Conceptual questions

The following are links to hints and suggestions on the conceptual problems that appear at the end of each chapter in the lecture book. Note that for virtually all of these questions we are not expecting you to just “know” the answer based on your intuition. In each case, instead, you need to perform some simple analysis first, and from this analysis, draw conclusions on the answers.

Our hope is that through such conceptual questions your intuition on dynamics will develop beyond just remembering methods and procedures. You will see similar conceptual questions on your quizzes and exams in this course.

Use these hints to guide you in your conclusions. Please post your comments and questions below regarding these conceptual questions.

Chapter 1
Chapter 2
Chapter 3
Chapter 4
Chapter 5
Chapter 6

9 thoughts on “Conceptual questions”

  1. I was wondering for C1.7 if the answer is decreasing in class we got increasing but when I am redoing it I am getting decreasing when I am projecting the vectors.

    1. Also, for C1.10 there is a note saying the relative velocity equation doesn’t hold because one plane is rotating, but I thought those equations always held. So is ‘both (a) and (b)’ not the correct answer?

      1. For C1.10:

        You are correct in that v_A/B is always the velocity of A relative to B, and v_B/A is always the velocity of B relative to B.

        However, v_A/B is equal to to velocity of A AS SEEN BY OBSERVER B only if B is not rotating, and v_B/A is equal to to velocity of B AS SEEN BY OBSERVER A only if A is not rotating. Since B is rotating as it moves on its path, then v_A/B is NOT equal to to velocity of A as seen by observer B.

        Does that help?

    2. For C.17:

      If you take the dot product of acceleration a_P with the path unit vector e_t you will get the negative number of -(6)*(10)/sqrt(40). Since that dot product represents the rate of change of speed, the speed of P is DECREASING. Therefore your answer is consistent with that result.

  2. If a no-slip disk is in contact with an accelerating floor or moving platform, does the acceleration of the contact point stop being purely toward the center and instead have both x and y components? In some problems, even when the disk’s center is accelerating, the contact point’s acceleration still points toward the center. In what situations does the no-slip contact point have two acceleration components instead of just a radial (toward-center) one?

    1. The answer to your first question is YES.

      Do not try to memorize results; instead, focus on understanding why these results occur. Remember to use the rigid body kinematics equations – they will give you the answer.

  3. For C4.18, why is the linear momentum in the x-direction not conserved for A+B? There are no forces imparted in the x direction at the instant of the collision. Does it have something to do with the movement of B being constrained along a circular path, which changes the linear momentum to angular? This problem looks like C4.15, which says that angular momentum would be conserved for this scenario, disproving that explanation.

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  4. For question C4.18, I’m confused why momentum is not conserved in the x-direction for the A+B system. According to the FBD given, the only force acting on the system is the applied by the rigid bar on the system, which is only in the negative y-direction. Since the sum of forces in the x-direction is zero, linear momentum should be conserved in the x-direction, yet the answer key says that linear momentum is not conserved in the x-direction.

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