# Difficult: Linear vs. Angular acceleration

1. Apr 4, 2009

### datdo

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

A board of mass m and length L is hinged to the ground. Attached to the board is a cup of negligible mass and is positioned Rc away from the hinge. Also on the edge of the board a ball rests. When the board is dropped from an angle of theta the ball is meant to be caught in the cup.

A) What is the minimum angle required for the ball to lag behind the board? I think cos(theta) > 1/2

B) Where must the cup be placed in order to catch the ball in terms of theta and L? I think the answer to this one was L/2cos(theta)

2. Relevant equations

F=ma

$$\tau=Frsin\theta$$

$$\tau=I\alpha$$

$$\alpha r=a$$

3. The attempt at a solution

I'm really lost.

I guess for the cup to catch the ball both objects must be in the same spot in time so if I find their positions with respect to time and find their intersection...

board:
$$\tau=.5mgLsin\theta$$
$$\alpha=\frac{3\tau}{mL^2}$$

cup:
$$a_c=\alpha R_c$$

ball:
$$a_b=g$$

I don't know where to go from here...

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Last edited: Apr 4, 2009
2. Apr 4, 2009

### Staff: Mentor

Why sinθ?

You neglected to point out that the ball starts out balanced on the edge of the board.

Hints:

For A: What's the acceleration of the edge of the board? How must it compare to the acceleration of the falling ball in order for the ball to fall freely?

For B: The ball falls straight down.

3. Apr 4, 2009

### datdo

its sin because torque is $$F\times r = Frsin\theta$$
the angular acceleration increases as the angle decreases
A)

$$\alpha=\frac{3gsin\theta}{2L}$$
end of the board:
$$a_e=\frac{3gsin\theta}{2}$$

$$g\leq\frac{3gsin\theta}{2}$$

$$\frac{2g}{3g}\leq sin\theta$$

$$\frac{\sqrt{5}}{3}\leq cos\theta$$? but isn't the answer...

B)
$$R_c=Lcos\theta$$

4. Apr 4, 2009

### Staff: Mentor

It would be sinθ if θ was the angle between r and F, not r and the horizontal.
Redo this with the correct angle.

Not exactly.

Good.

5. Apr 4, 2009

### datdo

understood.

$$\alpha=\frac{3gsin(\pi/2 -\theta)}{2L}$$
end of the board:
$$a_e=\frac{3gsin(\pi/2 -\theta)}{2}$$

$$g\leq\frac{3gsin(\pi/2 -\theta)}{2}$$

$$\frac{2g}{3g}\leq sin(\pi/2 -\theta)$$

$$\frac{2}{3}\leq cos\theta$$ this more likely....

B still confuses me..

6. Apr 5, 2009

### Staff: Mentor

Good. So what's the maximum angle?

What about it confuses you? The ball, which begins at the edge of the board, falls straight down. Find the horizontal distance between ball and hinge--that will tell you where to put the cup.

7. Apr 5, 2009

### datdo

$$cos^-^1\frac{2}{3}$$

I guess it just seems too simple...even for a mechanics problem...

8. Apr 5, 2009

### Staff: Mentor

Don't complain.

(I see that you tagged the thread with "Cooper Union". Are you a student there?)

9. Dec 18, 2009

### datdo

I could be...