# Net displacement for a point on a circular saw

1. Nov 22, 2009

### BHFCBabe

1. The problem statement, all variables and given/known data

When a carpenter shuts off his circular saw, the 10.0-inch diameter blade slows from 4440 rpm to zero in 2.5 s.
A.What is the angular acceleration of the blade? In rev/sec2
B.What is the distance traveled by a point on the rim of the blade during the deceleration? In feet.
What is the magnitude of the net displacement of a point on the rim of the blade during the deceleration? in inches

I have solved A and B, but cannot get C. The answers are in the back of the book so I know A and B are correct.
2. Relevant equations
1) $$\omega$$f=$$\omega$$o+2$$\alpha$$t
2) $$\omega$$f2=$$\omega$$o2+2$$\alpha\Delta\Theta$$

3. The attempt at a solution
A. 4440rev/1min * 1min/60sec=74 rev/sec
Using equation 1:
0=74+$$\alpha$$*2.5
$$\alpha$$=-29.6 rev/sec2

B. Using equation 2:
0=742+2*-29.4*$$\Delta\Theta$$
$$\Delta\Theta$$=92.5 Rev

1ft/12in*10pi in./1 rev * 92.5rev = 242.16 feet.

C. I know that it completed 92.5 revolutions. There are .5 revolutions left over.
10pi in/ 1 rev *.5 rev = 15.7 inches.

However, this answer is wrong. Why? The proper answer is 10 inches.

2. Nov 22, 2009

### Redbelly98

Staff Emeritus
Displacement is determined by the straight-line distance between two points, not the distance around the circumference.

p.s since nobody said it to you before: welcome to Physics Forums