Two horses pull a large crate of negligible mass at constant speed across a barn
ID: 584756 • Letter: T
Question
Two horses pull a large crate of negligible mass at constant speed across a barn floor by means of two light steel cables. A large box of mass 240 kg sits on the crate (see the figure below, which shows a top-down view of the crate and box). As the horses pull, the cables are parallel to the horizontal floor. The coefficient of friction between the crate and the barn floor is 0.25. (a) What is the work done by each horse if the box is moved a distance of 21 m? kJ (b) What is the tension in each cable if the angle between each and the direction the crate moves is 16°? kN
Explanation / Answer
m = 240 kg
d = 21 m
a) if at constant speed the total applied force of horses = the force of kinetic friction
the force of kinetic friction = (0.25)(Normal force)
Normal force = mg = (240)(9.81) = 2354.4 N
force of kinetic friction = force of horses = (0.25)(2354.4) = 588.6 N
total work done by horses = (588.6)(21) = 12360.6 J
work done by each horse = 12360.6/2 = 6180.3 J = 6.1803 kJ
b) The speed is constant , then a = 0 . So:
2 T cos16° = 0.25*240*9.8
T = 306.16 N = 0.306 kN
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