# Homework Help: Linkage kinetics problem

1. Jun 10, 2012

### Krappy

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

The linkage ABD is formed by connecting two 8.05-lb bars and a collar of negligible weight. The motion of the linkage is controlled by the force applied to the collar. Knowing that a tthe instant shown the angular velocity and angular acceleration of bar B are zero and 10 rad/s2 counterclockwise, respectively, determine the force P.

http://img685.imageshack.us/img685/5416/picxv.png [Broken]

2. Relevant equations

$$\sum \tau = I\alpha$$
$$\sum F = m a_G$$
$$\vec a_G = \vec a_B + \vec a_{G/B}$$

3. The attempt at a solution

Considering the vertical bar (1):
$$a_B = \alpha_1 L$$
$$\tau = LF = I\alpha_1 \Rightarrow F = \frac{I\alpha_1}{L} = \frac{mL\alpha_1}{3}$$

Now, the horizontal one (2):
$$a_{Gx} = a_B = \alpha_1 L$$
$$P\cos \theta - F = ma_{Gx} = m \alpha_1 L \Leftrightarrow P \cos \theta = m\alpha_1 L (1 + 1/3) \Leftrightarrow P = \frac{4m \alpha_1 L}{3\cos \theta}$$

I know this isn't correct (it doesn't match the solutions), but I can't figure out what I'm doing wrong.

Regards

Last edited by a moderator: May 6, 2017