A. 2.0 cm B. 4.0 cm C. 4.5 cm D. 6.0 cm E. 9.0 cm

Transcription

1 ENGPHY1 QUIZ 3 Kinetic Energy and Work Potential Energy and Conservation of Energy Linear Momentum and Collisions MULTIPLE CHOICE 1. Which of the following is NOT a correct unit for work? A. erg B. ft lb C. watt D. newton meter E. joule 2. A boy holds a 40-N weight at arm s length for 10 s. His arm is 1.5 m above the ground. The work done by the force of the boy on the weight while he is holding it is: A. 0 B. 6.1 J C. 40 J D. 60 J E. 90 J 3. Which of the following bodies has the largest kinetic energy? A. Mass 3M and speed V B. Mass 3M and speed 2V C. Mass 2M and speed 3V D. Mass M and speed 4V E. All four of the above have the same kinetic energy 4. A 2 kg ball is attached to a 0.80 m string and whirled in a horizontal circle at a constant speed of 6 m/s. The work done on the ball during each revolution is: A. 90 J B. 72 J C. 16 J D. 32 J E A block is attached to the end of an ideal spring and moved from coordinate x i to coordinate x f. The relaxed position is at x = 0. The work done by spring is positive if: A. x i = 2 cm and x f = 4 cm B. x i = 2 cm and x f = 4 cm C. x i = 2 cm and x f = 4 cm D. x i = 2 cm and x f = 4 cm E. x i = 4 cm and x f = 2 cm 6. A rock is lifted for a certain time by a force F that is greater in magnitude than the rock's weight W. The change in kinetic energy of the rock during this time is equal to the A. work done by the net force (F - W) B. work done by F alone C. work done by W alone D. difference in the potential energy of the rock before and after this time. 7. An 80-N crate slides with constant speed a distance of 5.0 m downward along a rough slope that makes an angle of 30 with the horizontal. The work done by the force of gravity is: A. 400 J B. 200 J C. 69 J D. 200 J E. 400 J 8. A car of mass m slides across a patch of ice at a speed v with its brakes locked. It the hits dry pavement and skids to a stop in a distance d. The coefficient of kinetic friction between the tires and the dry road is µ. If the car has a mass of 2 m, it would have skidded a distance of A. 0.5d B. d C. 1.41d D. 2d E. 3d 9. Three identical ideal springs (X,Y,Z) are arranged as shown. When a 4.0-kg mass is hung on X, the mass descends 3.0 cm. When a 6.0-kg mass is hung on Y, the mass descends A. 2.0 cm B. 4.0 cm C. 4.5 cm D. 6.0 cm E. 9.0 cm 10. The weight of an object on the moon is one-sixth of its weight on Earth. The ratio of the kinetic energy of a body on Earth moving with speed V to that of the same body moving with speed V on the moon is: A. 6:1 B. 36:1

2 C. 1:1 D. 1:6 E. 1:36 the mechanical energy E of the Earth-ball system as a function of the altitude y of the ball? 11. The amount of work required to stop a moving object is equal to: A. the velocity of the object B. the kinetic energy of the object C. the mass of the object times its acceleration D. the mass of the object times its velocity E. the square of the velocity of the object 12. A 50-N force is the only force a 2-kg crate that starts from rest. At the instant the object has gone 2 m the rate at which the force is doing work is: A. 2.5W B. 25 W C. 75 W D. 100 W E. 500 W 13. A force on a particle is conservative if: A. its work equals the change in the kinetic energy of the particle B. it obeys Newton s second law C. it obeys Newton s third law D. its work depends on the end points of every motion, not on the path between E. it is not a frictional force 14. Two particles interact by conservative forces. In addition, an external force acts on each particle. They complete round trips, ending at their starting points. Which must be the same for the two-particle system at the start and end of this trip? A. total kinetic energy B. potential energy C. mechanical energy D. total linear momentum E. none of the above 15. The wound spring of a clock possesses: A. kinetic but no potential energy B. potential but no kinetic energy C. both potential and kinetic energy in equal amounts D. neither potential nor kinetic energy E. both potential and kinetic energy, but more kinetic energy than potential energy 16. A ball is held at a height H above a floor. It is then released and falls to the floor. If air resistance can be ignored, which of the five graphs below correctly gives 17. A toy cork gun contains a spring with spring constant is 10.0 N/m. The spring is compressed 5.00 cm and then used to propel a 6.00-g cork. The cork, however, sticks to the spring for 1.00 cm beyond its unstretched length before separation occurs. The muzzle velocity of the cork is: A m/s B m/s C m/s D m/s E m/s 18. The string in the figure is 50 cm long. When the ball is released from rest, it swings along the dotted arc. How fast is it going at the lowest point in its swing? A. 2.0 m/s B. 2.2 m/s C. 3.1 m/s D. 4.4 m/s E. 6.0 m/s 19. A block slides across a rough horizontal table top. The work done by friction changes: A. only the kinetic energy B. only the potential energy C. only the internal energy D. the kinetic and potential energies E. the kinetic and internal energies

3 20. The x and y coordinates of the center of mass of the three-particle system shown below are: A. 0, 0 B. 1.3 m, 1.7m C. 1.4 m, 1.9m D. 1.9 m, 2.5m E. 1.4 m, 2.5m 21. The center of mass of Earth s atmosphere is: A. a little less than halfway between Earth s surface and the outer boundary of the atmosphere B. near the surface of Earth C. near the outer boundary of the atmosphere D. near the center of Earth E. none of the above 22. The center of mass of a system of particles has a constant velocity if: A. the forces exerted by the particles on each other sum to zero B. the external forces acting on particles of the system sum to zero C. the velocity of the center of mass is initially zero D. the particles are distributed symmetrically around the center of mass E. the center of mass is at the geometric center of the system 23. A man sits in the back of a canoe in still water. He then moves to the front of the canoe and sits there. Afterwards the canoe: A. is forward of its original position and moving forward B. is forward of its original position and moving backward C. is rearward of its original position and moving forward D. is rearward of its original position and moving backward E. is rearward of its original position and not moving 24. Two objects, P and Q, have the same momentum. Q has more kinetic energy than P if it: A. weighs more than P B. is moving faster than P C. weighs the same as P D. is moving slower than P E. is moving at the same speed as P 25. A 1.0-kg ball moving at 2.0 m/s perpendicular to a wall rebounds from the wall at 1.5 m/s. The change in the momentum of the ball is: A. zero B. 0.5N s away from wall C. 0.5N s toward wall D. 3.5N s away from wall E. 3.5N s toward wall 26. A man is marooned at rest on level frictionless ice. In desperation, he hurls his shoe to the right at 15 m/s. If the man weighs 720 N and the shoe weighs 4.0 N, the man moves to the left with a speed of: A. 0 B m/s C m/s D. 15 m/s E m/s 27. A projectile in flight explodes into several fragments. The total momentum of the fragments immediately after this explosion: A. is the same as the momentum of the projectile immediately before the explosion B. has been changed into kinetic energy of the fragments C. is less than the momentum of the projectile immediately before the explosion D. is more than the momentum of the projectile immediately before the explosion E. has been changed into radiant energy 28. Force: A. equals the negative integral (with respect to distance) of the potential energy function B. is the ability to do work C. is the rate of change of doing work D. equals the time rate of change of momentum E. has dimensions of momentum multiplied by time 29. A rocket exhausts fuel with a velocity of 1500 m/s, relative to the rocket. It starts from rest in outer space with fuel comprising 80 per cent of the total mass. When all the fuel has been exhausted its speed is: A m/s B m/s C m/s D. 880 m/s E. 400 m/s

4 30. The law of conservation of momentum applies to a system of colliding objects only if: A. there is no change in kinetic energy of the system B. the coefficient of restitution is one C. the coefficient of restitution is zero D. the net external impulse is zero E. the collisions are all elastic 31. Sphere X, of mass 2 kg, is moving to the right at 10 m/s. Sphere Y, of mass 4 kg, is moving to the left at 10 m/s. They collide head-on. The magnitude of the impulse of X on Y is the magnitude of the impulse of Y on X. A. twice B. half C. one-fourth D. four times E. the same as 32. Two bodies of unequal mass, placed at rest on a frictionless surface, are acted on by equal horizontal forces for equal times. Just after these forces are removed, the body of greater mass will have: A. the greater speed B. the greater acceleration C. the smaller momentum D. the greater momentum E. the same momentum as the other body 33. A 4.0-N puck is traveling at 3.0 m/s. It strikes an 8.0-N puck, which is stationary. The two pucks stick together. Their common final speed is: A. 1.0 m/s B. 1.5 m/s C. 2.0 m/s D. 2.3 m/s E. 3.0 m/s 34. An open cart on a level surface is rolling without frictional loss through a vertical downpour of rain, as shown above. As the cart rolls, an appreciable amount of rainwater accumulates in the cart. The speed of the cart will A. increase because of conservation of mechanical energy B. decrease because of conservation of momentum C. decrease because of conservation of mechanical energy D. remain the same because the raindrops are falling perpendicular to the direction of the cart's motion 35. How does an air mattress protect a stunt person landing on the ground after a stunt? A. It reduces the kinetic energy loss of the stunt person. B. It reduces the momentum change of the stunt person. C. It shortens the stopping time of the stunt person and increases the force applied during the landing. D. It lengthens the stopping time of the stunt person and reduces the force applied during the landing. PROBLEM SOLVING 1. A box is sliding with a speed of 4.50 m/s on a horizontal surface when, at point P, it encounters a rough section. On the rough section, the coefficient of friction is not constant, but starts at at P and increases linearly with distance past P, reaching a value of at 12.5 m past point P. (a) Use the work energy theorem to find how far this box slides before stopping. (b) What is the coefficient of friction at the stopping point? (c) How far would the box have slid if the friction coefficient didn t increase but instead had the constant value of 0.100? 2. A 10.0kg block is released from rest at point A. The track is frictionless except for the portion between points B and C, which has a length of 6.00 m. The block travels down the track, hits a spring of force constant N/m, and compresses the spring m from its equilibrium position before coming to rest momentarily. Determine the coefficient of kinetic friction between the block and the rough surface between points B and C. 3. A 1500-kg sedan goes through a wide intersection traveling from north to south when it is hit by a 2200-kg SUV traveling from east to west. The two cars become enmeshed due to the impact and slide as one thereafter. On-the-scene measurements show that the coefficient of kinetic friction between the tires of these cars and the pavement is 0.75, and the cars slide to a halt at a point 5.39 m west and 6.43 m south of the impact point. How fast was each car traveling just before the collision?

5 ANSWER KEY (including explanations to selected problems) 1. C 2. A 3. C 4. E Work done is zero since the Force is always perpendicular to the distance moved as you move incrementally around the circle. 5. E W = F d F and d are in the same direction (positive W) if d is towards equilibrium (x = 0). 6. A 7. D W = F g dsinθ 8. B 1 2 mv2 µ k mgd = 0 9. E 10. C KE is dependent on mass, not weight. 11. B 12. E 13. D 14. B 15. B 16. E No air resistance = no nonconservative force = ME is conserved C kx 2 i 2 = 1 kx 2 f mv2 18. C mgh = 1 2 mv2 19. E 20. C 21. D 22. B 23. E Initial momentum = 0 = final momentum m man (0) + m canoe (0) = 0 = m man v forward + m canoe v rearward The canoe compensates for the forward motion of the man so that the center of mass of the system doesn t move (momentum doesn t change). 24. B 25. D p = m v = 1.0 [2.0 ( 1.5)] 26. C m 1 (0) + m 2 (0) = m 1 v 1f + m 2 v 2f 27. A 28. D I = p = F t F = p t

6 29. B 30. D 31. E Same force (F XY = F YX ) and time of contact ( t ). Same impulse. 32. E Same force (F XY = F YX ) and time of contact ( t ). Same momentum change. 33. A m 1 v 1i + m 2 (0) = m 1 v f + m 2 v f 34. B m i v i = m f v f As m increase, v decrease. 35. D p = F t = constant t increases, so F decreases.

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