|
|
one particle is that from the other particle and we can categorize this as a situation in F 12 ! " F 21 . We can express this condition as Let us further analyze this situation by incorporating Newton’s second law. Over some time interval, the interacting particles in the system will accelerate. Thus, replac- a gives Now we replace the acceleration with its definition from Equation 4.5: If the masses m 1 and m 2 are constant, we can bring them into the derivatives, which gives (9.1) To finalize this discussion, note that the derivative of the sum m 1 v 1 # m 2 v 2 with respect to time is zero. Consequently, this sum must be constant. We learn from this discussion v for a particle is important, in that the sum of these quantities for an isolated system is conserved. We call this quantity linear momentum: d dt (m 1 v 1 # m 2 v 2 ) ! 0 d(m 1 v 1 ) dt # d(m 2 v 2 ) dt ! 0 m 1
d v 1 dt # m 2
d v 2 dt ! 0 m 1 a 1 # m 2 a 2 ! 0 F 21 # F 12 ! 0 S E C T I O N 9 . 1 • Linear Momentum and Its Conservation 253 v 2 m 2 m 1 F 21 F 12 v 1 Figure 9.1 Two particles interact with each other. According to Newton’s third law, we must have F 12 ! " F 21 . The linear momentum of a particle or an object that can be modeled as a particle of mass m moving with a velocity v is defined to be the product of the mass and velocity: (9.2) p ! m v Linear momentum is a vector quantity because it equals the product of a scalar quan- v. Its direction is along v, it has dimensions ML/T, and its SI unit is kg · m/s. If a particle is moving in an arbitrary direction, p must have three components, and Equation 9.2 is equivalent to the component equations As you can see from its definition, the concept of momentum 1 provides a quantitative distinction between heavy and light particles moving at the same velocity. For example, v quantity of motion; this is perhaps a more graphic description than our present-day word momentum, which Using Newton’s second law of motion, we can relate the linear momentum of a par- ticle to the resultant force acting on the particle. We start with Newton’s second law " F ! ma ! m d v dt p x ! mv x
p y ! mv y
p z ! mv z 1 In this chapter, the terms momentum and linear momentum have the same meaning. Later, in Chapter 11, we shall use the term angular momentum when dealing with rotational motion. Definition of linear momentum of a particle |