11 thoughts on “Exams”

1. On 1B for Spring 2018 Exam, why is the forces on AB and BC 100 and 15, and -100, -15. Wouldn't it be -100 for ABx and 100 for BCx

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   1. It is the force acting on the member at the point, not the force the member is exerting on the point.

      For member BC, there needs to be a force in the direction opposite to the 200N force to keep it in equilibrium. So the force exerted on it, at point B is -100N.

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2. In question 2B for spring 2018, shouldn't h decrease?

   It is established in the first part of this question that h=3.81m.

   When theta=0, for slipping and tilting to be true at the same time, sum of Moments = mg(1)-hP = 0, as well as sum of Forces in X = friction-P = (0.3)mg - P = 0.

   That means P = 0.3mg
   => mg = 0.3mgh
   => 1=0.3h
   => h=3.33 < 3.81

   Which means h should decrease, as opposed to the indicated increase in the answer key.

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   1. I also got the same answer as you. I am not sure why it would be increasing because indeed h=3.33 (when theta=0) < 3.81 (when theta=10). Have you gotten it confirmed by any professor if the answer is correct?

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3. How was mg determined to be 100 for question 2B on spring 2018

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   1. I'm pretty sure it was not defined as 100, it was just left as mg. The answer for P is not in the answer key... but it is 0.122*mg

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4. Can anyone explain how they got an answer for 2A on the SP18 final?

   1

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   1. sum of Moments about A = (a+b)F-N+c(F_f) = 1.5F-N+0.1(mu)N = 1.5F-N+(0.1)(0.3)N = 0
      =>1.5F = 1.03N
      =>N = 1.456F
      =>F_f = (mu)N = 0.436F

      sum of Moments about B = r(F_f)-rW = 0
      =>W = F_f
      =>10 = 0.436F
      => F = 22.8N

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5. In Question 3D of Spring 2018,

   Is the maximum tensile stress 6400 because the normal stress at B is -6400, though I have seen nothing that substantiates this claim.

   Or is there some other reason?

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   1. No, the reason depends on what your y value is when you use the (sig) = -My/I equation. For solving (sig)M, y is +1in while for solving (sig)max, y is -1 because the maximum tensile stress will be at the bottom of the cantilever beam. We also know this because the moment is positive at section B.

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6. Sp16 4D, how do you know the location of x'?

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