Sir Lost-a-Lot dons his armor and sets out from the castle on his trusty steed (
ID: 1461833 • Letter: S
Question
Sir Lost-a-Lot dons his armor and sets out from the castle on his trusty steed (see figure below). Usually, the drawbridge is lowed to a horizontal position so that the end of the bridge rests on the stone ledge. Unfortunately Lost-a-Lot's squire didn't lower the drawbridge far enough and stopped it at = 20.0° above the horizontal. The knight and his horse stop when their combined center of mass is d = 1.00 m from the end of the bridge. The uniform bridge is =8.50 m long and has a mass of 1 900 kg. The lift cable is attached to the bridge 5.00 m from the hinge at the castle end and to a point on the castle wall h = 12.0 m above the bridge. Lost-a-Lot's mass combined with his armor and steed is1 150 kg.
(a) Determine the tension in the cable.
N
(b) Determine the horizontal force component acting on the bridge at the hinge.
(c) Determine the vertical force component acting on the bridge at the hinge.
magnitude
Explanation / Answer
We need the angle the cable makes with the bridge.
The angle between the bridge and the wall (vertical) is 70º.
The law of cosines gives us
C² = 5² + 12² - 2*5*12*cos70º = 128
C = 11.3 m length of cable
Now we can use the law of sines to get the angle between the bridge and cable:
sin / 12m = sin70º / 11.3m
sin = 0.997
= arcsin0.997 = 85.46º
(a) Now sum the moments about the hinge:
M = 0 = (1900kg * ½*8.5m + 1150kg * 7.5m)*9.8m/s²*cos20º - T*sin85.46º*5m
where T is the cable tension. Solving, find
T = 30,842 N
(b) ß = 180º - 70º - 85.46º = 24.54º angle between cable and wall
horizontal force( Fx)
Fx = T*sin24.54
Fx= 12,809 N
away from wall
(c) vertical force Fy = (1900 + 1150)kg * 9.8m/s² - T * cos24.54 = 1,833 N
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