Standard convention for rotation:
CCW is positive, CW is negative
matches the xyz coordinate if +z comes out toward us
(+x is toward the right, +y is up)
Examples:
if torque is in +z direction, it will cause angular acceleration
in the +z direction
this gives an increase in angular velocity in the +z direction
if the object is initially at rest, it will start to spin CCW
Q 12: The angular velocity of a bicycle wheel points north while
its angular acceleration points south.
(a) In which direction is the bicycle moving?
(b) Is it speeding up or slowing down?
Problems with torque
Lift the metal rod
volunteer lifts at the center
draw a force diagram
choose an axis and sum the torques
choose another axis and sum the torques
another volunteer lifts at the end
draw a force diagram
choose an axis and sum the torques
Exercise 12.27
A phonograph tonearm assembly must be counterweighted to prevent the
needle from scratching the record.
Details:
cartridge mass = 7 g, 20 cm from the pivot
tonearm mass = 12 g, 26 cm long
counterweight mass = 60 g, x from the pivot
Find the position of the counterweight for the assembly to be balanced.