Dr. SB's Topper Point

Q1. A turntable must spin at 33.3 rpm (3.49 rad/s) to play an old-fashioned vinyl record. How much torque must the motor deliver if the turntable is to reach its final angular speed in 2.0 revolutions, starting from rest? The turntable is a uniform disk of diameter 30.5 cm and mass 0.22 kg. Answers; 0.0012 N m

Q2. Four masses are arranged as shown. They are connected by rigid, mass less rods of lengths .75 m and .50 m. What torque must be applied to cause an angular acceleration of 0.75 rad/s squared about the axis shown? Answers ; 1.5 N m
masses are 4kg,5kg,3kg,2kg axis is parallel to 0.5 m rod.

Q3. A bucket of water with a mass of 2.0 kg is attached to a rope that is wound around a cylinder. The cylinder has a mass of 3.0 kg and is mounted horizontally on the frictionless bearing. The bucket is released from rest. a) find its speed after is has fallen through a distance of .80 m. What are b) the tension in the rope and c) the acceleration of the bucket?
Answers ; a) 3.0 m/s b) 8.4 N c) 5.6 m/s squared
Q4. A block moves at 5m/s in the positive x direction and hits an identical block, initially at rest. A small amount of gunpowder had been placed on one of the blocks. The explosion does not harm the blocks but it doubles their total kinetic energy. After the explosion the blocks move along the x axis and the incident block has a speed in m/s of:

Q5. A spoked wheel with a radius of 40.0 cm and a mass of 2.00 kg is mounted horizontally on frictionless bearings. JiaJun puts his .500 kg guinea pig on the outer edge of the wheel. The guinea pig begins to run along the edge of the wheel with a speed of 20.0 cm/s with respect to the ground. What is the angular velocity of the wheel? Assume the spokes of the wheel have negligible mass. Answers; 0.125 rad/s

Q6. A painter (mass 61 kg) is walking along a trestle, consisting of uniform plank (mass 20.0 kg, length 6.00 m) balanced on two sawhorses. Each sawhorse is placed 1.40 m from an end of the plank. A paint bucket (mass 4.0 kg, diameter 28 cm) is placed as close as possible to the right-hand edge of the plank while still having the whole bucket in contact with the plank. a) How close to the right hand edge of the plank can the painter walk before tipping the plank and spilling the paint? b) How close to the left hand edge can the same painter walk before causing the plank to tip? (Hint; as the painter walks towards the right hand edge of the plank and the plank starts to tip clockwise, what is the force acting upward on the plank from the left hand sawhorse support) Answers; a)0.96 from RH edge b) 0.58 m form LH edge

Q7. A bicycle has wheels of radius 0.28 m. Each wheel has a rotational inertia of 0.075 kg·m2 about its axle. The total mass of the bicycle including the wheels and the rider is 74 kg. When coasting at constant speed, what fraction of the total kinetic energy of the bicycle (including rider) is the rotational kinetic energy of the wheels?

Q8. Two ice skaters of = mass grab hands and spin in a circle once very four seconds. Their arms are each 0.76m long, and they each have a mass of 55.0 kg. How hard are they pulling on one another?