Activity Description

First students are given a review question about the constancy of the speed of light in all reference frames. Then the operation of the light clocks with the light path perpendicular to the direction of motion is demonstrated for the clocks at rest with respect to each other. Students then make a prediction on their own in the form of a comparison between the tick rates of the two clocks when the upper clock will move at relatvie velocity of 0.5c. with respect to the lower. They follow this prediction with small group discussion and then a second prediction.

The activity is repeated for two clocks with the light path parallel to the direction of relative motion.

Target Conceptual Difficulty

Difficulty in applying the constancy of the speed of light to different physical situations

PowerPoint

Comparison Task - Relativistic Clocks

Simulation

Comparison Task - Reltavistic clocks

Presentation Notes

The first part of the task is a review concept test on the constancy of the speed of light.

Then the first simulation should be shown with the clocks perependicular to the relative motion. The operation of the clocks should be shown with the clocks at rest. The studetns shoudl then go throught the prediction cycle of first making the prediction on their own, then having a small group discussion and making the prediction again. Following this a value of &beta = 0.5 should be entered in the text box and the button "set value and play" should be clicked. Once the correct answer is demonstrated to the students then you can have a very nice discussion about why the moving clock is slower.

The activity is then presented in the same way for the clocks that are parallel to the relative motion.

Student Results

The results for these two activities will be tabulated together. Not too surprisingly students had a lot of difficulties with time dilation. For a light clock where the pulse was transverse to the direction of propagation, the students, after small group discussion, predicted it would keep the same time as a stationary clock. For the light clock with the pulses parallel to the direction of travel, the students were about evenly split about whether the clock would run fast or slow. It seems that mid-instruction students' ideas about time dilation were quite muddled.
Questions 1) and 2) are for the transverse clock and question 3) and 4) are for the parallel clock.
1.) When the green clock moves with respect to the red clock it will
Responses
(percent) (count)
Keep the same time as the red clock 36.36% 8
Run faster than the red clock 31.82% 7
Run slower than the red clock 27.27% 6
Cannot be determined 4.55% 1
Totals 100% 22

2.) When the green clock moves with respect to the red clock it will
Responses
(percent) (count)
Keep the same time as the red clock 72.73% 16
Run faster than the red clock 22.73% 5
Run slower than the red clock 4.55% 1
Cannot be determined 0% 0
Totals 100% 22

3.) When the green clock moves with respect to the red clock it will
Responses
(percent) (count)
Keep the same time as the red clock 28.57% 6
Run faster than the red clock 52.38% 11
Run slower than the red clock 19.05% 4
Cannot be determined 0% 0
Totals 100% 21

4.) When the green clock moves with respect to the red clock will
Responses
(percent) (count)
Keep the same time as the red clock 8.70% 2
Run faster than the red clock 43.48% 10
Run slower than the red clock 47.83% 11
Cannot be determined 0% 0
Totals 100% 23

Pen and Paper Actvivities

Pen and Paper version of this activty

Follow up exercises (non-tiper) from homework assignments, test reviews and test items