Rotational motion 3
1 The center of mass of the three lead balls shown is at....
a ... b ... c ... d
2 The total moment of inertia of the three lead balls rotating about the center of the large ball is calculated to be approximately....
3 The actual measured moment of inertia of the three lead balls rotating about the center of the large mass is found to be 81.25 ± 0.05 units. The value is higher than the approximate calculation because....
a The formula for moment of inertia is always approximate.
b None of the masses are point masses.
c The large mass is not a point mass.
d The two smaller masses are not point masses.
Questions 4 -10 apply to a wheel made from expanded polystyrene (which has very little mass) and six 250 g lead balls as shown below.
Which of the following statements is incorrect? The center of mass of the wheel can be found by....
a.... balancing the wheel in a horizontal position on the point of a pencil.
b.... measuring x and y for each mass and calculating the position of the center of mass using any two axes.
c.... hanging the wheel from different points and noting where the verticals cross.
d.... calculating the moment of inertia.
5 It is important to ensure that the axle passes through the center of mass so that....
a ... we can apply a constant torque.
b ... the wheel will have constant angular acceleration (a).
c ... we can calculate the moment of inertia.
d ... we can find the final angular KE.
6 When finding the torque applied by a hanging weight it is necessary to....
a.... use a micrometer to find the radius of the axle.
b.... prevent the axle slipping on the rails.
c.... measure the radius of the axle accurately.
d.... find the radius of the axle in cm.
7 When applying a torque to the axle by hanging a weight it is best to wind the cotton in a single layer on the axle to ensure that....
a.... the torque is constant.
b.... you do not get the cotton tangled.
c.... the set up looks neat.
d.... the weight does not swing from side to side.
8 Why has the polystyrene been cut away as much as possible?
a To reduce its mass to a minimum.
b To make the wheel look artistic.
c To save on materials.
d To balance the wheel.
9 Which statement is incorrect?
a The total torque on the wheel is given by mgr where mg is the weight of the hanging mass.
b The final kinetic energy of the wheel is equal to the PE lost.
c The wheel will accelerate (a) more quickly if it slips on the rails.
d The acceleration a is equal to ra where r is the radius of the axle.
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