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P211 Final Sample 1
17) A mass of 5 kg is hanging from the beam shown in the figure, at a distance of 2 m from a wall.
The beam is connected to the wall via a hinge free to rotate frictionlessly. The beam has a uniform
mass distribution, a length of 5 m and a total mass of 20 kg. What is the magnitude of the force F at
point A necessary for the system to be in equilibrium? (Hint: there must be no net force on the
system or torque about the hinge.)
5 m
A
F
20 kg
5 kg
2 m
Hinge
Wall
A) 39N B) 78 N C) 103 N
D) 118 N E) 490 N
18) A skater extends her arms horizontally, holding a 2 kg mass in each hand. She is rotating about a
vertical axis with an angular velocity of 1 rev/s. If she pulls her hands in towards the center of her
body, approximately what will the final angular velocity be if her own moment of inertia remains
approximately constant at 5 kg m 2 , and the distance of the masses from the axis changes from 1 m to
0.2 m?
A) 12 rev/s B) 9 rev/s C) 6 rev/s
D) 3 rev/s E) 1.7 rev/s
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