124s09-exam1

# Calculate the net torque about point o for the two

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Calculate the net torque about point O for the two forces applied as in Figure 1. The rod and both forces are in the plane of the page. Take positive torques to be counterclockwise. a) 12 N · m b) -12 N · m c) 28 N · m d) -28 N · m e) -52 N · m 2. An artist constructs the mobile shown in Figure 2. In the illustrated con- figuration, the mobile is perfectly balanced. If the artist decides to make the star twice as massive, and not change the length of any crossbar or the location of any object, what does she have to do with the mass of the smiley face to keep the mobile in perfect balance? Note that she may have to change masses of other objects to keep the entire structure balanced. a) make it eight times more mas- sive b) make it four times more mas- sive c) make it twice times more mas- sive d) nothing e) impossible to tell 3. The flywheel of an engine has moment of inertia 3.0 kg/m 2 about its rotation axis. What constant torque is required to bring it up to an angular speed of 10rad/s in 10s, starting from rest? a) 1.5 N · m b) 3 N · m c) 6 N · m d) 30 N · m e) 150 N · m

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4. A 1.0kg solid disc 1m in diameter is rolling without slipping on a horizontal floor at a steady 2rad/s. What is the total kinetic energy of the disc? 5. Several objects start at rest at the same height and roll without slipping down the left part of the track shown in Figure 3. The right part of the track is frictionless. Which object climbs higher along this frictionless part? 6. A small (point-like) block on a frictionless, horizontal surface has a mass of 1kg. It is attached to a massless cord passing through a hole in the surface (see Figure 4). The block is originally revolving at a distance of 0.3 m from the hole with an angular speed of 1rad/s.
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