Problem statementSolution video |

**DISCUSSION THREAD**

**Discussion**

Note that the plate rotates about point O. Therefore, O is the center of the circular paths of points A and B. From the animation above, we see that the velocities of A and B are tangent to their circular paths, as expected. The accelerations of A and B are NOT perpendicular to the paths of A and B since the speeds of A and B are increasing in time (and consequently, A and B each have positive tangential components of acceleration).

Initially, the acceleration for these two points is nearly aligned with velocity, since the speeds are small and therefore the centripetal components of acceleration are small. Near the end of the first revolution of the plate, the speeds have increased to the point where the centripetal components of acceleration dominate, and acceleration is nearly perpendicular to the path.

**Solution hints**

For Part a) of this problem, it is recommended that you use the rigid body kinematics equations using point O as the reference point, since the velocity and acceleration of O are zero. That is, you should use v_B = v_O + Ω x r_B/O and a_B = a_O + Ω_dot x r_B/O - Ω^2*r_B/O. Repeat the process for finding the velocity and acceleration of A.

For Part b) of this problem, it is recommended that you use the rigid body kinematics equations with point A first. This will give you the equations needed to find Ω_dot. Then, use the rigid body kinematics equations to find the acceleration of B.

Any questions?? Please ask/answer questions regarding this homework problem through the "Leave a Comment" link above.

For part a, should I treat corners a and b as separate points and find their individual velocity and acceleration? Or should I find the velocity and acceleration of edge AB?

The former. "Points" A and B have velocity and acceleration. Edges ("lines") do NOT have velocity and acceleration.

How do we find a numerical value for omega_dot when we do not have a numerical value for the given acceleration of A in part B?

Andrew: Write down the vector equation of the acceleration of A referenced to the fixed point O. You know that a_Ax = 0. This vector equation gives two scalar equations in terms of two unknowns: a_Ay and Omega_dot. Solve.