Evaluation Activities Sample
Dr. Aaron Warren
Purdue University – North Central
Activity #1 (Based on Ch.4, Problem 36 in Serway & Faughn College Physics)
1. A block with mass m
1
= 10.0kg on a horizontal table is connected to a cable that passes over a pulley and is then fastened to a hanging object with mass m
2
= 4.0kg (see figure below). The coefficient of kinetic friction is 0.30. After the masses are set in motion, what will be the acceleration of the system? m
1 m
2
2. Perform a unit analysis for your prediction in #1, and explain why this analysis is useful.
3. Perform a special-case analysis for your prediction in #1 by completing the steps below. a. Choose a special case that simplifies the problem situation, and explain what you intuitively believe the acceleration should be for that special case. (Possible
Hints: What should the acceleration be if m
2
shrunk to 0kg? Or if m
1
grew to be infinite? Or if m
1
shrunk to 0kg?)
Possible Responses:
1) If m
2
became 0kg, then the acceleration should be 0m/s
2
because there would be nothing pulling on m
1
.
2) If m
1
became infinite, then the acceleration should be 0m/s
2
because no finite amount of force would be able to make it move.
3) If m
1
became 0kg, then the acceleration should be 9.8m/s 2 because m
2 would be in freefall. b. Plug your chosen special-case value into your prediction for #1. Does your mathematical prediction match your intuitive prediction? If not, discuss what you think may be causing the mismatch. c. Explain why this special-case analysis is useful.
Ideal Response: Special-case analysis allows me to test my work, correct my own mistakes, and gain confidence in my work if it passes the analysis. It also helps me to integrate the new material with what I already know.

Activity #2: Transfer to a Semi-Realistic Context
Geneticists and genetic engineers often use a method called gel electrophoresis to estimate the sizes of fragments of DNA (and sometimes to estimate the sizes of proteins).
Here is a sketch of a typical system:
Gel
+
+
+
+
Fragments of DNA, negatively charged.
The right side is positively charged, and the left side is negatively charged. Fragments of
DNA are embedded within the gel. These fragments are negatively charged (a chemist would say they’re polyanionic ). The gel makes it harder for large fragments to move, so the fragments become separated as smaller fragments are able to travel faster. The distribution of fragments is then mapped so that the geneticists can know how many fragments of each size there are.
1. Your research advisor wants the smallest DNA fragments to reach the right side of the gel within 45 minutes. The smallest DNA fragments (1000bp) in your sample require an electric force of 9.6x10
-14
N to make it to the right side in 45 minutes. These fragments each have a mass of about 1.0x10
-21 kg, and a charge of -3.2x10
-10 μC. What should the magnitude of the net electric field be? (Assume that the electric field is constant within the gel).
2.
Before you give your answer to your boss, you decide to evaluate your work to make sure it’s reasonable. a.
First, do a unit analysis of your work in both parts of #1. b.
Next, do a special-case analysis. From prior experience, you know that a
1,000bp fragment in a 225V/m electric field will travel through the gel in 60 minutes. Test whether your solution to #1 is consistent with this. What assumptions about the gel must you make during your special-case analysis?