Planning for the activity

[admin]

Time Requirements
• One class period.
Materials Needed
For each group:
• dynamics cart
• modeling clay
• spring scale, 0-10 newton range
• tape, masking, 3/4" x 60 yds
• ticker tape timer
• weight, 100 g slotted mass
Advance Preparation Pand Setup
Two matters need to be considered in advance: (1)
the pairs of objects to be used by each group in the
collisions, including how the masses will be varied
and how the masses will be caused to stick together
upon colliding, and (2) how the velocity will be
measured before and after the collision.



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Regarding the objects to be collided, two
possibilities seem to exist, air track gliders or
laboratory carts. At a minimum, three collisions are
desired, involving mass ratios of 1:1, 2:1 and 1:2
(the moving mass is listed first in the ratios listed –
see the data table in For You To Do for details).
Masses of 1 and 2 kg are recommended, but
certainly not required. Instead, one laboratory cart
could collide with another identical laboratory cart
to provide a 1:1 mass ratio, and a 2:1 (and 1:2) ratio
could be obtained by loading one cart to double its
mass. Similarly, air track gliders could be rigged to
provide 1:1 and 2:1 mass ratios.
The colliding objects need to stick together upon
colliding to move as a single object after the
collision. Stick-on patches are very convenient for
this purpose, but double-stick tape or modeling clay
also works.
Whatever objects are used, have the students
measure and use their masses, in kilograms, to call
attention to and engage students in using the unit
of momentum, (kg)m/s. If the masses are not to be
1 and 2 kg, have students change the values listed
in the data table in For You To Do to the values to
be used in your class.
The speed, in meters per second, must be measured before and after the collision. It is necessary to measure the speed of the incoming mass before the collision and the speed of the combined masses after the collision. One way to accomplish this would be to use a sonic ranging device to monitor the speed of Object 1 (the mass moving before the
collision) before, during, and after the collision.  Other possibilities for measuring the speeds include stop action video, strobe photography, a ticker-tape timer or a spark timer. The particular method to be used depends on the equipment available at your school. Whatever method of measuring speeds is used, students should record speeds in m/s.  For better data if friction is involved (as when using laboratory carts) it would be best to use the speed values which occur just before and just after the collision; this would help to avoid changes in speed  (deceleration) as a source of error.  You may wish to provide additional mass ratios for students to use for staging additional collisions. A 3:1 mass ratio run “both ways,” 3:1 and 1:3, gives particularly interesting results; if organized on bifilar supports to collide head-on as pendulums, hard wooden or metal spheres having a 3:1 mass ratio provide a cyclically repeating sequence of collisions.
 


PAGE 105

“Nonsticky” collisions present problems for
measuring speed because both masses move
simultaneously at different speeds. This is difficult,
but not impossible to overcome with ordinary
equipment and may present an interesting challenge
to interested students.

[Tracy Greeley-Adams]

The web site Becky listed under the general suggestions was perfect.  I showed it as the students were answering the WDYT and it really helped show or remind the students what a pole vault was.  Great job Becky!