Force Vectors:
Mr. McConnell
Objective: To support a known mass with string and compare theoretical tension to
experimental tension using a force vector analysis.
Materials: Twine, clamp, spring scales, masses (200g and 500g), protractor
Procedure:
1. Take a piece of string that is approximately a meter in length and tie a loop on
each end and one in the middle.
2. Clamp a spring scale to the table so it is parallel to the surface of the table. This
scale will measure T1.
3. Attach one of the end loops to the spring scale.
4. Hang a 200 gram or 500 gram mass from the middle loop while placing a second
spring scale on the end loop opposite the clamp. This scale will measure T2.
5. By holding onto only the second spring scale, the weight can be positioned so that
the string that extends toward the clamp is parallel to the surface of the table.
6. Once this is done, a protractor can be used to measure an angle of 120 degrees
between the horizontal string and the one leading toward the second spring scale.
7. Record the experimental tension in each string.
8. Do the same for the remaining angles then switch weights and repeat all the other
angles for the other mass.
Data:
500 g
angle
120
135
150
T1exp
T2exp
200 g
angle
110
135
165
T1exp
T2exp
Analyze: For each setup draw a free body diagram labeling all known forces. Solve for
the theoretical tension, T1the and T2the for each mass and angle. I should see all 6 force
diagrams with weights labeled. I want to see at least 2 sets of sample calculations similar
to what is done in class to solve for the unknown magnitudes of T1the and T2the and the
answers for all theoretical values should be summarized in a table with the percent error
calculated on each experimental value. Remember, theoretical T1 and T2 are your
calculated resultants: do not use your experimental (spring scale) measurements to
calculate the theoretical. Discuss errors in measured values and their effect on the results.
Questions:
1. For each setup state whether the actual angle at which the strings were situated was
too big or too small based on comparing the theoretical tensions to the experimental
tensions.
2. If the angle was to be made bigger and get closer to 180 degrees, what happens to the
values of T1 and T2?
3. As the angle between the lines approaches 90 degrees, what values do T1 and T2
approach?