Before I explain what we did today in class, I want to once again say how awesome it is to have a local Physics/Physical Science Share group. The Phox Valley Share Groups had another share session tonight, and it was awesome… a few science jokes, NGSS Evidence statements, holograms, rotary motion, a possible community outreach event, and a new take on an old favorite. One of our members (Ryan Peterson @BrillionNerd) and a few of his students built a new and improved version of the Monkey and the Hunter that uses a Vernier DCU in conjunction with a light sensor and laser pointer… TOTALLY AWESOME.
I’m sure most you know are already familiar with the WB discussion for the N2L experiment with the modified Atwood machine. What I want to tell you about is one extra bit I have added.
We get to the point in the discussion where the slope unit of acceleration vs. net force graph are not really recognized (I know you know them, but my students don’t). At this point I have not explained what is ‘inside a Newton’, we just use N. So we have that acceleration is proportional to net force. We also discuss ho acceleration is inversely proportional to the mass of the system. The slope units for the linear graph are also not recognized. I explain that this is OK, because we want to join to the two parts of the experiment. We end up with one relationship… acceleration is proportional to Net force/mass of the system. To turn his into an equation, we need the proportionality constant (or slope) of a graph… acceleration vs. net force/mass of the system. The students use a calculated column in Logger Pro to add this ‘new data’. Here is what it looks like:
Notice the slope… 1.009m/s/s/(N/kg). The slope value is 1.0 and the only way to get that is if what is on the y-axis is equal to what is on the x-axis…. so (m/s/s) = (N/kg) and after a bit of rearranging, N=(m/s/s)*kg… the definition of a Newton of force falls very nicely into our lap!
Now we can go back and explain the meaning of the slope of the other two linear graphs and formally write the equation for N2L that is so familiar to all of us.
We did a horizontally launched projectile ranking task. To check it we used a little homemade device that I can’t recall where I saw. Here is the ranking task:
Tomorrow I’ll include a picture of the device to check it.