# Day 117: Are you kidding? It’s that easy? and The Sledder Lab

Today we summarized the four main energy models; Eelas, Ek, Eg, and Ediss. After this we finally looked at two COE sample problems.  The first one was this:

It was the same problem I gave the students to introduce the energy unit.  At the time, they solved it (or attempted to) with kinematics and dynamics.  It is about 15 steps to get it done.  I mentioned at the time, we could probably do it in in 2 or 3 steps.  So today was the day to show them that.  They caught on right away… decide your initial and final situations, draw the energy bars so you can write the energy equation, then solve.  For this problem it is Eg (initial) = Ediss (final).  From there I STRONGLY suggested they use substitution… that COE equation boils down to h=μk*Δd.  (There is another step in this, it is to use theΔd and the 70cm rough patch length to find the final resting spot, but that was not thee important part of the lesson.)

In many of the classes I heard comments like the title of the post and that’s actually pretty cool.  One of the reasons I like substitution is that it really helps kids with conceptual questiions like… what happens to the distance is slides if I double the mass?  Well, nothing, mass cancels out.

The second problem was predicting the velocity of a pendulum at the bottom of it’s swing.  This one is equally simple and then we actually check it with a bowling ball pendulum hanging from the ceiling.  We use a Vernier ‘laser-gate’ with one gate timing to check the prediction.  It’s typically very close, but smaller, because there is a tiny bit f energy dissipated.  It provides a chance for us to do this demo:

General Physics:

We discussed the N2L assessment the students took yesterday, then did a fair amount of discussion to get ready for the ‘deployment’ activity.  The assessment was a sledder on a frictionless hill, not realistic at all, because there IS friction.  The goal of the experiment is to determine the coefficient of friction for a sled and the snow.  Yep, we’re going sledding tomorrow.

Through the course of the discussion, we see that we need the angle of the hill,  the combined mass of the sledder and sled, and the acceleration.  To determine the acceleration, we ill be using the LabQuest2 x-accelerometer.  It was absolutely incredible today using the LabQuest Viewer software and our wifi network to show hte students how to set-up and use the LabQuest2.   Way to go Vernier, as always, you guys ROCK.