Homework H.4.J

HINTS

STEP 1 - FBD: Draw a SINGLE free body diagram (FBD) of the system including Block A, Block B and the cable. From this, determine which forces do work on this system.
STEP 2 - Kinetics:  Write down the work/energy equation. Determine the work done by the forces that you identified  above in STEP 1.
STEP 3 - Kinematics: Review the constrained motion kinematics from Section 1.D of the course lecture book. To this end, you will write down an expression for the length of the cable in terms of sA, sB and constants. Differentiate this expression to relate the speeds of A and B.
STEP 4 - Solve

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10 thoughts on “Homework H.4.J”

1. ecasetti says:

How do we solve for s_B1 and s_B2? Because we need to know it for the initial and final potential energy.

2. CMK says:

Keep in mind that you need to use only the CHANGE in potential energy for the work energy equation. Typically, we define either the initial or final positions as DATUM lines for calculating the gravitational potentials. For this problem, you see that the change in potential energy for Block A is zero. For Block B, use the known change in position for A to calculate the positive change in potential for B.

Does this help?

3. Chick-fil-A Official says:

Is the velocity of block B zero when vA1 = 15m/s? If not, how could we get the velocity of block B in phase 1.

1. CMK says:

You can use your kinematics equation from Step 3. Simply set s_A = 0 in that equation, and you will find that v_B = 0.

4. stahchem says:

Can we assume that the pulley has a negligible diameter?

1. CMK says:

Yes, assume that the pulley is small enough that they amount of wrap of cable around the pulley is small compared to the motion of A and B.

5. Yes says:

do we just use the initial starting point of B as the Datum Line

1. Yes says:

are we allowed to assume B and A start at the same point and so Sb2 - h = Sa2

2. Jack Christiansen says:

I believe so, because only the change in height of B matters.