Ohio University Physics Contest - Fall 2005

Transcription

1 YOUR NAME: November 5, 2005 Ohio University Physics Contest - Fall 2005 Your High School: Have you studied Physics in school before this school year? Yes No Are you currently enrolled in a Physics class? Yes No What year are you in? Junior Senior INSTRUCTIONS: Do not open the test until we officially start the exam. Clearly mark the answer on the test. Unless otherwise stated, there is only 1 correct answer to the multiple choice questions. You may work on the front or back of the paper, but we will be grading the final answer, not the work. Pace yourself. Don t spend too much time on any one question. Good Luck! 1 Final Score:

2 1. (2 pts) In a motion diagram the position of an object is shown at a series of even time intervals. Given the motion diagram below, which graph best represents the position of the ball over time? 1s 2s 3s 4s 5s 6s 7s 8s 9s x=0 +x position (m) (a) position (m) (b) position (m) (c) position (m) (d) time(s) time(s) time(s) time(s) 2. (2 pts) Consider the box shown to the right. It has three strings coming out of the top of the box and concrete-filled tin cans in the center. The cans are all the same mass. You lift each string in turn so that you are supporting the can in static equilibrium off the floor of the box. In situation X, a string is fastened to the wall, passes over the pulley, and is attached to the can. You are pulling up on a string attached directly to the pulley. In situation Y you are pulling on a string attached directly to the can. In situation Z the pulley is attached directly to the can. You are pulling on a string that wraps around the pulley and is fastened to the top of the box. Which situation requires the greatest upward force? (a) Situation X (b) Situation Y (c) Situation Z (d) All 3 require same force (e) X and Z require X Y Z 3. (2 pts) A child sits in a wagon which is moving horizontally at a constant velocity. She throws an apple straight up into the air (from her point of view). Ignore air resistance. The apple falls: (a) behind the wagon. (b) in front of the wagon. (c) in the wagon. 2

3 4. (2 pts) Consider 3 different experiments (I through III). In each experiment you hang a spring from a hook and place an unknown mass on the spring. The table below lists the trial, the spring constant of the spring in units of N/m, and the distance the spring stretched from equilibrium. Which of the trials used the greatest mass? (a) Trial I (b) Trial II (c) Trial III (d) Both Trial I and Trial II (e) Both Trial II and Trial III (f) All three trials had the same mass Trial Spring Constant (N/m) Displacement (cm) I 500 N/m 4.0 II 1000 N/m 2.0 III 250 N/m (2 pts) Two blocks are sliding across a frictionless surface towards each other. Block A has a mass of 3kg is traveling with a velocity of 2m/s to the right while block B has a mass of 5kg and is traveling with a velocity of 1m/s to the left. When they collide, they stick together. What is the direction of motion of the combined blocks after the collision? 3kg 2m/s 1m/s 5kg (a) to the right (b) to the left (c) they remain at rest 6. (2 pts) 4 blocks are sliding across a frictionless surface towards each other as shown in the figure. Their sides are covered in velcro so that they all stick together after they collide. What direction of motion of the combined blocks after the collision? 3kg 2kg 3kg 5kg 2m/s 1m/s 1m/s 1m/s (a) to the right (b) to the left (c) they remain at rest 3

4 7. (2 pts) A block rests on a horizontal frictionless surface and is attached to a spring. The graph represents the displacement of the block as a function of time. A displacement to the right of equilibrium is considered positive. A displacement to the left of equilibrium is considered negative. At what point(s) are both the KE of the block and the Potential Energy stored in the spring positive, non-zero numbers? X(m) E 0 A B D F G t(s) C (a) A (b) F (c) D (d) A and D (e) B and G (f) B, F and G (g) C and E 8. (2 pts) You hang a series of objects from a spring scale (one at a time) and completely submerge them in a beaker of water (density = 1000 kg/m 3 ). The objects properties are given in the following table (V is the volume of object D): Object Density (kg/m 3 ) Volume Height A V 5 cm B V 7 cm C V 2 cm D 3600 V 3 cm Which object has the greatest buoyant force when completely submerged? (a) A (b) B (c) C (d) D Water Scale Beaker 9. (2 pts) You ve been given the opportunity to make a video of physics demonstrations on the Moon, where the acceleration due to gravity is 1/6 that of Earth. One of your many goals is to demonstrate how friction behaves on the moon by dragging a wooden block on a piece of plywood and measuring the frictional force. You try this out at home on Earth and find a force of 15 N. On the moon, you will find: (a) A larger frictional force. (b) The same frictional force. (c) A smaller frictional force. (d) No frictional force. 4

5 10. (2 pts) In this experiment Block A slides down a frictionless ramp of height h. At the bottom of the ramp, Block A collides with Block B, they stick together, and the pair go flying off the edge of the table. They hit the floor a distance d from the edge of the table. In Trial 1, the mass of Block A is M and the mass of Block B is 2M. In trial 2, everything is identical except the mass of Block A is 2M and the mass of Block B is M. In which trial is the resultant distance d the greatest? A B (a) Trial 1 (b) Trial 2 (c) Trial 1 and Trial 2 have the same d 11. (2 pts) Which of the following types of electromagnetic radiation has the highest frequency? (a) ultraviolet (b) gamma rays (c) radio waves (d) microwaves 12. (2 pts) Consider a 2.0-m long wooden plank with a weight of 40N. A fulcrum is placed 0.5 m from the left hand end. You then apply an extra force on the left-hand side to hold the plank level. What is the force that the fulcrum exerts upwards on the board? 0.5m d Fulcrum 2m (a) 20N (b) 40N (c) 60N (d) 80N (e) 160N 13. (2 pts) You and a friend are standing in front of a wall which has 4 flat mirrors hanging flat against the wall. In which mirror would you most likely see your friend s reflection? Mirror 1 Mirror 2 Mirror 3 Mirror 4 You Your Friend (a) Mirror 1 (b) Mirror 2 (c) Mirror 3 (d) Mirror 4 (e) all 4 mirrors 5

6 14. (2 pts) An ambulance is driving East as a speed of 35m/s. When the ambulance and observer are stationary, the siren has an observed frequency of 400Hz. There are four other cars on the road driving at the speeds and in the directions shown. Car 3 is at rest on the side of the road. Which driver hears the highest frequency for the siren? 45m/s 30m/s m/s 35 m/s m/s (a) Driver of Car 1 (b) Driver of Car 2 (c) Driver of Car 3 (d) Driver of Car 4 (e) All of the drivers hear the same frequency 15. (2 pts) You are leaning with one ear against a solid wood fence. Your friend is pounding nails in the fence 200m away. You: X See the hammer hit the wood Y Hear the hammer hit the wood through the air with one ear Z Hear the hammer hit the wood through the wood with the other ear. What is the correct order in which these three things happen? (a) First X, then Y, then Z. (b) First Z, then Y, then X. (c) First X, the Y and Z at the same time. (d) First X, then Z, then Y. (e) First Y, then Z and X at the same time. 6

7 16. There are two speakers at the front of the room. Each is outputting a 250Hz pure tone. λ represents the wavelength of the 250Hz tone. Consider three situations: I You are 2.5λ away from speaker A; 3λ away from speaker B and at the speakers the waves are exactly in phase. II You are 2.5λ away from speaker A; 3.5λ away from speaker B and at the speakers the waves are exactly in phase. III You are 2.5λ away from speaker A; 3λ away from speaker B and at the speakers the waves are exactly out of phase. In which situation(s) will you hear destructive interference (soft or no noise)? (a) I only (b) II only (c) III only (d) I and II (e) II and III 17. By wiggling the end of a rubber hose at a frequency of 5 Hz, a wave train is launched down the hose. The velocity of the wave is determined to be 10 m/s. Now you double the frequency at which you wiggle the end while keeping all other characteristics the same. What will the velocity of the new wave traveling down the spring be? (a) 20 m/s (b) 5 m/s (c) 80 m/s (d) 10 m/s (e) 1 m/s 18. (2 pts) Three parallel rays enter a plano-convex lens from the left. This is a strange lens. It is made of a pocket of air enclosed in a large chunk of glass. The semi-circular shape represents the boundary between the air lens and the surrounding glass. Which diagram best represents the behavior of the rays as they pass into and out of the lens? (a) (b) (c) (d) (e) (f) 7

8 19. Each bulb in the circuit to the right has a resistance of 5Ω. A 3-A fuse is located in the circuit as shown (remember that a fuse acts like a zero resistance wire that burns out if too much current in this case 3A flows through it). (a) (2 pts) You begin to turn up the voltage being applied to the circuit. At what voltage does the fuse blow? (a) 6V (b) 15 (c) 30V V A Fuse B C D (d) 45VV (e) 60V (b) (2 pts) Which bulb(s) go out when the fuse blows? A B C D E 20. (2 pts) You are approaching a stop light and slowing down. Which figure best represents the orientation of the fuzzy dice hanging from your rearview mirror? (a) Windshield (b) Windshield (c) Windshield vertical Forward 21. (2 pts) It was realized in the 1920 s that even though photons are massless, they have momentum. In fact the momentum of a photon is indirectly proportional to the wavelength, λ, of the photon: p = h/λ, where h is Planck s Constant and p is the momentum of the photon. It has been proposed that a large reflective sail could use the momentum transfered from photons to accelerate slowly out of the solar system using sunlight. Consider a slightly different experiment. One of these very light sails is traveling towards you at a speed of m/s. You shine a pulse of light directly at the sail that contains photons at a wavelength of 400nm. This pulse brings the sail to a stop. Now you shine a pulse of 600nm photons at the sail in an attempt to get it to sail away from you at a speed of m/s. How many photons will you need in the pulse of light? (neglect drag forces) (a) (b) (c) (d) (e) (f)

9 22. (4 pts) Joe and Tom are having a cup of coffee. The coffee in both of their cups initially has a tempearteure of 80 C. Joe takes some milk out of the fridge and adds it to his coffee. Then they start discussing a physics problem for about 5 minutes (which of course doesn t give them a chance to drink any of their coffee during that time). After this discussion, Tom pulls the milk back out of the refrigerator and adds the same amount of milk into his cup as Joe added earlier. At this point, whose coffee will be colder? Discuss whether you have enough information to answer this question, and if you do, how you arrive at your answer. How would your answer change if the the milk was already at room temperature when added to the coffee? (Please write coherently and concisely in complete, well-structured sentences.) 9