Page 439 - College Physics For AP Courses
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Chapter 10 | Rotational Motion and Angular Momentum 427
Before the collision, then, linear momentum is
After the collision, the disk and the stick's center of mass move in the same direction. The total linear momentum is that of
the disk moving at a new velocity plus that of the stick's center of mass, which moves at half this speed because . Thus,
(10.133)
(10.134)
(10.135)
(10.136)
(10.137)
Gathering similar terms in the equation yields,
so that
Substituting known values into the equation,
Discussion
First note that the kinetic energy is less after the collision, as predicted, because the collision is inelastic. More surprising is that the momentum after the collision is actually greater than before the collision. This result can be understood if you consider how the nail affects the stick and vice versa. Apparently, the stick pushes backward on the nail when first struck by the disk. The nail's reaction (consistent with Newton's third law) is to push forward on the stick, imparting momentum to it in the same direction in which the disk was initially moving, thereby increasing the momentum of the system.
Applying the Science Practices: Rotational Collisions
When the disk in Example 10.15 strikes the stick, it exerts a torque on the stick. It is this torque that changes the angular momentum of the stick. A greater torque would produce a greater increase in angular momentum. As you saw in Example
10.12, the relationship between net torque and angular momentum can be expressed as , where represents
the change in angular momentum and represents the time interval that it took for the angular momentum to change.
How can you test this? Design an experiment in which you apply different measurable torques to a simple system. The torque applied to the stick can be varied by changing the mass of the disk, the initial velocity of the disk, or the radius of the impact of the disk on the stick. How will you measure changes to angular momentum? Recall that angular momentum is defined as .
The above example has other implications. For example, what would happen if the disk hit very close to the nail? Obviously, a force would be exerted on the nail in the forward direction. So, when the stick is struck at the end farthest from the nail, a backward force is exerted on the nail, and when it is hit at the end nearest the nail, a forward force is exerted on the nail. Thus, striking it at a certain point in between produces no force on the nail. This intermediate point is known as the percussion point.
An analogous situation occurs in tennis as seen in Figure 10.27. If you hit a ball with the end of your racquet, the handle is pulled away from your hand. If you hit a ball much farther down, for example, on the shaft of the racquet, the handle is pushed into your palm. And if you hit the ball at the racquet's percussion point (what some people call the “sweet spot”), then little or no force is exerted on your hand, and there is less vibration, reducing chances of a tennis elbow. The same effect occurs for a baseball bat.
