Lab 2

We are familiar with the concept of force: it is a push or pull . But in order to make force into a quantity that has scientific meaning, we need to construct a scale and assign a unit . To do this, loop an elastic band around the upright rod on your lab bench and pull it a few centimeters until you can feel the elastic band is stretching a bit. If you pull the elastic band with a given amount of force, it should stretch to the same length. We will use this fact to create our scale. Choose a standard length for stretching the elastic band (probably between 5 and 12 cm – it’s up to you!) and record it below. Now, every time you stretch an elastic band to that length, you should have the same force. We’ll call this amount of force 1 Standard-Length Elastic Band Stretch (1 SLEBS) . Standard length: Put two elastic bands on the upright rod and pull them simultaneously. How does the force required to pull them both to the standard length compare to the force for only one elastic band? How many SLEBS of force are required to pull two elastic bands to the standard length? How many elastic bands would be required if you wanted to create a force of 7 SLEBS? ° Question A-1 (5 pts) As it turns out, there are some shortcomings to using the SLEBS scale for measuring force. Talk with your lab partner and come up with a list of at least three reasons why the SLEBS scale might not be the best way to create a scale for measuring force. Explain the three reasons below: standard length 8.5cm 1. It takes more force to pull. 2. To pull two bands that are parallel it is be 2 SLEBS. 3. It would take 7 rubber bands for 7 SLEBS.

In order to be useful, a force probe should report data that is proportional to the quantity it is measuring. In order to make a linear relationship, we need to zero the force sensor. Save your experimental file, then close Logger Pro. Re-open the file Rubber Bands (L03A1-2b) . This time, zero the force sensor by pressing the ∅ (slash-through zero) button when there is no force applied to the force sensor. Now, repeat the data collection you carried out in Activity A-2. Apply a linear fit and insert your graph below. Save a copy of your Logger Pro file with the data and give it the name “Activity A-4”. Upload this file to Canvas (Assignments → Lab 02 Logger Pro Files – Force and Motion). ° Question A-2 (5 pts) Forgetting to zero your sensor often creates a systematic error . In science, error is not a mistake: instead, error is the difference between a measured value and the true value. For example, if you measure that π = 3.76 instead of 3.14 then the error is 0.62. A systematic error is when all of your measurements have the same error. For example, measurements of length are 0.5 cm larger than the true value, or measurements of mass are all half the true value. Systematic errors are commonplace in science, and it is very important to account for them. Why is it important to think about sources of systematic error when doing physics experiments? Write a paragraph response, including some examples you can think of. It is important to consider sources of systematic errors because the errors will skew your results and lead to invalid conclusion. Some examples of systematic errors would be not calibrating your scale when weighing something, not calibrating the spectrophotometer before measuring a sample, and using imprecise tools (yard stick to meter tiny rubber band).

° Activity C-1 (5 pts) (Corresponds to Activity 2-2 in Real Time Physics) Cut a piece of string that is about 1.5 m long and tie loops in both ends. Put one loop on the hook of the force probe, pass the string over the pulley, and hang a 10 g mass from the other end. Open the experiment file Speeding Up Again (L03A2-2) . Be sure to zero your force sensor with no force being applied to it. When you are collecting data, loosely hold the force probe cable in the air so that it is not pulling on the cart. Save a copy of your Logger Pro file with the data and give it the name “Activity C-1”. Upload this file to Canvas (Assignments → Lab 02 Logger Pro Files – Force and Motion). Adjust the axes and insert your graph below: ° Question C-1 (5 pts)

° Activity C-3 (5 pts) (Corresponds to Extension 2-4 in Real Time Physics) Repeat the experiment from Activity C-1 five more times, for a total of seven times, using the following masses: 50 g, 70 g, 100 g, 150 g, 200 g (you can use the brass slotted mass set, in a small blue box, for some of these masses). Record your data in the following table: Hanging mass (g) Hanging mass (kg) acceleration (m/s 2 ) force (N) 10 0.010 20 0.020 50 70 100 150 200 Save copies of your Logger Pro files with the data with the names “Activity C-3-50”, “Activity C-3-70”, “Activity C-3-100”, “Activity C-3-150”, “Activity C-3-200”. Upload these files to Canvas (Assignments → Lab 02 Logger Pro Files – Force and Motion). ° Activity C-4 (5 pts) (Corresponds to Activity 2-5 in Real Time Physics) 0.1513 0.0813 0.257 0.185 0.050 0.8446 0.4353 0.070 1.174 0.5717 0.100 1.563 0.7888 0.150 1.585 1.237 0.200 1.777 1.637

mass. What relationship do you expect to see? Adjust the axes and insert your graph below: Save a copy of your Logger Pro file with the data and give it the name “Activity C-5”. Upload this file to Canvas (Assignments → Lab 02 Logger Pro Files – Force and Motion). Finally, solve the “physics problem” with variables to determine the theoretical prediction for the relationship between the acceleration (a) and the hanging mass (m). First, draw free-body diagrams and write equations for the forces on the hanging mass (m) and the cart (M). Then, use those two equations to eliminate the tension (T), and solve for the acceleration. Insert your answer in the form “a = …” below. Does this match your prediction, the experimental results, or neither? I expect a proportional relationship. ↑ Fr T a + X M & & Y ↑ T -xm = mg - T ⑨ Fe mc = my - T & M Fxm = Ma l T = Ma Fg = my F + ne + = (m + m)c my - T + T = (m + m)c = m + M