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Imagine rolling a bowling ball down one lane at a bowling alley and rollinga playground ball down another lane at the same speed. The more-massivebowling ball, shown in Figure 6-1, will exert more force on the pins becausethe bowling ball has more momentum than the playground ball. When wethink of a massive object moving at a high velocity, we often say that theobject has a large momentum. A less massive object with the same velocity hasa smaller momentum.On the other hand, a small object moving with a very high velocity has alarge momentum. One example of this is hailstones falling from very highclouds. By the time they reach Earth, they can have enough momentum tohurt you or cause serious damage to cars and buildings.Momentum is so fundamental inNewton’s mechanics that Newtoncalled it simply “quantity of motion.”The symbol for momentum, p,comes from Leibniz’s use of the termprogress, defined as “the quantity ofmotion with which a body proceedsin a certain direction.”SAMPLE PROBLEM 6AMomentumPROBLEMA 2250 kg pickup truck has a velocity of 25 m/s to the east. What is themomentum of the truck?SOLUTIONGiven: m = 2250 kg v = 25 m/s to the eastUnknown: p = ?Use the momentum equation from page 208.p = mv = (2250 kg)(25 m/s)p = 5.6 × 10 4 kg •m/s to the eastCALCULATOR SOLUTIONYour calculator will give you theanswer 56250 for the momentum.The value for the velocity has only twosignificant figures, so the answermust be reported as 5.6 × 10 4 .MomentumPRACTICE 6A1. An ostrich with a mass of 146 kg is running to the right with a velocity of17 m/s. Find the momentum of the ostrich.2. A 21 kg child is riding a 5.9 kg bike with a velocity of 4.5 m/s to the northwest.a. What is the total momentum of the child and the bike together?b. What is the momentum of the child?c. What is the momentum of the bike?3. What velocity must a car with a mass of 1210 kg have in order to havethe same momentum as the pickup truck in Sample Problem 6A?Copyright © by Holt, Rinehart and Winston. All rights reserved.Momentum and Collisions209
Figure 6-2When the ball is moving very fast,the player must exert a large forceover a short time to change theball’s momentum and quickly bringthe ball to a stop.A change in momentum takes force and timeFigure 6-2 shows a player stopping a moving soccer ball. In a given time interval,he must exert more force to stop a fast ball than to stop a ball that is movingmore slowly. Now imagine a toy truck and a real dump truck starting fromrest and rolling down the same hill at the same time. They would accelerate atthe same rate, so their velocity at any instant would be the same, but it wouldtake much more force to stop the massive dump truck than to stop the toytruck in the same time interval. You have probably also noticed that a ballmoving very fast stings your hands when you catch it, while a slow-movingball causes no discomfort when you catch it.From examples like these, we see that momentum is closely related to force.In fact, when Newton first expressed his second law mathematically, he wroteit not as F = ma, but in the following form.F = ⎯ ∆ ⎯p∆tchange in momentumforce = ⎯⎯⎯time intervalWe can rearrange this equation to find the change in momentum in termsof the net external force and the time interval required to make this change.IMPULSE-MOMENTUM THEOREMF∆t =∆p or F∆t =∆p = mv f − mv iforce × time interval = change in momentumNSTATOPIC: MomentumGO TO: www.scilinks.orgsciLINKS CODE: HF2061impulsefor a constant external force, theproduct of the force and the timeover which it acts on an objectThis equation states that a net external force, F, applied to an object for acertain time interval, ∆t, will cause a change in the object’s momentum equalto the product of the force and the time interval. In simple terms, a small forceacting for a long time can produce the same change in momentum as a largeforce acting for a short time. In this book, all forces exerted on an object areassumed to be constant unless otherwise stated.The expression F∆t =∆p is called the impulse-momentum theorem. Theterm on the left side of the equation, F∆t, is called the impulse of the force Ffor the time interval ∆t.The equation F∆t =∆p explains why follow-through is important in somany sports, from karate and billiards to softball and croquet. For example,when a batter hits a ball, the ball will experience a greater change in momentumif the batter follows through and keeps the bat in contact with the ball fora longer time. Follow through is also important in many everyday activities,such as pushing a shopping cart or moving furniture. Extending the timeinterval over which a constant force is applied allows a smaller force to cause agreater change in momentum than would result if the force were applied for avery short time.210Chapter 6Copyright © by Holt, Rinehart and Winston. All rights reserved.
Page 1 and 2: Copyright © by Holt, Rinehart and
Page 3: 6-1Momentum and impulse6-1 SECTION
Page 7 and 8: Stopping distances48 km/h (30 mi/h)
Page 9 and 10: (a)(b)Now consider a falling egg. W
Page 11 and 12: NSTATOPIC: RocketryGO TO: www.scili
Page 13 and 14: SAMPLE PROBLEM 6DConservation of mo
Page 15 and 16: F 1 is the force that m 2 exerts on
Page 17 and 18: 6-3Elastic and inelastic collisions
Page 19 and 20: PRACTICE 6EPerfectly inelastic coll
Page 21 and 22: PRACTICE 6FKinetic energy in perfec
Page 23: SAMPLE PROBLEM 6GElastic collisions
Page 26 and 27: CHAPTER 6SummaryKEY IDEASSection 6-
Page 28 and 29: CONSERVATION OF MOMENTUMReview ques
Page 30 and 31: 43. A 0.10 kg ball of dough is thro
Page 32 and 33: 56. A 55 kg pole-vaulter falls from
Page 34 and 35: PROCEDURECBL AND SENSORSConservatio
Page 36: 12. Find the average of these three

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