition of speed, so sure.

14. May 15, 2012

### Staff: Mentor

The equation you want from your list is:
s = vit + 1/2at2

(s is distance, of course.)

Just plug in a = 0.

15. May 15, 2012

### freshbox

Ah... yea why didn't i see that hehe.. i understand already, thanks for the help..

Btw if you don't mind can you take a look at https://www.physicsforums.com/showthread.php?t=605830..I just want to clarify some doubts..

Since the question says that "Gear A is accelerated uniformly from rest making 20 revolutions in 12secs"

Gear A: Vi=0, t=12s

Can I say that Gear B/Drum C is also Vi=0, t=12s since they are all connected together.