Engineering Mechanics Dynamics Fifth Edition Bedford Fowler Solutions Manual -

[ v_B = \frac{v_A}{\cos\theta} ]

Thus: Rope from fixed pulley to A shortens at rate ( v_A ). Rope from left fixed point to B lengthens at rate ( v_B \cos\theta ). Since total rope length constant: ( v_A = v_B \cos\theta ).

Wait, check: If A moves down 1 m, rope segment from fixed pulley to A shortens by 1 m. That rope length change must come from two places: (1) horizontal movement of B, (2) change in diagonal length from left fixed point to B. That diagonal length change rate = ( v_B \cos\theta ) (because only horizontal motion of B changes the diagonal length at rate ( v_B \cos\theta )). [ v_B = \frac{v_A}{\cos\theta} ] Thus: Rope from

This example focuses on a common but subtle topic: and relative velocity , which often trips students up. Sample Problem (Inspired by Bedford & Fowler, Ch. 2-3) Problem: Block A is pulled down the inclined plane at a constant speed ( v_A = 2 \text{ m/s} ). The rope system shown (a single continuous rope, fixed at the top left, passing through a movable pulley attached to block B, and then down to block A) causes block B to move horizontally. Determine the velocity of block B when the rope segment between the fixed pulley and block B makes an angle ( \theta = 30^\circ ) with the horizontal. The rope is always taut and inextensible.

Constraint: Total rope length ( L = \underbrace{y_B} {\text{horizontal top left to B}} + \underbrace{\sqrt{y_B^2 + H^2}} {\text{diagonal from B up to fixed pulley?}} ) — This gets messy. Let's do the : Two movable pulleys. Wait, check: If A moves down 1 m,

For ( \theta = 30^\circ ), ( \cos 30^\circ = 0.866 ):

Therefore:

Let ( s_A ) = distance of A along incline from fixed pulley at top right (positive down incline). Let ( y_B ) = horizontal distance of B from left fixed anchor (positive right).