<p> RI-1 An object's acceleration vs. time is:</p><p> a </p><p> t </p><p>Which graph best represents the object's velocity vs. time? v v Pink Yellow </p><p> t t </p><p> v Blue v Green </p><p> t t </p><p>Purple: None of these! </p><p>Correct answer is Yellow. RI-2 A student is asked the following question on an exam: A car is moving at 90mph and suddenly brakes with a constant acceleration a = -2g to a full stop. How many second does the car take to stop?</p><p>Which equation can be used to answer the question:</p><p>1 2 Yellow: x  xo  vo t  2 a t</p><p>Blue: v  vo  at 2 2 Green: v  vo  2a(x  xo )</p><p>Answer: v  vo  a t . We know the initial velocity vo, the final speed (v=0) and the acceleration a. We seek the time t.  RI-3 Consider the vector A</p><p> y Which equation is correct? </p><p>Yellow: A x  A cos</p><p>Blue: A x  A cos</p><p> x Green: A x  A sin Pink: A  A sin  x</p><p>A </p><p>Answer: A x  A sin RI-4 The "multiflash photograph" below shows a ball rolling along a surface. The camera flashed once a second, and the time is shown above each image.</p><p>Which graph below best represents the ball's velocity as a function of time?</p><p>Correct answer is Purple. RI-5    What is the direction of vector C  A  B?</p><p>Ax = 1, Ay = 1</p><p>Bx = -2, By = 2 </p><p> y</p><p>Blue Pink</p><p> x Green Yellow</p><p>Purple if along x or y axes.</p><p>Correct answer is Blue. Cx = Ax + Bx = +1 - 2 = -1, Cy = Ay + By = +1 + 2 = +3. The vector C points to the upper left, in the second quandrant. RI-6. A particle is moving with constant acceleration. Its velocity vector at two different times is shown below. What is the direction of the acceleration? y V 1 Blue Purple: None of these!</p><p>Green Yellow V 2 Pink x</p><p>Answer: The direction of the acceleration is the same as the  v2 direction of v , which is up. Remember the vector v is the vector you vector-add to vector v1 to get vector v2. v </p><p> v1 </p><p>Australia!</p><p> g V 2 V 1 RI-7. </p><p>A projectile is fired at an initial speed of vo at an angle of  above the horizontal. Which of the following expressions could possibly be the correct expression for the range of the projectile? (Hint: check the units).</p><p>2 Pink : x  2 g v0 tan  Yellow : x  2 g vo sin(2) v 2 g Green : x  o sin 2 Blue : x  sin 2   2   g vo</p><p> v2 Answer: Only the equation x  o sin2 has the dimensions of length on both sides of g the equation. RI-8. A car moves around a figure-eight track at a constant speed in the direction shown. Over which part of the track is the magnitude of the car's acceleration smallest? B D</p><p>C A</p><p>Blue: from A to B Green: from C to D Yellow: Neither.</p><p>Answer: Neither! When moving along the straight portions of the track (B to C or D to A) the acceleration is zero. RI-9. An elevator is going up at a constant speed. Near the top floor, it starts to slow to a stop. During the period that it is slowing down, its acceleration is</p><p>BLUE: downward</p><p>YELLOW: upward. </p><p>PINK: in some other direction.</p><p>Answer: Acceleration is downward even though the velocity is upward. Use the "delta-v" argument to see this. RI-10 A bucket containing a brick is swung in a circle at constant speed in a vertical plane as shown. The bucket is swung fast enough that the brick does not fall out.</p><p>The net force on the brick as it is swung has T maximum magnitude at position. Pink: Top. Green:Bottom. Yellow: Right Purple: The net force has the same magnitude at all positions. N R</p><p>Answer: The net force has the same magnitude at all positions. Since this is circular motion with constant speed, the magnitude of the acceleration (a=v2/R) is B constant and so is Fnet=ma.</p><p>Consider the normal force exerted on the brick by the bucket when the bucket is at the three positions shown: R, T, B. The magnitude of the normal force is a minimum at position... Pink: Top. Green:Bottom. Yellow: Right Purple: The normal force has the same magnitude at all positions.</p><p>T Answer: The normal force is minimum at position T(top). At both positions B and T, the normal force N vector added to the weight mg mg N is the net force Fnet. Since Fnet and mg are constant in magnitude, N is minimum when it Ffric points in the same direction as mg . </p><p>R N N mg </p><p>B mg </p>