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Multiple Choice Questions The momentum of an object is equal to

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					Chapter 6, Conservation of Linear Momentum


Multiple Choice Questions


   1. The momentum of an object is equal to
         a. the distance traveled times the acceleration of the object.
         b. its mass times its acceleration.
         c. its mass divided by its acceleration.
         d. mass x velocity.
         e. the distance traveled by an object times its kinetic energy.

Ans: d


   2. Which of the following is FALSE about linear momentum?
        a. Its characteristics follow from Newton’s laws of motion.
        b. It is a scalar quantity only.
        c. Its units are kgm/s.
        d. It matches our intuition regarding the inertia of objects in motion.
        e. All of the above are true.

Ans: b


   3. Which of the following has the greatest momentum?
        a. a speeding bullet
        b. a sprinter in the 100-meter dash
        c. a motor cycle
        d. a pick-up truck
        e. not enough information given to answer this

Ans: e


   4. Which of the following does not have linear momentum?
        a. an airplane at 30,000 feet
        b. a neutrino
        c. a car stuck in a traffic jam
        d. a speeding bullet
        e. All of the above have momentum.

Ans: c


   5. The mass of a truck is 4000 kg. It has a velocity with a magnitude of 10 m/s.
      What is its momentum?
Chapter 6, Conservation of Linear Momentum


          a.   400 kg m/s
          b.   2400 kg m/s
          c.   40000 kg m/s
          d.   14000 kg m/s
          e.   Not enough information is available to tell.

Ans: c


   6. The momentum of the moving truck in the above problem
         a. is a vector quantity.
         b. is more than the momentum of a 125 kg man shot out of a cannon with a
            high velocity of 320 m/s.
         c. is inversely proportional to the objects inertia.
         d. all of the above are true.
         e. a and b only are true

Ans: a


   7. A 1200 kg car moves down the road at 20 m/s. What is its momentum?
         a. 60 kg m/s
         b. 6000 kg m/s
         c. 18000 kg m/s
         d. 24,000 kg m/s
         e. 54,000 kg m/s

Ans: d


   8. If the above car accelerates from its velocity of 20 m/s with a constant
      acceleration of 2 m/s2 for 8 seconds, what will the new momentum be?
           a. 12400 kg m/s
           b. 19200 kg m/s
           c. 36860 kg m/s
           d. 43200 kg m/s
           e. 49300 kg m/s

Ans: d


   9. The momentum of a truck rolling down a hill while in a neutral gear
         a. gives you an idea of the amount of force required to bring the truck to a
            halt.
Chapter 6, Conservation of Linear Momentum


          b. will be the same as its momentum at the same height on an adjacent hill if
             the truck rolls to the bottom of the first and up the second on a smooth
             road, provided we neglect any effects of friction.
          c. will be conserved throughout its journey down the hill.
          d. will decrease gradually as the truck rolls down the hill, provided the truck
             loses its load of apples one at a time as it speeds down the hill.
          e. a and b

Ans: e


   10. Two marble balls move directly toward each other. Which of the following must
       be true if the total momentum of the two balls is zero?
           a. The velocity of the two balls must be the same.
           b. The total energy of the system must equal zero.
           c. The momentum of the system cannot be conserved since the balls move
               toward each other, not away from each other as would be required for the
               momentum to equal zero.
           d. The scalar products of the mass times the velocity of each ball must be
               equal to each other.
           e. all of the above

Ans: d


   11. A skater moves with a constant velocity of 12 m/s. If her momentum is 600 kg
       m/s, what is her mass?
          a. 72 kg
          b. 60 kg
          c. 50 kg
          d. 46 kg
          e. There is not enough information to answer this.

Ans: c


   12. How much does the skater in the above problem weigh?
         a. 588 N
         b. 490 N
         c. 420 N
         d. 366 N
         e. 348 N

Ans: b
Chapter 6, Conservation of Linear Momentum


   13. A 150 kg football player runs down the field with a constant momentum of 1200
       kg m/s for 6 seconds. How far does he travel?
          a. 25 m
          b. 32.5 m
          c. 38 m
          d. 48 m
          e. 60m

Ans: d


   14. An old 10,000 kg truck moves east at 9 m/s. How fast would a 0.1 kg bullet have
       to travel to have the same momentum as the truck?
            a. 900,000 m/s
            b. 560,000 m/s
            c. 190,000 m/s
            d. 90,000 m/s
            e. The bullet could never have the same momentum since it is simply too
               small.

Ans: a


   15. A change in momentum is called
          a. a Newton.
          b. an impulse.
          c. total momentum.
          d. an internal force.
          e. none of the above

Ans: b


   16. Newton’s second law of motion states that force equals mass times acceleration.
       From knowing this we are able to derive the following equation regarding
       momentum.
          a. pc = mv
          b. (mv) = F x t
          c. mv =mc2t
          d. mg = mv
          e. none of the above

Ans: b


   17. An impulse can involve
Chapter 6, Conservation of Linear Momentum


          a.   a large force acting over a long period of time
          b.   a small force acting over a long period of time
          c.   a small force acting over a short period of time
          d.   all of the above
          e.   a and b only

Ans: d


   18. Which of the following situations does NOT involve an impulse?
         a. a tug boat that pushes a cruise ship to its dock, only slowly altering the
             larger ships course in the process
         b. a car that runs into a brick wall
         c. the moving fist of a boxer mid-jab, at a location just inches away from her
             opponents face
         d. All of the above are impulses according to a physicist.
         e. a and b only

Ans: e


   19. An extra heavy hockey puck with a mass of 1kg is at rest when it is struck by a
       hockey stick. The velocity of the puck after contact with the stick is 24 m/s. If the
       stick was in contact with the puck for 0.5 s, what is the force in Newtons that has
       been applied to the puck?
           a. 48 N
           b. 30 N
           c. 24 N
           d. 12 N
           e. not enough information is given to answer

Ans: a


   20. Which of the following are examples of an impulse?
         a. a car traveling at a constant velocity of 60 mph
         b. an apple falling from a tree that has reached its halfway point in its fall
         c. a golf ball that is being struck by a golf club
         d. an oil tanker in dry dock
         e. b and c only

Ans: e


   21. A 120 kg motorcycle runs into a brick wall while traveling at 40 m/s. It comes to
       a complete halt within 0.5 seconds of the first contact with the wall, crumpling in
Chapter 6, Conservation of Linear Momentum


         the process. The wall remains intact. What is the force that the wall exerts on the
         cycle?
             a. 15000 N
             b. 9600 N
             c. 4800 N
             d. 3600 N
             e. cannot be answered with the information available

Ans: b


   22. A closed system consist of four particles in motion. Which of the following is
       true?
           a. The total momentum of the system must sum to zero since the directions
              of the motion of the individual particles cancel each other out.
           b. Two of the particles collide, requiring the total momentum of the system
              to change regardless of the type of collision.
           c. A fifth particle enters the system and collides with one of the particles, but
              the momentum of the system is conserved nonetheless.
           d. All of the above are true.
           e. All of the above are false.

Ans: e


   23. A cue ball strikes the eight ball in the final shot of a game of pool. The eight ball
       moves off at 6 meters per second toward the corner pocket. If the mass of the two
       balls is the same, and the cue ball’s speed prior to contact with the eight ball was
       9 m/s, what is the speed of the cue ball after it has hit the 8-ball? Ignore any
       effects due to the rotation of the two balls.
           a. ¾ m/s
           b. 0.67 n/s
           c. 1.5 m/s
           d. 3 m/s
           e. 4.4 m/s

Ans: d


   24. If you were stuck out on perfectly smooth, friction free, frozen pond with nothing
       but a sack of apples on your back, how might you get closer to the shore so you
       could go home?
           a. Drop the apples so you could have less weight, and thus, make it easier to
               walk on the frictionless surface.
           b. Place the apples directly over your head and walk off.
           c. Throw the apples in a direction opposite to the direction you want to go.
Chapter 6, Conservation of Linear Momentum


           d. all of the above
           e. a or b

Ans: c


   25. Two ice skaters stand facing each other. One weighs 85 kg; the other weighs 50
       kg. If the larger skater pushes off the smaller skater straight backwards so that she
       moves with a velocity of 7 m/s, what will happen to the larger skater provided the
       ice is perfectly smooth?
           a. She will stay in the same place since she was the one who exerted the
                force.
           b. She will move in the same direction as the smaller skater but with a slower
                velocity.
           c. She will move backwards opposite in direction to the smaller skater at 11
                m/s.
           d. She will move backwards at 5.2 m/s.
           e. She will move backwards at 4.1 m/s.

Ans: e


   26. What is meant by the term “conservation” in physics?
         a. The concern with the preservation of various species in nature
         b. Quantities must always keep (“conserve”) the same values
         c. The initial net value of a quantity must be the same as the final net value
              of a quantity

Ans: a


A perfectly elastic collision is one in which the colliding objects bounce off of each other
without losing any energy. A perfectly inelastic collision is one in which the colliding
objects stick completely together. We will examine these ideas with sleds on an air track.

   27. In which type of collision is linear momentum conserved?
           a. Elastic
           b. Inelastic
           c. Both a & b
           d. Neither a nor b

Ans: c



                            A                                  B
Chapter 6, Conservation of Linear Momentum




   28. Sleds A and B have the same mass. Sled A heads toward B with an initial speed v
       to the right while B is initially at rest. What is the most likely result of an elastic
       collision between A and B?
           a. A comes to a complete stop and B moves to the right with speed v a
           b. A and B stick together and move to the right with speed v
           c. A and B stick together and move to the right with less than speed v
           d. A and B stick together and move to the right with more than speed v
           e. A bounces off B and heads back to the left with speed slightly less than v
                while B moves slowly to the right

   29. How might you describe the amount of time the two carts spend in contact with
       each other (i.e. during the collision)?
          a. Very brief
          b. Moderate
          c. Permanent (they stick together)

Ans: a


   30. Suppose the same conditions apply as above in question #3, except that the
       collision is now inelastic. What is the most likely result of the collision?
           a. A comes to a complete stop and B moves to the right with speed v
           b. A and B stick together and move to the right with speed v
           c. A and B stick together and move to the right with less than speed v
           d. A and B stick together and move to the right with more than speed v
           e. A bounces off B and heads back to the left with speed slightly less than v
               while B moves slowly to the right

Ans: c


   31. Suppose that the collision between A and B is elastic and A has only ½ of the
       mass of B. What is the most likely result of the collision?
          a. A comes to a complete stop and B moves to the right with speed v
          b. A and B stick together and move to the right with speed v
          c. A and B stick together and move to the right with less than speed v
          d. A and B stick together and move to the right with more than speed v
          e. A bounces off B and heads back to the left with speed slightly less than v
              while B moves slowly to the right

Ans: e
Chapter 6, Conservation of Linear Momentum


   32. Suppose that the collision between A and B is inelastic and A has twice the mass
       of B. What is the most likely result of the collision?
           a. A comes to a complete stop and B moves to the right with speed v
           b. A and B stick together and move to the right with speed v
           c. A and B stick together and move to the right with less than speed v
           d. A and B stick together and move to the right with more than speed v
           e. A bounces off B and heads back to the left with speed with speed slightly
              less than v while B moves slowly to the right

Ans: c


   33. In a car accident, a moving van plows into a stationary car. Which of the
       following is most likely true? Assume the van’s initial velocity is described as
       forward.
           a. The collision is elastic, the van stops and the car moves forward
           b. The collision is inelastic, the van and car move together forward with a
               speed greater than the initial speed of the van
           c. The collision is elastic, the van bounces off the car & moves backward and
               the car moves forward
           d. The collision is inelastic, the van and car move together forward with a
               speed less than the initial speed of the van

Ans: d


   34. Why do physicists usually specify “initial” and “final” values for momentum
       conservation problems?
          a. It is often difficult to measure an impulse during a collision
          b. It is always necessary to specify each individual object’s initial and final
              momentum
          c. Only initial and final momenta are conserved; momentum is not conserved
              during a collision

Ans: a


   35. Impulse is described as the change of momentum of an object. What is another
       way of stating the concept of momentum? Consider the examples above with the
       air carts.
           a. The “internal” force during a collision multiplied by the time of collisiona
           b. The “external” force during a collision multiplied by the time of collision
           c. The “internal” force present during an experiment multiplied by the time
                from the beginning of the experiment to the end of the experiment
           d. The “external” force present during an experiment multiplied by the time
                from the beginning of the experiment to the end of the experiment
Chapter 6, Conservation of Linear Momentum



Ans: a


   36. To promote safety during collisions, many automotive vehicles now have
       “crumple zones”. What is the purpose of a crumple zone?
           a. The crumple zone should be rigid so as to spread the impulse of a collision
              throughout the entire vehicle
           b. The crumple zone should be rigid so as to bounce the vehicle away from
              the collision
           c. The crumple zone should be soft so as to seal up the occupants of the
              vehicle to protect them from harm
           d. The crumple zone should be soft so as to absorb the impulse of the
              collision

Ans: d


   37. Another safety innovation in vehicles in recent years is the front seat airbag.
       Airbags are used for adults but not for children. Why would anyone be against
       airbags for children?
           a. Children should always remain in the back seat so that they are less of a
              distraction for the driver
           b. Airbags inflate so quickly that the impulse they provide can be dangerous
              to children
           c. Automakers balk at the extra cost of installing extra airbags

Ans: b


   38. Boxing instructors will tell students to back their heads away from an incoming
       blow. What is the advantage of doing this?
          a. The relative velocity of the punch will decrease and thus the impulse
              received will decrease
          b. The time over which the punch is delivered will increase and thus the
              force received will decrease
          c. The distance over which the punch is delivered will decrease and thus the
              impulse received will decrease

Ans: b


Uranium-238 is a radioactive element that splits into thorium-234 and helium-4 (also
called an alpha particle). The numbers with the elements refer to their relative masses.
Chapter 6, Conservation of Linear Momentum




                      He-4
                                           U-238                    Th-234




   39. A uranium-238 nucleus is sitting motionless in a rock. It decays into thorium-234
       and helium-4, which rush off in opposite directions at high speeds. Which of the
       following statements is true? Let the positive direction be to the right.
           a. The final momentum of the system is positive
           b. The final momentum of the system is negative
           c. The final momentum of the system was zero

Ans: c


   40. For the U-238 decay above, which of the following happens to the products (Th-
       234 and He-4), and why?
          a. Both products move with equal speeds in opposite directions in order to
               conserve momentum
          b. The Th-234 nucleus moves much faster than the He-4 nucleus because it
               carries away more mass
          c. The He-4 nucleus moves much faster than the Th-234 nucleus because it
               carries away less mass
          d. The figure is misleading; both products move in the same direction

Ans: c



                             A                                B


   41. Two carts of equal mass on an air track collide as discussed in earlier questions
       (the figure has been reproduced above). Which statement is true?
           a. No forces will be exerted on either object during the collision
           b. Only A feels a force during the collision
           c. Only B feels a force during the collision
           d. Both carts will feel a force during the collision

Ans: d
Chapter 6, Conservation of Linear Momentum



   42. Which quantity can you observe that most clearly justifies your previous answer?
         a. The accelerations of the carts during the collision
         b. The masses of the carts
         c. The positions of the carts, before and after the collision
         d. The velocities of the carts, before and after the collision

Ans: a


   43. In which type of collision will the force (during the collision) acting on B be the
       largest?
           a. Elastic
           b. Inelastic
           c. Neither; no force is felt by B

Ans: a


Two carts on an air track undergo an elastic collision. They are not necessarily the same
mass. Cart C has a positive initial velocity and D is initially stationary.


                            C                                  D
                            ?                                  ?

   44. What is true about the relative masses of C and D if the final momentum of C is
       positive?
          a. C must be less massive than D
          b. C must be more massive than D
          c. C must be equal in mass to D
          d. Not enough information is given to answer the question

Ans: b


   45. In which case would C experience the largest amount (magnitude) of force?
           a. C bounces off of D and has a negative velocity    a
           b. C bounces off of D and comes to a stop
           c. C bounces off of D and has a positive velocity

Ans: a


   46. A karate expert conducts a common demonstration where she splits a series of
       boards with her hand. In which case would she impart the largest force to the
Chapter 6, Conservation of Linear Momentum


         boards? Assume her hand is in contact with the boards for the same amount of
         time in each case. (Note that in reality she might not wish to do this)


                Direction of                        Direction of
                incoming hand                       outgoing hand?


                                                          ?
                Before the collision                After the collision

            a. Her hand bounces off the boards and goes backward
            b. Her hand comes to a stop after hitting the boards
            c. Her hand goes continues going forward through the boards

Ans: a


You are an accident investigator. A minivan and a compact car have collided with the
only damage to both front ends of the vehicles. Both drivers claim that they were stopped
and the other vehicle ran into them. Skid marks from both vehicles are visible on the
street. The two vehicles are still in contact after the accident.

   47. Which type of collision was this?
         a. Elastic
         b. Inelastic
         c. Cannot tell from the information given

Ans: b




   48. Skid marks are visible behind the car and in front of the minivan. Which vehicle
       was actually moving before the collision?
          a. The car
          b. The van

Ans: a


True or False Questions
Chapter 6, Conservation of Linear Momentum




True or False


   1. The law of conservation of momentum states that momentum can never change.

Ans: False


   2. In a static system, the total momentum will be constant.

Ans: True


   3. A change in the total momentum of a system occurs only when an external force
      is applied.

Ans: True


   4. A large force always creates a larger change of momentum than a small force,
      regardless of the time involved in exerting the force.

Ans: False


   5. An impulse must always, by necessity, involve a change in velocity.

Ans: False


   6. The total momentum of a multi-particle system can still be conserved after a
      collision between its members.

Ans: True


   7. A system that contains more than two moving objects can never have a total
      momentum of zero.

Ans: False


   8. One way a seatbelt functions is by spreading out the total force exerted on your
      body over a longer time period than would otherwise be the case.
Chapter 6, Conservation of Linear Momentum



Ans: True


   9. The conservation of momentum means that a fireworks shell before its explosion
      will have the same momentum as the sum of the momentum of the pieces of the
      shell has after the explosion.

Ans: True


   10. The conservation of linear momentum led to the proposal of the existence of the
       neutrino many years before its actual discovery.

Ans: True

				
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