Chapter 9 | Linear Momentum and Collisions 405 9 | LINEAR MOMENTUM AND COLLISIONS Figure 9.1 The concepts of impulse, momentum, and center of mass are crucial for a major-league baseball player to successfully get a hit. If he misjudges these quantities, he might break his bat instead. (credit: modification of work by “Cathy T”/Flickr) Chapter Outline 9.1 Linear Momentum 9.2 Impulse and Collisions 9.3 Conservation of Linear Momentum 9.4 Types of Collisions 9.5 Collisions in Multiple Dimensions 9.6 Center of Mass 9.7 Rocket Propulsion Introduction The concepts of work, energy, and the work-energy theorem are valuable for two primary reasons: First, they are powerful computational tools, making it much easier to analyze complex physical systems than is possible using Newton’s laws directly (for example, systems with nonconstant forces); and second, the observation that the total energy of a closed system is conserved means that the system can only evolve in ways that are consistent with energy conservation. In other words, a system cannot evolve randomly; it can only change in ways that conserve energy. In this chapter, we develop and define another conserved quantity, called linear momentum, and another relationship (the impulse-momentum theorem), which will put an additional constraint on how a system evolves in time. Conservation of momentum is useful for understanding collisions, such as that shown in the above image. It is just as powerful, just as important, and just as useful as conservation of energy and the work-energy theorem. 406 Chapter 9 | Linear Momentum and Collisions 9.1 | Linear Momentum Learning Objectives By the end of this section, you will be able to: • Explain what momentum is, physically • Calculate the momentum of a moving object Our study of kinetic energy showed that a complete understanding of an object’s motion must include both its mass and its velocity ( K = (1/2)mv2 ). However, as powerful as this concept is, it does not include any information about the direction of the moving object’s velocity vector. We’ll now define a physical quantity that includes direction. Like kinetic energy, this quantity includes both mass and velocity; like kinetic energy, it is a way of characterizing the “quantity of motion” of an object. It is given the name momentum (from the Latin word movimentum, meaning “movement”), and it is represented by the symbol p. Momentum The momentum p of an object is the product of its mass and its velocity: →p = m →v . (9.1) Figure 9.2 The velocity and momentum vectors for the ball are in the same direction. The mass of the ball is about 0.5 kg, so the momentum vector is about half the length of the velocity vector because momentum is velocity time mass. (credit: modification of work by Ben Sutherland) This OpenStax book is available for free at http://cnx.org/content/col12031/1.5 Chapter 9 | Linear Momentum and Collisions 407 As shown in Figure 9.2, momentum is a vector quantity (since velocity is). This is one of the things that makes momentum useful and not a duplication of kinetic energy. It is perhaps most useful when determining whether an object’s motion is difficult to change (Figure 9.3) or easy to change (Figure 9.4). Figure 9.3 This supertanker transports a huge mass of oil; as a consequence, it takes a long time for a force to change its (comparatively small) velocity. (credit: modification of work by “the_tahoe_guy”/Flickr) Figure 9.4 Gas molecules can have very large velocities, but these velocities change nearly instantaneously when they collide with the container walls or with each other. This is primarily because their masses are so tiny. Unlike kinetic energy, momentum depends equally on an object’s mass and velocity. For example, as you will learn when you study thermodynamics, the average speed of an air molecule at room temperature is approximately 500 m/s, with an average molecular mass of 6 × 10−25 kg ; its momentum is thus p ⎛ −25 ⎞⎛ m⎞ −22 kg · m molecule = ⎝6 × 10 kg⎠⎝500 s ⎠ = 3 × 10 s . For comparison, a typical automobile might have a speed of only 15 m/s, but a mass of 1400 kg, giving it a momentum of ⎛ ⎞⎛ ⎞ kg · m p ⎝ ⎠ m car = 1400 kg ⎝15 s ⎠ = 21,000 s . These momenta are different by 27 orders of magnitude, or a factor of a billion billion billion! 408 Chapter 9 | Linear Momentum and Collisions 9.2 | Impulse and Collisions Learning Objectives By the end of this section, you will be able to: • Explain what an impulse is, physically • Describe what an impulse does • Relate impulses to collisions • Apply the impulse-momentum theorem to solve problems We have defined momentum to be the product of mass and velocity. Therefore, if an object’s velocity should change (due to the application of a force on the object), then necessarily, its momentum changes as well. This indicates a connection between momentum and force. The purpose of this section is to explore and describe that connection. Suppose you apply a force on a free object for some amount of time. Clearly, the larger the force, the larger the object’s change of momentum will be. Alternatively, the more time you spend applying this force, again the larger the change of momentum will be, as depicted in Figure 9.5. The amount by which the object’s motion changes is therefore proportional to the magnitude of the force, and also to the time interval over which the force is applied. Figure 9.5 The change in momentum of an object is proportional to the length of time during which the force is applied. If a force is exerted on the lower ball for twice as long as on the upper ball, then the change in the momentum of the lower ball is twice that of the upper ball. Mathematically, if a quantity is proportional to two (or more) things, then it is proportional to the product of those things. → The product of a force and a time interval (over which that force acts) is called impulse, and is given the symbol J . Impulse → Let F (t) be the force applied to an object over some differential time interval dt (Figure 9.6). The resulting impulse on the object is defined as → → (9.2) d J ≡ F (t)dt. This OpenStax book is available for free at http://cnx.org/content/col12031/1.5 Chapter 9 | Linear Momentum and Collisions 409 Figure 9.6 A force applied by a tennis racquet to a tennis ball over a time interval generates an impulse acting on the ball. The total impulse over the interval is tf − ti t t (9.3) → f → → f → J = ∫ d J or J ≡ ∫ F (t)dt. ti ti Equation 9.2 and Equation 9.3 together say that when a force is applied for an infinitesimal time interval dt, it causes → an infinitesimal impulse d J , and the total impulse given to the object is defined to be the sum (integral) of all these infinitesimal impulses. To calculate the impulse using Equation 9.3, we need to know the force function F(t), which we often don’t. However, a result from calculus is useful here: Recall that the average value of a function over some interval is calculated by xf f x 1 f x dx ( )ave = x∫ ( ) Δ xi where . Applying this to the time-dependent force function, we obtain Δx = xf − xi t (9.4) → f → F = 1 F (t)dt. ave Δt∫ ti Therefore, from Equation 9.3, → → (9.5) J = F ave Δt. The idea here is that you can calculate the impulse on the object even if you don’t know the details of the force as a function of time; you only need the average force. In fact, though, the process is usually reversed: You determine the impulse (by measurement or calculation) and then calculate the average force that caused that impulse. 410 Chapter 9 | Linear Momentum and Collisions → To calculate the impulse, a useful result follows from writing the force in Equation 9.3 as F (t) = m →a (t) : tf tf → → → ⎡ → → ⎤ J = ∫ F (t)dt = m∫ a (t)dt = m⎣ v (tf) − v i⎦. ti ti → → → For a constant force F ave = F = m a , this simplifies to → → → → → → J = m a Δt = m v f − m v i = m( v f − v i). That is, → (9.6) J = mΔ →v . Note that the integral form, Equation 9.3, applies to constant forces as well; in that case, since the force is independent of time, it comes out of the integral, which can then be trivially evaluated. Example 9.1 The Arizona Meteor Crater Approximately 50,000 years ago, a large (radius of 25 m) iron-nickel meteorite collided with Earth at an estimated speed of 1.28 × 104 m/s in what is now the northern Arizona desert, in the United States. The impact produced a crater that is still visible today (Figure 9.7); it is approximately 1200 m (three-quarters of a mile) in diameter, 170 m deep, and has a rim that rises 45 m above the surrounding desert plain. Iron-nickel meteorites typically have a density of ρ = 7970 kg/m3 . Use impulse considerations to estimate the average force and the maximum force that the meteor applied to Earth during the impact. Figure 9.7 The Arizona Meteor Crater in Flagstaff, Arizona (often referred to as the Barringer Crater after the person who first suggested its origin and whose family owns the land).