Problem statementSolution video |

**DISCUSSION THREAD**

**Discussion**

The animation above shows the motion of the mechanism over a range of input angles of link OA. For a given position, envision the location of the instant centers (ICs) for links AB and CD. Do the directions and magnitudes for the velocities of points B, C and D agree with the location of these ICs?

Shown below if a freeze-frame of the mechanism motion at the position for which you are asked to do analysis. From this figure, where are the two ICs for links CD and BC? In particular, how does the position of the IC for AB relate to the relative sizes of the speeds of points A, C and B? What is the angular velocity of link AB at this position?

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Is it possible that link AB does not have an instant center ? And if possible, does it mean that it is not rotating ?

Frederic is correct. At this instant, link AB is only translating

Frederic is correct. At this instant, link AB is only translating

A rigid body moving in a plane always has an instant center. The instant center might be at infinity, which is probably what you are thinking here. If it is at infinity, then the angular velocity of the body is zero. Why is that?

If link AB is only translating, how are we supposed to find the velocity for v_c and v_b?

If a rigid body is only translating, every single point on the body will have the same velocity. Points A, B, and C are all on the same rigid body, meaning they will all have the same velocity in the same direction. You can find v_A using v = omega * r and this will also give you v_B and v_C

How would AB only be translating if the points are moving at different speeds? You can also see that its rotating from the animation

Keep in mind that the "I" in IC stands for "instantaneous." At the "instant" shown in the frozen frame bar AB is in pure translation (since all points are moving with the same velocity). Immediately before and immediately after that time AB picks up a non-zero angular velocity, and AB is no longer in pure translation.