Practice Problem 2-2 (a)

Practice Problem 2-2 (a)

Given: A student weighing W = 120 lb rides a skateboard with the trucks (the part the wheels are mounted to) L = 32 inches apart. Assume that her feet are positioned inward of the wheels at equal distance of L/4, and she applies half of her weight at each point. Assume the board is simply supported.

Req'd: Determine the reaction force at the front truck R_A.

Sol'n: The FBD of the skateboard is first drawn. The reaction force at A can be calculated by setting the sum of the moments acting about B to zero:

SM_B = (R_A)(L) – (W/2)(3L/4) – (W/2)(L/4) = 0

Solving for R_A:

R_A = [(60 lb)(24 in.) + (60 lb)(8 in.)]/(32 in.)

R_A = 60 lb

Note: Both the load and geometry of this system is symmetric. Thus R_A = R_B = W/2.

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Updated: 05/24/2009 DJD