Kinematics of Rotational Motion Part II

[freshbox]

Homework Statement

I cannot get the answer for part c, -13.02rad/s^2

The Attempt at a Solution

52.08=260.42+α(10)
-208.34=10α
α= -20.83rad/s^2 (Correct)


0=52.08+α(10)
-52.08=10α
α=-5.208rad/s^2 (Wrong)


Please help, thanks..

 

[Doc Al]

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

 

[tiny-tim]

hifreshbox!

(try using the X² button just above the Reply box )

52.08=260.42+α(10)
-208.34=10α
α= -20.83rad/s²(Correct)


0=52.08+α(10)
-52.08=10α
α=-5.208rad/s² (Wrong)

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 ​

 

[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

 

[tiny-tim]

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

 

[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?

 

[Doc Al]

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?

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?

 

[freshbox]

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

 

[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..

 

[Doc Al]

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

 

[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

 

[freshbox]

 

[Doc Al]

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

What formula? The attachment was blank.

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

That's the basic definition of speed, so sure.

 

[Doc Al]

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

(s is distance, of course.)

Just plug in a = 0.

 

[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.