16. 18. 19. Two people are initially standing still on frictionless ice. They push on each other so that one person, of mass 120 kg, moves to the lett at 2 m/s, while the other person, of mass 80 kg, moves to the night at 3 m/s. What 1s the velocity of their center of mass? (A) Zero (B) 0.5 m's to the left ({C) 1 m/'s to the nght (D) 2.4 m's to the left (E)} 2.5 m's to the right B . An object having an initial momentum that may be represented by the vector above strikes an object that 1s imitially at rest. 'Which of the following sets of vectors may represent the momenta of the two objects after the collision? o (A) _ (B) .' h. (C) (D) (E) / 'I,_l / Floor A 2 kg ball colhdes with the floor at an angle 8 and rebounds at the same angle and speed as shown above. Which of the following vectors represents the impulse exerted on the ball by the floor? (A) (D) {EJI h PNy o O The momentum p of a moving object as a function of time is given by the expression p = kr', where k is a constant. The force causing this motion 1s given by the expression (A) 3k (B) 3k72 (C) kf*/3 (D) kt* (E) kt'/d4 Duestions 20-21 20. 22, 23. OE.U m/s 15kg . $1.0 ms O4okg Two pucks moving on a frictionless air table are about to collide, as shown above. The 1.5 kg puck 1s moving directly east at 2.0 m/s. The 4.0 kg puck 15 moving directly north at 1.0 m/s. What 1s the total kinetic energy of the two-puck system before the collision? {h}ql I (B) 5.0 (C) 701 (D) 101 (Ey111 . What 1s the magnitude of the total momentum of the two-puck system after the collision? (A) LOkg'm/s (B)3.5kgrmis (C)50kgm/s (D)7.0kgemis (E) 5.5+5 kegems ( R As shown above, two students sit at opposite ends of a boat that is initially at rest. The student in the front throws a heavy ball to the student in the back. What is the motion of the boat at the time immediately after the ball is thrown and, later, after the ball 1s caught? (Assume that air and water friction are negligible.) Immediately After the Throw After the Catch (A) Boat moves forward Boat moves forward (B) Boat moves forward Boat moves backward (C) Boat moves forward Boat does not move (D) Boat moves backward Boat does not move (E) Boat moves backward Boat moves forward A person holds a portable fire extinguisher that ejects 1.0 kg of water per second horizontally at a speed of 6.0 m's. What horizontal force in newtons must the person exert on the extinguisher in order to prevent it from accelerating? (A)ON (B)eN (C)ION (D)ISN (E)36N 24. 25. 26. Center of Mass of System Center f d L A person is standing at one end of a uniform raft of length L that is floating motionless on water, as shown above. The center of mass of the person-raft system is a distance 4 from the center of the raft. The person then walks to the other end of the raft. If friction between the raft and the water is negligible, how far does the raft move relative to the water? (B)L {ij (D)d (E)2d Objects | and 2 have the same momentum. Object | can have more kinetic energy than object 2 if, compared with object 2, it (A) has more mass (B) has the same mass {C) is moving at the same speed (D) 15 moving slower (E} 1s moving faster A 5 kg object is propelled from rest at time =0 by a net force F that always acts in the same direction. The magnitude of F in newtons 15 given as a function of t in seconds by F = 0. 5. What is the speed of the object at =457 (A)05m's (B)0EBm's (C)20m's (D)40m's (E)E0m's = = I I i i i I I i I i i e i i | I 1 i 1 Top View 27. Two balls with masses m and 2m approach each other with equal speeds v on a horizontal frictionless table, as shown in the top view above. Which of the following shows possible velocities of the balls for a time soon after the balls collide? i 1 (A) . (B) (C) # F e - 1\\ (D) Tabletop 28, Three identical disks are initially at rest on a frictionless, horizontal table with their edges touching to form a triangle, as shown in the top view above. An explosion occurs within the tnangle, propelling the disks horizontally along the surface. Which of the following diagrams shows a possible position of the disks at a later time? (In these diagrams, the triangle is shown in its onginal position.) (A) ()