Physics 121k Exam 3 7 Dec 2012

Transcription

1 Answer each question and show your work. A correct answer with no supportin reasonin may receive no credit. Unless directed otherwise, please use =10.0 m/s 2. Name: 1. (15 points) An 5.0 k block, initially at rest is draed alon a horizontal frictionless surface by a force F = 20.0 N (as shown in the fiure below). What is the manitude of the normal force of contact exerted by the floor upward on the block? A. 70 N B. 65 N C. 60 N D. 55 N E. 45 N F N 2. (15 points) Considerin the previous problem, and assumin that the coefficient of kinetic friction between the floor and the block is µ k = 0.1, how fast will the block be movin by the time it has travelled 10 meters to the riht? A. 4.2 m/s B. 6.7 m/s C. 7.5 m/s D. 8.3 m/s E m/s Dept. of Physics Pae 1 of 7 Fall 2012

2 Answer questions 3 5 usin the fiure below which shows a perfect physics sprin with constant k = 10 N/m, and a mass m = 4.0 k initially at rest on a frictionless surface. 3. (15 points) The block of mass m = 4.0 k is pushed up aainst the sprin, compressin it by an amount s = 3.0 m. How much potential enery is stored in the sprin at this point? A. 15 J B. 33 J C. 45 J D. 48 J E. 93 J 4. (15 points) Now the block is released from rest, and the sprin pushes the block to the riht. At the instant the block is at the lowest point, what is the upward normal force actin on the block? (You may assume that the lowest point looks like the cross section of a circle of radius 2.5 m.) A. 24 N B. 64 N C. 84 N D. 94 N E. 104 N F. 124 N 5. (15 points) The block continues slidin and when it ets to the top level portion, there is a rouh surface (µ k = 0.1), and the block slides a distance d before comin to rest; how lare is this distance, d? A m B m C m D m E. 6.0 m Dept. of Physics Pae 2 of 7 Fall 2012

3 6. (15 points) Earth and Moon are separated by rouhly d EM = m. At the instant shown, where is the center of mass of the Earth-Moon system. Assume that x = 0 is at the center of Earth. (M E = k, M m = k, and R E = m) A m B m C m D m E m 7. (15 points) Usin your knowlede of Newton s law of Gravitation, and circular motion, calculate the centripetal acceleration (in m/s 2 ) of the moon while it orbits the earth. A m/s 2 B m/s 2 C m/s 2 D m/s 2 E m/s 2 8. (15 points) Suppose that an evil remlin moved the earth to half it s present distance from the earth and arrane it s tanential velocity perfectly so that the moon orbited the Earth in a perfect circle at this new radius. If the moon s current period is 27.5 days, what would the new period be after the evil remlin s meddlesome deed? A. 3.2 days B. 6.4 days C. 8.3 days D. 9.7 days E days Dept. of Physics Pae 3 of 7 Fall 2012

4 9. (15 points) A solid sphere of mass m = 6.0 k and radius R = 0.1 m is released from rest, and rolls without slippin down a entle hill and arrives at the bottom (10.0 meters vertically below its startin heiht) and then rolls (without slippin) alon a level surface. How fast is the center of mass of the sphere movin when it ets to the bottom of the hill? A. 10 m/s B. 12 m/s C. 14 m/s D. 16 m/s E. 18 m/s 10. (15 points) A solid thin disk of mass M and radius R rotates about an axis perpendicular to the plane of the pae and passin throuh the point P as shown below. The axis of rotation is 2 R from the center of mass of the disk. What is the moment of inertia of 3 this disk about the axis throuh the point P? A MR2 B. 8 9 MR2 C MR2 D MR2 E. 5 7 MR2 Dept. of Physics Pae 4 of 7 Fall 2012

5 11. (15 points) A solid thin disk of mass M and radius R rotates on frictionless bearins with initial anular velocity ω about an axis perpendicular to the plane of the pae and passin throuh the center of mass of the disk. Some time later, someone drops two (both initially with no anular velocity) disks (each of mass 2M; yes, the smaller disks are made of more dense material than the bottom disk) on top of the bottom disk. If the radii of these two disks are 3R and 2 R, what will be the final anular velocity of the 4 3 system? A. 1 2 ω B. 1 6 ω C. 1 5 ω D. 1 4 ω E. 1 3 ω Dept. of Physics Pae 5 of 7 Fall 2012

6 12. (15 points) The Tardis and an evil black Dalek are suspended by a massless physics rope that passes over two frictionless pulleys (each of mass k and radius 0.50 m) as shown. If the system is released from rest, calculate manitude of the acceleration of the Tardis. A. 2 B. 6 C. 5 D. 4 E (15 points) What is the tension, T, in the section of rope between the two pulleys? A N B N C N D N E N F N Dept. of Physics Pae 6 of 7 Fall 2012

7 Exam Summary Pae Question Points My Answer Total: 195 Dept. of Physics Pae 7 of 7 Fall 2012