PLC Activity #6
Newton's 2nd Law & Equilibrium
Due: see website for due date
How to get credit for this activity
First, sign-in at the computer and sign-out when the activity is complete. Show your work
and results to a PLC tutor so they can check your work and initial the signoff sheet. Be
prepared to answer questions about the activity or your results.
You will need two spring scales with attached strings, a metal block and clamps for this
activity.
Part 1: Use a Standard Coordinate System to determine tension T in strings
a. Use a spring scale to determine the mass of the block.
b. Use the two spring scales to setup two angled cords (45o above the horizontal) that
are used to support the block.
c. Draw a FBD for the block and setup Newton's equations (∑Fx and ∑Fy).
 Is the block in equilibrium or nonequilibrium?
 Determine the tension Tthy for one of the strings?
d. Read off the tension Texpt from one of the spring scales and use a percent difference
to compare the measured tension Texpt with Tthy. How do they compare?
Part 2: Use a Rotated Coordinate System to determine the weight of the hanging mass
Note: no new measured are required for Part 2.
a. Using the same setup as in Part (1), rotate your coordinate system so that the x- and
y-axes line up with each string.
b. Draw a FBD for the block and setup Newton's equations (∑Fx and ∑Fy).
 Determine the weight (mg)thy of the handing mass?
c. From your weight measurement in Part (1a), use a percent difference to compare the
measured weight (mg)expt with (mg)thy. How do they compare?