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Exploration 9.3: Compare Relative Motion in Different Frames

Please wait for the animation to completely load.

An object may appear to have one motion to one observer and a different motion to a second observer, depending on how the two observers are moving relative to one another. This Exploration lets you view objects from different frames of reference.

There is a river (shown in green) in the center of the screen (the yellow dots are stationary with respect to the water) and two red boats are moving with respect to the river. There is a rectangular barge that is stationary with respect to the river and changes color based on your reference frame. There is also a person (shown in blue) walking on the ground (shown in gray with black dots stationary relative to the ground) near the river.

You can easily change your frame of reference by moving the mouse to different regions and to different objects. For example, if you move the mouse within the river, YOU will become an observer moving with the water. The long thin rectangle (the barge) in the water moves with the water and changes color to represent the reference frame you are in. In addition,

  1. Suspend the animation and then turn on "show information." Change the horizontal and vertical speeds of the person so that she swims straight across the river (as seen from the ground). Once you are satisfied with your choice of velocity, deselect the "show information" check box, click in the animation (things are easier to see this way), and resume the animation.
  2. What ground velocity do you have to give her to accomplish this task?
  3. Now move the mouse around to change reference frames. Does the person still swim straight across the river in other reference frames? If not, what velocity must you give her (relative to the ground) to accomplish this as seen from the boats? What velocity must you give her (relative to the ground) to accomplish this as seen from the river?

Download PDF Worksheet

Exploration authored by Fu-Kwun Hwang and Mario Belloni.
Applet authored by Fu-Kwun Hwang, National Taiwan Normal University.

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