Question

Assume that Ian sits on a freely rotating stool holding a bicycle wheel with its axle vertical so that it rotates in a counterclockwise direction when viewed from above. If Ian turns the wheel over, he will............

a. rotate clockwise because angular momentum is conserved.

b. rotate counterclockwise because angular momentum is conserved.

c. none of above

d. not rotate because the two torques cancel

e. not rotate because the angular momentum is lost.

Answer 1

Initial angular momentum is upward since the cycle wheel is rotating counterclockwise. When he turns the wheel over, his new angular momentum is pointing downward. But since the initial angular momentum is upward, his net final angular momentum must be upward to conserve angular momentum. To balance this, he must rotate counterclockwise so as to counter the downward angular momentum and give a net upward angular momentum.
So the answer is b. rotate counterclockwise because the angular momentum is conserved.