Physics 201 Exam 2 Write also your name in the appropriate box of ...

Name: ______________________________________________________________ Total Points: _______(Last)(First)Physics 201Exam 2Write also your name in theappropriate box of the scantron

Name: ______________________________________________________________ Total Points: _______(Last)(First)Multiple choice questions [60 points]Answer all of the following questions. Read each question carefully. Fill the correctbubble on your scantron sheet. Each question has exactly one correct answer. Allquestions are worth the same amount of points.1.A car travels east at constant velocity. The net force on the car is:A. eastB. westC. upD. downE. zero since the acceleration is 0 and F net = ma.

Name: ______________________________________________________________ Total Points: _______(Last)(First)2.A circus performer of weight W is walking along a "high wire" asshown. The tension in the wire is:Hint: Draw a free body diagram for the circus performer.A. approximately WB. approximately W/2C. much less than WD. much more than WDraw a free body diagram for the performerIt follows (with T the magnitude of the tension in the rope)−T rPLeftRopeW2 WPEsince θ is small.2sinθPEWPE+ T sinθ= 0 ⇒ T = ⇒ T >>E. depends on whether he stands on one or two feetW rPEθT rPRightRope

Name: ______________________________________________________________ Total Points: _______(Last)(First)3.A 1-N pendulum bob is held at an angle θ from the vertical by a 2-Nhorizontal force F as shown. The tension in the string supporting thependulum bob (in newtons) is:A. cos θB. 2/cos θC. sin θD. tan θE.5Draw a free body diagram for the bobT rBSF rW r BE22TBS = WBE+ F = 5N

Name: ______________________________________________________________ Total Points: _______(Last)(First)4.A 70 N block and an 35-N block are connected by a string as shown.If the pulley is massless and the surface is frictionless, the magnitudeof the acceleration of the 70-N block is:(Recall the similar computation done in lab)A. 1.6 m/s2B. 3.3 m/s2for the 35 N blockrWrr35E+ T35S= m35a35⇒ T35S= W35E− m35a35= 35 − m35a35And for the 70 N blockr r r rT70S + W70E+ N70T= m70a70⇒ T70S= m70a70Use thatT70 S= T35Sand a 70= a 35= a to get35m70a = 35 − m35a⇒ a == 3.3m/ s(70 + 35) / 9.82C. 4.9 m/s2D. 6.7 m/s2E. 9.8 m/s2

Name: ______________________________________________________________ Total Points: _______(Last)(First)5.A horizontal shove of at least 200-N is required to start moving a 800-N crate initially at rest on a horizontal floor. The coefficient of staticfriction is:A. 0.25200N is the minimum force to accelerate thebox. Thus it is equal to the maximum forceof static friction:200 = µs800 ⇒ µs= 0.25B. 0.125C. 0.50D. 0.75E. 1.00

Name: ______________________________________________________________ Total Points: _______(Last)(First)6.A box with a weight of 50 N rests on a horizontal surface. A personpulls horizontally on it with a force of 10 N and it does not move. Tostart it moving, a second person pulls vertically upward on the box. Ifthe coefficient of static friction is 0.4, what is the smallest verticalforce for which the box moves?A. 4 NB. 10 NC. 14 N25 NDraw a FBDD.f r BTN rF r vBEW r BTthe normal force acting on the box isNBT= WBE− FvWhen the box starts moving, the frictionF r hforce is equal to F hfBT= FhBut fBT= µSNBT= µS( WBE− Fv)Fh10⇒ Fv= WBE− = 50 − = 25NµS0.4E. 35 N

Name: ______________________________________________________________ Total Points: _______(Last)(First)7.Which of the following five acceleration versus radius graphs iscorrect for a particle moving in a circle of radius r with acceleration aat a constant speed of 10 m/s?A. 1B. 2C. 3D. 4E. 5 sinceva =r2

Name: ______________________________________________________________ Total Points: _______(Last)(First)8.A person riding a Ferris wheel is strapped into her seat by a seat belt.The wheel is spun so that the centripetal acceleration is g. Select thecorrect combination of forces that act on her when she is at the top. Inthe table, W PE = weight, down; F PB = seat belt force, down; and F PS =seat force, up.A. W PE = 0, F PB = mg, F PS = 0W PE = mg, F PB = 0, F PS = 0Draw a FBDF r PSB.rWPEr+ FPSr+ FPBF r PBW r PEr= maalong the vertical ( positive is down)mg − FPS+ FPB= mg ⇒ FPB= FPSB is the only possible answerC. W PE = 0, F PB = 0, F PS = mgD. W PE = mg, F PB = mg, F PS = 0E. W PE = mg, F PB = 0, F PS = mg

Name: ______________________________________________________________ Total Points: _______(Last)(First)9.A crate moves to the right on a horizontal surface as a woman pullson it with a 10-N force. Rank the situations shown below according tothe work done by the 10-N force, least to greatest. The displacementis the same for all cases.3, 2, 1A. W = Fd cosθUseandθ= 0, θ2≈ 45, θ3B. 2, 1, 3C. 2, 3, 1D. 1, 3, 2E. 1, 2, 31=9010.A man pulls a sled along a rough horizontal surface by applying aconstant force F r at an angle θ above the horizontal. In pulling thesled a horizontal distance d, the work done by the man is:A. FdB. Fd cos θC. Fd sin θD. Fd/cos θE. Can’t tell without knowing the coefficient of kinetic friction.

Name: ______________________________________________________________ Total Points: _______(Last)(First)11.A particle is initially at rest on a horizontal frictionless table. It is actedupon by a constant horizontal force F. Which of the following fivegraphs is a correct plot of work W as a function of particle speed v?Hint: use the work energy theoremA. IB. IIC. IIID. W = Kf− Ki ⎫⎪1 21 2IV Kf= mv ⎬ ⇒ W = mv2 ⎪ 2Ki= 0 ⎪⎭E. V

Name: ______________________________________________________________ Total Points: _______(Last)(First)12.The velocity of a particle moving along the x axis changes from v i to v f .For which values of v i and v f is the total work done on the particlenegative?A. v i = 2m/s, v f = 5m/sB. v i = –2m/s, v f = 5m/sC. v i = –5m/s, v f = 2m/sD. v i = 2m/s, v f = –5m/sE. v i = –2m/s, v f = –5m/s⎫W = Kf− Ki ⎪⎪21Kf= mvf ⎬ ⇒ W = m(vKi=1 2 2f− vi212mv2i⎪⎪⎪⎭2) ⇒ W< 0 ifvf

Name: ______________________________________________________________ Total Points: _______(Last)(First)PROBLEM [40 points]Winkin, Blinkin, and Nod are identical triplets, each having amass m = 30.0 kg. They clasp arms in a line and go out on toan ice-covered pond.Winkin grabs onto a post anchored in the ice with his freearm, and starts to go around the post. He soon reaches aconstant speed, where he makes one complete revolution in atime T = 6.28 s.The distance from the center of each boy to the end of his arm is 0.50 m, so that Nod’s body is 2.5m from the post and Blinkin’s body is 1.5 m from the post. At the instant shown, the boys are linedup along the x-axis ( î ), and skating at constant speed in the negative y direction ( − ĵ , out of thepage). The z-axis ( kˆ ) is vertical.1). [10 pts] If the boys’ arms stay rigid, what is Nod’s velocity as he goes around thepost? Measure his velocity at the center of his body which is 2.5 m from the post.Give the direction, units and magnitude. Explain.Nod is moving on a circle at constant speed. Nod makes one revolution = 2πr in T.2 rv = π = 2.5m/ sTAt the instant shown the direction of Nod’s velocity is − ĵ (as mentioned above)2). [10 pts] What is Nod’s acceleration vector at the instant shown (direction, units, andmagnitude). Explain.The acceleration is toward the center of the circle and has magnitude v 2 /r (Nod is moving on acircle at constant speed).2r v2a Nod= ( −iˆ)= −2.5ˆi m / sr

Name: ______________________________________________________________ Total Points: _______(Last)(First)3). [15 pts]What is the force F rBNthe force with which Nod pulls on Blinkin? Give thedirection, units and magnitude. Explain.r rAccording the Newton’s 3 rd law F BN= −FDraw a FBD for NodNBF rNBN r NINEW rrFNB= marN= −30 × 2.5×iˆ= −75ˆi NThusr= 75iˆNF BN4). [10 pts] What is the force F rBWthe force with which Winkin pulls on Blinkin? Give thedirection, units and magnitude. Explain.Draw a FBD for BlinkinF rBWN r BIBEW rF rBNThe velocity of Blinkin isrv B= −1.5 ˆjm / sThe acceleration of Blinkin isThusrFBWr+ FBNr= maBr⇒ FBWra B= −1.5ˆi m / s2= 30 × ( −1.5ˆ)i − 75ˆ i = −120ˆi N