The tub of a washer goes into its spin-dry
cycle, starting from rest and reaching an an-
gular speed of 7.5 rev/s in 7.3 s . At this point
the person doing the laundry opens the lid, and a safety switch turns off the washer. The
tub slows to rest in 11.1 s .
During the whole process of starting up
and stopping, through how many revolutions
does the tub turn? Assume constant angular
acceleration while it is starting and stopping.
Answer in units of rev

Question

The tub of a washer goes into its spin-dry
cycle, starting from rest and reaching an an-
gular speed of 7.5 rev/s in 7.3 s . At this point
the person doing the laundry opens the lid, and a safety switch turns off the washer. The
tub slows to rest in 11.1 s .
During the whole process of starting up
and stopping, through how many revolutions
does the tub turn? Assume constant angular
acceleration while it is starting and stopping.
Answer in units of rev

anonymous · Answer

I've tried and got the wrong answer of 69

anonymous · Answer

given data variables:
$$
\omega_0=0\qquad \omega_1=7.5{\rm rev/s}=7.5\times2\pi\,{\rm rad/s}\qquad t_1=7.3s\
\omega_2=0\qquad t_2=11.1s\
$$
1. how many revolutions = what is $\theta$ from t=0 to t1?
$$\omega_1^2=\omega_0^2+2\alpha\theta\
{\rm where}\quad \alpha=\frac{\omega_1-\omega_0}{t}=?$$
plug in and find theta
2. same thing from t=t1 to t2