Angular momentum Conservation Gyroscopes PHYS116 A02 101811 Lecture

Angular momentum, Conservation, Gyroscopes PHYS-116 A-02, 10/18/11, Lecture 14 Momchil Velkovsky

A solid bowling ball rolls down a ramp. Which of the following forces exerts a torque on the bowling ball about its center? ¡ ¡ ¡ A. the weight of the ball B. the normal force exerted by the ramp C. the friction force exerted by the ramp D. more than one of the above E. The answer depends on whether the ball rolls without slipping.

A solid bowling ball rolls down a ramp. Which of the following forces exerts a torque on the bowling ball about the point of contact with the ramp? ¡ ¡ ¡ A. the weight of the ball B. the normal force exerted by the ramp C. the friction force exerted by the ramp D. more than one of the above E. The answer depends on whether the ball rolls without slipping.

A lightweight string is wrapped several times around the rim of a small hoop. If the free end of the string is held in place and the hoop is released from rest, the string unwinds and the hoop descends. How does the tension in the string (T) compare to the weight of the hoop (w)? 1. 2. 3. 4. A. T = w B. T > w C. T < w D. not enough information given to decide

Don’t get confused: Rolling Friction is rarely considered

Angular Momentum This is always valid! The difficulty is to calculate L. If no net (external) torque, L is conserved! A special case: Axis of symmetry

A spinning figure skater pulls his arms in as he rotates on the ice. As he pulls his arms in, what happens to his angular momentum L and kinetic energy K? ¡ ¡ A. L and K both increase. B. L stays the same, K increases. C. L increases, K stays the same. D. L and K both stay the same.

This is how a car’s clutch works!

A 6. 0 -kg particle moves to the right at 4. 0 m/s as shown. The magnitude of its angular momentum about the point O is: 1. 2. 3. 4. 5. zero 288 kg × m 2/s 144 kg × m 2/s 249 kg × m 2/s

Angular momentum of a system orientation, falling cats ¡ ¡ http: //video. nationalgeographic. com/video/ player/animals/mammalsanimals/cats_domestic_ninelives. html Friction and rotating chair

Bullet in a bar L What is final w ? As we saw, the bullet has angular momentum wrt, the axis! No external torques! M m, v

Gyroscopic precession

A rotating flywheel df

Next time: Thursday 10/20/2011 Midterm 2 ! ¡ Same format as Midterm 1 ¡ Practice tests (with solutions) are posted in Lectures on OAK ¡ If you have questions: Practice session on Wednesday. (Come to my office, we’ll find a big room. ) ¡ For Tuesday 10/25 – read Ch. 12 ¡