Physics: The End of Momentum

This week, our momentum transfer unit is coming to a close.  After the exciting introduction to the unit with the egg drop challenge, we explored many more momentum-y (not a word) scenarios through a few weeks of whiteboarding activities targeting the application of the impulse-momentum equation.  Before we transition to energy, I love to close this unit with one of my FAVORITE mini challenges.  Ready for all the details?  Yes!

The Compelling Introduction

The students walk into the room and I have one Vernier track and two carts (one with a plunger and one without) set up on the table at the front of the room.  At one end of the track, I have mounted a force sensor quite sophisticatedly (using tape).  I have LoggerPro projected behind me displaying an empty force vs time graph.  After the kids are settled, I say something like, "Let me tell you a story."  I pause for a bit of drama.  I hold up the plunger-less green carts from the track in front of me and continue.  "This may look like a cart to you.  But for the tiny people who drive it, this is their car, their mode of transportation.  Unfortunately for these tiny people, the car manufacturers in their land are rather behind the times in terms of safety.  They don't make seatbelts, padded dashboards, bumpers, or airbags.  These little people risk their lives every day when they drive these doomed contraptions.  You all are bumper manufacturers determined to protect your tiny passengers from almost certain death.  Let's see what you are working with."  I place the cart on the track right up against the cart with the plunger at about the halfway mark on the track (usually a bit closer to the force sensor end than the free end).  I pick a reference point on the ruler so that every subsequent test will be from the same position.  Releasing the plunger and pressing record on LoggerPro, I launch the cart into the force sensor (like a crash-test dummy situation) creating a very dramatic looking Force (N) vs time (s) graph with just one sharp, narrow peak (the max force being anywhere from around 20N to 30N depending on how far away the cart was at the start).  I explain that this impact force is nowhere near survivable for our little people and I task them with bringing this down to 1N!  At this point, they usually exclaim that this is impossible!

The Guidelines

In order to get this impact force down, I explain that they are going to design a bumper using only paper and scotch tape.  I continue by specifying that the bumper must not incorporate friction (drag on the track) and must only contact the force sensor during impact (if you miss the sensor, the force can't be read!).  I also mention that they can test their designs as many times as they want during this one class period with the goal of the final design achieving the 1N or less impact force.  All of the extra carts and materials (paper, tape, scissors) are lined up on the table in the back of the room.  I let them work either alone or with only one other person.

What Happened?

The students rush to the materials and immediately get started.  During our hour together, some decide to test repeatedly, documenting every force value on the front board.  Others are more reserved, testing only a few times.  I love watching all of the different design approaches and observing how most students immediately hone in on increasing impact time as the key to success.  This activity also brings out the competitive spirit in many of the students.  I am always amazed at how determined they become to get the lowest force in the class.

Below are a few examples of the designs the students created.  While these are admittedly not the best designs for navigating winding roadways or congested traffic, they absolutely address the objective.  Several groups got the impact force quite near and even below 1N!

I would be remiss if I didn't show the data.  I love how the force vs time graphs below show the gradual transition from a force-heavy impulse to a time-heavy impulse.  This ties so well into all of the momentum work we have done!  You can also check out the very short video of what the collision looks like!

Will I do this again?

Yes.  Not only does it conclude our momentum unit quite nicely but...