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Newtons Second Law of Motion - DOC - DOC

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					                                                                                           Experiment

                                                                                                    2
               Graphing Constant Acceleration
The most useful examples of motion to study are examples of constant acceleration. Many
complicated motions can be studied by breaking them into separate time intervals, each having a
different constant acceleration. Many motions that derive directly or indirectly from the pull of
gravity are good examples of constant acceleration, as long as air resistance is negligible. We
will use a track that can be tilted to generate constant accelerated motion on a cart.

Make as many predictions as possible as you work through this lab. When you correct your
predictions use a different color pencil, and make sure you and your partners understand the
corrected graphs.


OBJECTIVES/PURPOSE:
    For the special case of one-dimensional motion with a constant (or nearly constant)
      acceleration, student will learn:
    - how to relate graphs plotting position (d) , velocity (v), and acceleration (a) vs time(t).
    - how to read, draw, and analyze such graphs, using slope and area concepts.
      the meaning of positive and negative positions, displacements, velocities, and
      accelerations.


PRE-LAB PREPARATION/PRELIMINARY QUESTIONS
1. Before starting, sketch your predictions of distance, velocity, and acceleration vs. time graphs
   for all the cases you will be trying out in the lab. Use the axes provided in pages 3, 4, 6, & 8.
   The cases that you will be trying out in the lab are:
    -   an object at rest (v=0)
    -   an object moving in the positive direction and slowing down (+v & -a)
    -   an object moving in the positive direction with a constant speed (+v & a=0)
    -   an object moving in the positive direction and speeding up (+v & +a)
    -   an object moving in the negative direction and speeding up (-v & -a)
    -   an object moving in the negative direction and slowing down (-v & +a)
    -   an object moving in the negative direction, slowing down, then turning around (-v & +a)
    -   an object moving in the positive direction, slowing down, then turning around (±v & -a)

MATERIALS
    Power Macintosh or Windows PC                     Vernier Motion Detector & protective cage
    Universal Lab Interface                           dynamics cart & track
    Logger Pro                                        color pencil (to correct graphs)

PROCEDURE
    Part l Setting up the Computer
A. Connect the Motion Detector to PORT 2 of the Universal Lab Interface (ULI) and turn on the
   ULI first with the switch in the back.
    1. Log on the computer using your student password. Then click on the LoggerPro logo.


Physics with Computers                                                                              1-1
Experiment 2

   2. Place the Motion Detector at one end of the track. Make sure that the motion detector is
   behind the protective bar.
      It is very important NOT to let the motion detector be struck by the moving cart
      during the course of this experiment, as this can damage the motion detector! One
      member of the team should be assigned the task of protecting the motion detector.
   3. Prepare the computer for data collection by opening file Exp06 in the Computers in
   Physics folder. You do this first clicking File Menu, then Open, then Computers in Physics,
   and finally Exp06.
   4. The screen should show three graphs: one of position, one of velocity, and one of
   acceleration vs. time. It will be necessary to change the scales of these graphs. The scales can
   be changed easily by clicking a number, changing its value, then pressing the “enter” key. To
   start, set the maximum distance at 2 m, the maximum velocities at ±1 m/s, and the maximum
   accelerations at ±1 m/s2. Finally, change the Sampling rate under the Experiment Menu to 10
   per sec and the Averaging rate to 3 so your graph will be smoother.
   B. The following facts will help you understand your graphs better. So keep them in mind
      when doing this experiment and refer to them when necessary.
   1. The detector is the origin and it is programmed to record all positions in front of it as
   positive positions. Since the detector cannot detect anything behind it, it is not possible to
   generate negative positions in these examples. But a change in position (called the
   displacement or ∆d) will be positive when moving away from the motion detector (+∆d) or
   negative when moving toward the motion detector (-∆d)
   2. As a result of the way the positive direction is defined by the motion detector, moving
   away from the motion detector is moving in the positive direction and represent positive
   velocities (+v). Moving toward the motion detector is moving in the negative direction and
   represent negative velocities (-v).
   3. The motion detector does not read anything closer than 0.5 m. You should attempt to
   keep the cart further than this in all cases. The motion detector also gives you a couple of
   seconds of lead-time (two warning beeps) before it actually starts recording data after
   clicking the Collect button.
   4. In some of these experiments you will be giving the cart an initial push to get it started.
   You should practice a run before collecting data to make sure that the cart will do what you
   want it to do and not run off the track or get too close the motion detector. Work together
   with the members of your group to figure out the best strategy for generating a good run.
   5. Because we are interested in studying constant accelerations in this experiment, you
   should avoid recording any motion during an initial push or a bounce of the cart at the end of
   the track. Concentrate on the motion occuring when the cart is freely moving on the track. If
   you have any motion that is not constant acceleration, simply ignore it or redo that graph.
   6. When reading and analyzing a graph the features of interest are: the values of the graph,
   the slope (or slopes) of the graph, and the area (or areas) defined by the graph and the
   horizontal axis. Pay attention to these features as you study the graphs and answer the
   questions.

                                             Positive direction




1-2                                                                           Physics with Computers
                                                                                Constant Acceleration

Part Il Comparing an object at REST (A) to an object SLOWING DOWN (B).
In these two cases you need to have more friction than provided by the wheels. Use the screw on top of
the cart to push the friction bar down so it slightly touches the track. When you produce a good example,
copy the part of the graph corresponding to the freely moving cart over the predictions that you drew
beforehand

A. Place the cart somewhere in the middle               B. Place the cart 0.5 m from motion
   of the track. Press Collect to start the                detector. Press Collect to start the motion
   motion detector and wait until it Stops.                detector and give the cart enough of a
                                                           push so that it will coast to the end of the
                                                           track from the friction along the way.

                (+)---> direction


                                                                        (+) ---> direction




   d                                                       d




                                             t                                                        t
   v                                                       v




                                             t                                                        t




   a                                                       a




                                             t                                                        t




Physics with Computers                                                                              1-3
Experiment 2

Part IIl Comparing an object moving with CONSTANT VELOCITY (C) to an object SPEEDING
   UP (D) , both moving away from the detector (the positive direction).

We want to minimize friction in the rest of these cases. Use the screw to retract the friction bar back into
its original place. After generating a good example of each case, copy the part of the graph
corresponding to the freely moving cart over the predictions that you drew below in the pre-lab seection.

C. Give the cart a slight push to get it                  D. Elevate the track enough to produce a
   started. The cart should continue to slide                visible acceleration of the cart down the
   without slowing down. If the cart shows                   track and away form the motion detector.
   signs of slowing, elevate the track                       Press Collect to start the motion detector
   slightly to cancel the effect of friction in              and let the cart go. Stop the detector
   the wheels. Press Collect to start the                    before the cart bounces back
   motion detector and wait until it Stops.




                                                                    (+)
                (+)---> direction

   d
                                                             d




                                              t
                                                                                                         t
   v
                                                             v




                                              t
                                                                                                         t




   a
                                                             a




                                              t
                                                                                                         t


1-4                                                                                  Physics with Computers
                                                                              Constant Acceleration



Before proceeding to the next part, answer the following questions about case A,B,C & D.
   Remember that “speed” refers to the magnitude or value of the velocity regardless of
   direction.
    1. What feature of the graphs (such as shape, trend, + value, etc...) indicates that the cart is
       moving away from the detector …

        a) in the position graphs?



        b) in the velocity graphs?



        c) in the acceleration graphs?

    2. What feature of the graphs (if any) indicates that the speed of the cart is changing in some
       way…

        a) in the position graphs?



        b) in the velocity graphs?



        c) in the acceleration graphs?



    3. What feature of the graphs (if any) indicates that the speed of the cart is increasing … (as
       opposed to decreasing)

        a) in the position graphs?



        b) in the velocity graphs?



        c) in the acceleration graphs?




Physics with Computers                                                                            1-5
Experiment 2


      Part IV Comparing an object SLOWING DOWN as it approaches the motion detector with an
               object SPEEDING UP as it approaches the motion detector (-direction)
       This is a good time to recheck the predictions you made in the pre-lab before going on with the
experiment. Proceed to generate the best example of each case and copy the part of the graph
corresponding to the freely moving cart over your predictions.

E. On the tilted track, place the cart at the           F. Place the motion detector at the bottom
   bottom. Press Collect to start the motion               of the track. Press Collect to start the
   detector and give the cart a slight push                motion detector and release the cart from
   up toward the motion detector. Before                   the top so that is speeds toward the
   the cart reaches the highest point or gets              detector. Before the cart gets to the
   too close to the detector, Stop recording.              detector Stop recording, and stop cart!




          (+)                                                                             (+)


                                                           d

   d




                                                                                                     t
                                                           v
                                             t
   v


                                                                                                     t

                                             t



                                                           a

   a


                                                                                                     t

                                             t


1-6                                                                               Physics with Computers
                                                                                           Experiment


Answer the following questions about cases E &F. Make sure you understand these concepts!
                                                                                                2
4. What feature of the graphs (if any) indicates that the cart’s speed is increasing vs decreasing..

        a) in the position graphs?




        b) in the velocity graphs?




        c) in the acceleration graphs?




5. Explain why both graphs show negative velocities values even though the cart moves in
   different directions on the track?




6. Explain what it physically means to have a (-)velocity that is increasing in magnitude and
   what it means to have a (-)velocity that is decreasing in magnitude.




7. How can an object have a (+)acceleration when it is slowing down? How can an object have a
   (-)acceleration when it is speeding up?




Physics with Computers                                                                            1-7
Experiment 2


    Part V Comparing objects TURNING AROUND as they move up and down the incline
G. . With the motion detector at the bottom        H. Put the motion detector back at the top
   of the track, place the cart at the bottom         of the tilted track and place the cart at
   next to it. Press Collect to start the motion      the bottom. Press Collect to start the
   detector and give the cart a slight push           motion detector and give the cart a slight
   up, away from the motion detector.                 push up, toward the motion detector.
   Allow enough time for the detector to              Allow enough time for the detector to
   record the up and down motion of the               record the up and down motion of the
   cart. Stop when the cart is near its               cart. Stop recording when cart is near its
   starting point.                                    starting point.




                                                              (+)
                                  (+)

                                                      d
   d




                                                                                             t
                                             t
                                                      v
   v




                                                                                             t
                                             t




                                                      a
   a




                                                                                             t
                                             t




1-8                                                                        Physics with Computers
                                                                             Constant Acceleration

Answer the following questions about cases G and H. Make sure you understand these concepts!
8. What feature of the graphs (if any) indicates that the cart is turning around

        a) in the position graphs?



        b) in the velocity graphs?



        c) in the acceleration graphs?



9. What feature (if any) of the graphs indicates that the cart is first slowing down and then
    speeding up…
        a) in the position graphs?



        b) in the velocity graphs?



        c) in the acceleration graphs?



10. Explain why the sign (+ or-) of the acceleration doesn’t change when the cart turns around
    and goes from slowing down to speeding up?




11. Why do the two graphs look different even though they represent the same motion of the
    cart? (Note how the graphs are mirror images of each other).




12. When the cart turns around the velocity is zero. Explain why the acceleration is NOT zero.




13. We assumed that friction between track and cart was negligible. Do you see any evidence in
    the graphs that friction was not negligible? Describe the evidence.




Physics with Computers                                                                          1-9
Experiment 2



SUMMARY OF RESULTS
1. In the following chart summarize all the cases by filling in with “+” and “-”, and “I”
   (increasing), “D” (decreasing), or “C” (constant) for each graph:

         A      B      C      D       E      F       G-up     G-down        H-up     H-down

    d +C        +I

    v 0         +D

    a    0      -C



2. Do motion diagrams for all the cases A->H. In each case draw three arrows representing the
   velocities near the start, near the middle, and near the end of the motion; and also draw
   acceleration arrows representing the change in the velocities during the motion. For the last
   two cases draw a set of arrows for the up motion and another set for the down motion.

         No arrows here since v=0 and a=0!                  +v
A                                            B


                                                             -a

C                                            D




E                                            F




G                                            H




1 - 10                                                                        Physics with Computers
                                                                           Constant Acceleration

EXTENSIONS
1.Sketch your best guess of the graphs that describe the following scenarios.

Scenario 1: Move steadily (with constant speed) away from the detector for about three meters,
   then stop for a while, finally return to your original position at a faster, but steady pace.

        d                         v                            a




Scenario 2: Starting about 4 m from the detector, move steadily (with constant speed) toward the
   motion detector for about 2 m, then slow down steadily (with constant acceleration) until you
   come to a stop before the detector.
        d                         v                            a




Scenario 3: Starting near the detector, accelerate steadily away from the detector for about 2 m,
   then hold a constant speed for 1 meter, finally slow down steadily until coming to rest at
   about 4 m from detector.

        d                        v                            a




2. What would be the changes in the position, velocity, and acceleration graphs if the motion
   were being measured from a different origin (0 position). For example, assume that the
   computer monitor is the origin rather than the detector.




3. Explain why we never get negative positions in these graphs.




Physics with Computers                                                                       1 - 11
Experiment 2

    4. You throw a ball up in the air over a motion detector set on the floor. Sketch the motion
       graphs for the ball going up and down after it leaves your hand.
         d                       v                            a




5. You fire a toy rocket vertically and record its motion with a detector on the ground. At first
   the rocket accelerates upward due to its fuel, but eventually the fuel runs out and the rocket
   returns to earth after reaching a peak height. Sketch graphs of the rocket motion including the
   fuel-driven stage of the rocket motion as well as the free-falling stage afterwards. Focus on
   the differences from the case above.
         d                       v                            a




1 - 12                                                                       Physics with Computers