Blog Post – Circular Motion
To introduce the
topic of circular motion, a rubber stopper was swung around in a
vertical circle at “a constant speed”.
We were asked if the stopper was accelerating, and then we discussed
ways to interpret this question. Using a
schema and FBD’s created at the extreme right and left endpoints, it became
clear that there was an unbalanced force acting always toward the center of the
motion. From this, we used our knowledge
of unbalanced forces to state that it must be accelerating.
Demonstration: Throwing the stopper. In a wide area, Don practiced throwing the
rubber stopper at a participant and it was observed that the stopper traveled
along a tangent line of the circular motion.
From this, we drew a motion map showing the velocity and acceleration of
an object moving through uniform circular motion; we then wrote our first two
rules:
1)
Velocity
arrows always point tangent to the circular path.
2)
Acceleration arrows always point toward the
center of the circular path.
Other
Demonstrations: A clever way of
showing how velocity is related to the distance an object is from the center of
rotation involved 4 people walking shoulder to shoulder around a central
object. The 1st person walks
slowly near the central object, and then a person is added one at a time. By the time the 4th person is added,
that person needs to run in order to keep up.
It was discussed why the person on the outside leans inward and then it
was demonstrated how an object hung from a string also “leans inward” when
moving in a circle.
Worksheets: We then worked on a few problems on a
worksheet. These problems are always
tricky for students! After white
boarding three examples (merry go round, graviton, and roller coaster), it was
demonstrated how an object resting on a plate can be swung in a vertical circle
without falling off, provided the speed is high enough.
Practicum: Our final practicum involved using a
pendulum made of sewing thread (low tensile strength); We could choose our own weight and angle of
release. We needed to allow the weight
to pass through equilibrium without breaking the string, and then by increasing
the angle by 10 degrees, the string was to break. A very good, but difficult practicum!
How I feel about
it: Very fun topic with lots of
interesting and engaging demonstrations.
However, this topic is often difficult for students to fully
understand. I’m interested in seeing if
student understanding improves with a modeling approach. I’ve done most of the demonstrations before,
but haven’t always done a thorough job of developing how we know the object is
accelerating. I really liked the
demonstration of the 4 people walking shoulder to shoulder!
How I plan to
implement: I see myself
following this unit design pretty much as it is written. Very powerful demonstrations and the discussion
techniques employed seem to do a good job explaining centripetal
acceleration. I expect I will need to
show numerous examples of solving problems with circular motion.
Difficulties I see
coming: I imagine students will
have difficulty applying Newton’s 2nd law to cars going over hills
and cresting the bottom of a hill.
However, with numerous examples and the white boarding discussion
techniques, I believe students will overcome these difficulties. I am excited to try the practicum with my
students to see if they figure it out!
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