# Day 75: Working Backwards and The Speeder and the State Trooper

Today was a WB session on a set of 5 ‘static equilibrium’ problems.  The students had been working on them for nearly one week, a few each night.  I truly believe it is important for the students to present the solutions to problems they have solved, (key word here is solved) which is one of the reasons I still WB problems.  It allows me to listen to the explanations and do some on-the-fly-formative assessment on the level of understanding.

Theses are the longest and most involved problems I have given them, here are two examples:  Sometimes these problems can look intimidating, especially one like #3 above.  To help with this, I have told my students I want them to become like Pavlov’s Dogs, no, not to salivate when they hear a bell, but when they are presented with a problem/question that has the word force in it, the classically conditioned response should be to draw a force diagram WITH the net force equations.  From there I drive at solving the problem by ‘working’ backwards.

SIDEBAR 2: Our chemistry and physics teachers use the acronym PIPES for our problem solving approach.  P- State the problem, essentially this is what you are solving for, the unknown. I- Information, what are the givens. P/E: Plan and explain HOW you will solve the problem, what steps will be followed. S- Solve it showing all your work.  The work backwards approach comes in at our P/E step.

Here is an example P/E and working backwards from problem 4 above:

P/E: 5. Find Ft from the Fnet x equation Fnet x=-Ft + (-) Ff + Fg x=0

4. Find Fg x from Trig (sine of the angle) and Fg

3. Find Ff from Ff – COF*Fn

2. Find Fn from Fnet y equation: Fnet y = Fn +(-)Fg y=0

1. Find Fg y from trig (cosine of the angle) and Fg

So start with step 1 on the bottom, then go up to step 2 …

Some students have ‘bought into’ the approach, but others are resistant, I’m not sure why though.

General Physics:

Today was the first part of a challenge lab we call the Speeder and the State Trooper. Here is the basic scenario: So today was about using the motion detector to determine the velocity of the dune buggy and the acceleration of the phan cart.  Once the data is gathered, the students can choose to solve it graphically ( with a position-time graph) or with the kinematic equations.