|
|
S E C T I O N 1 1 . 2 • Angular Momentum 341 Angular Momentum of a System of Particles In Section 9.6, we showed that Newton’s second law for a particle could be extended to This equation states that the net external force on a system of particles is equal to the where the vector sum is over all n particles in the system. Let us differentiate this equation with respect to time: d L tot dt # # i
d L i dt # # i
! i L tot # L 1 % L 2 % & & & % L n # # i
L i #
F ext # d p tot dt Quick Quiz 11.3 Recall the skater described at the beginning of this sec- tion. Let her mass be m. What would be her angular momentum relative to the pole at Quick Quiz 11.4 Consider again the skater in Quick Quiz 11.3. What would be her angular momentum relative to the pole at the instant she is a distance d from A particle moves in the xy plane in a circular path of radius v. Solution The linear momentum of the particle is always L is given by where we have used ! # 90° because v is perpendicular to r. This value of L is constant because all three factors on the The direction of L also is constant, even though the di- rection of p # mv keeps changing. You can visualize this by applying the right-hand rule to find the direction of L # r " p # mr " v in Figure 11.5. Your thumb points upward L Hence, we can write the vector expression L # (mvr)ˆk. If the particle were to move clockwise, L would point downward and into the page. A particle in uniform circular motion has a constant angular momentum about an axis through the center of its path. mvr L # mvr sin 90( # Example 11.3 Angular Momentum of a Particle in Circular Motion x y m v O r Figure 11.5 (Example 11.3) A particle moving in a circle of ra- dius r has an angular momentum about O that has magnitude mvr. The vector L # r " p points out of the diagram. ▲ PITFALL PREVENTION 11.2 Is Rotation Necessary for Notice that we can define angu- |