Interactive Physics Worksheet #3 — “Pushing Game”
Overview
In “Pushing Game” you have control over the direction and magnitude of the net force applied to a body that is free to move in two dimensions—i.e., a plane. You may wish to think of the body as a hockey puck on a frictionless level field of ice, however, in later parts of this worksheet we will consider the body to be a race car negotiating a track with curves.
Please “punch in” and “punch out” of the computer lab using the time card at right and write your answers to each question on the worksheet itself in the space provided.
Getting Started
Locate and open the Interactive Physics module—“Pushing Game”—in the Physics 121 folder. When the window opens you will see a scene like that shown in Figure 1.
Fig 1: The startup screen from the “Pushing Game.”
The “body” is the circular object at the lower left. The magnitude and direction of the force on the body are controlled by the “Mag” and “Dir” sliders and indicated graphically by the vector labeled “FT” attached to the body. The velocity of the body is indicated in terms of x- and y-components (“Vx” and “Vy”), magnitude and direction (“|V|” and “Angle”), and graphically by a vector labeled “V” attached to the body.
The direction (“Dir”) control takes a little getting used to; its slider is calibrated in terms of the angle counterclockwise from the +x-axis and runs from 0° to 360°. Get familiar with this slider: Turn on the display of the coordinate axes by clicking the mouse cursor on “Show Axes.” Click on the “Run” control button. Position the mouse cursor over the direction slider control. “Drag” the slider control up and down (i.e., press the mouse button and hold it down while moving the mouse up and down) and observe the effect on the force vector FT. Figure 2 provides 5 drawings that are missing some combination of net force vectors, slider control indicators, and numerical readouts.
Q1: Provide the missing information in each case.
Fig. 2: Drawings of the “Dir” control and the body with missing information.
Reset the simulation and adjust the force to 4.00 N at 0.00 degrees. Click on “Run” and watch the object move off to the right. Notice that small circular dots are left on the path of the object at 1.00 second intervals.
Q2: How far does the object move during the first second (i.e. from t = 0 to t = 1.0 s), during the second second (i.e., from t = 1.0 s to 2.0s), during the third second, etc.? What pattern do you see? What do you suppose this pattern means?
Q3: How far does the object move during the first second (i.e., from t = 0 s to 1.0 s), during the first two seconds (i.e., from t = 0 s to 2.0 s), during the first three seconds, etc.? Do you see a pattern? What pattern do you get if you divide all of these numbers by 2?
Reset the simulation and adjust the force direction to 30 degrees. Run it and watch.
Q4: How has the motion changed from that observed before?
Reset the simulation and turn the display of the axes off. Run the simulation and just play with it for a while until you get comfortable with the sliders. (Note that you can stop the simulation any time you want, make adjustments in the force, and restart from where you left off if the action gets too fast. Also note that you can use the “Tape Player” controls at the bottom of the window to go back to any point in the simulation and start from there.)
Now get the object moving and then try to apply a force to it in such a way that its speed |V| (represented by the length of the velocity vector) does not increase or decrease.
Q5: Do you think this should even be possible? If so, how would you do it? If not, why not?
Q6: What can you say about the direction of a force that makes the speed increase?
Q7: What can you say about the direction of a force that makes the speed decrease?
With the body moving at a moderate speed, try to exert a force on it that is always at right angles to its velocity vector. (This is a little difficult, just do your best.)
Q8: What kind of path does the object follow when you do this? Can you explain why the path should look like that?
With the body moving in some arbitrary direction, try to stop it.
Q9: Can you? What do you have to do?
Reset the simulation and turn on the display of “Track #1.” Make the path of the object follow the track as closely as possible. (Hint: You will probably want to take advantage of the “Tape Player” here. If your path wanders off, just stop the simulation, go back to an earlier frame, and restart. When you finally get to the end of the track, reset the simulation and rerun it to see how well you did.)
Q10: What was the most difficult part of the track to follow? Why? What did you have to do to get past that part?
THE FINAL TEST! Turn off the display of Track #1 and turn on Track #2. Now you are a race car driver. Your assignment is to get to the “Finish Line” in the shortest time possible while keeping the path of the object within the boundaries of the track. (The object itself may extend outside of the boundaries, but try to keep the recorded path inside.)
Q11: What kinds of maneuvers help you get around the track quickly?
Q12: What limits the speed that you can travel as you go around the track?
Q11: What’s your best time?
Q12: What would you say is the most important thing you have learned from this module?
Q13: Any suggestions for making it better?
AJM:10/2/95