# Homework Help: Circulation motion problem?

1. Oct 29, 2012

### Ishida52134

1. The problem statement, all variables and given/known data
A giant wheel, 40m in diameter, is fitted with a cage and platform on which a man can stand.
The wheel rotates at such a speed that when the cage is at X (as shown) the force exerted by
the man on the platform is equal to his weight. The speed of the man is:
X is located at the top of the vertical wheel.
A. 14m/s
B. 20m/s
C. 28m/s
D. 80m/s
E. 120m/s

2. Relevant equations
Fn + Fg = ma

3. The attempt at a solution
Well it states the normal force is mg. So I got 2mg = m(v^2/r)
which I got that v = 28 m/s.
But the answer key says it's b.

2. Oct 29, 2012

### Ishida52134

any ideas

3. Oct 29, 2012

### omiver4

your method is correct I believe you just made a computational error.

4. Oct 29, 2012

### Saitama

I see no image.

5. Oct 29, 2012

### Ishida52134

it's 2mg = mv^2/r
so v = sqrt (2gr) which is sqrt(784) = 28.

I don't know how to put an image up.
Basically it's just a vertical circle and point X is at the top of it.

Last edited: Oct 29, 2012
6. Oct 29, 2012

### Ishida52134

any ideas

7. Oct 29, 2012

### tehstone

It appears to me that you are using the diameter in your calculation rather than the radius.

2mg = m*v^2/r
v = sqrt(2gr)
v = sqrt(2*(9.8m/s^2)*20m)
v = sqrt(392) = 19.798 which rounds up to 20 m/s

If you do the same math with r = 40 you get 28 m/s. However, the question states that the diameter is 40, so the radius must be 20.

8. Oct 29, 2012

### rl.bhat

Take g = 10 m/s^2. Then 2gr = ? ( 2r = diameter)

9. Oct 30, 2012

### omiver4

the reason you keep getting sqrt(784) is because you are using 40m for radius when you should be using 20m

10. Oct 30, 2012

### Ishida52134

ohhhh lol thanks.