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Physics Lab
Coupled Motion
So far in physics, we have analyzed the motion of single objects; either in equilibrium or in a state of
acceleration. It is time to step up our game and begin the study of coupled motion. In coupled motion,
objects are connected to one another. The forces acting between these objects (usually some sort of
tension) act together to affect the overall motion, or lack thereof, of the object.
In this lab, you will determine how the tension acting between objects is established and how that
resulting tension affects the overall motion of the object. Examples of both equilibrium and
nonequilibrium forces will be analyzed.
Hypothesis: Before you begin this lab, write an if...then... because statement relating the tension acting
between objects and the overall acceleration of the system. (Hint: It may be helpful here to read the procedure
2 bricks
2 spring scales
Pulley attached to
Hanging mass set
Part A: Tension Between Objects on a Flat Surface
1. Using a spring scale, determine the mass of each brick. Record the masses.
2. Attach two brick to one another with a string and a spring scale between them. Attach a string
and spring scale to one of the bricks. See diagram below.
3. Apply a horizontal pulling force to the free string until the bricks begin to move at a constant
velocity. Record the spring scale readings from both spring scales as FT1 and FT2.
4. Repeat step 3 twice more.
5. Draw free body diagrams for both bricks in the data section for Part A. Label all forces. Include
numerical values where possible. The coefficient of friction between a paper wrapped brick and
the lab table is roughly 0.28.
Part B: Hanging Masses
1. Set up apparatus as shown in the diagram. Tie a spring scale into the system on both the
horizontal and vertical lengths of string. Ask your instructor if you are unsure of how to do this.
Hang mass until the brick just starts to move. Record the mass of the brick (from Part A and the
total mass needed to just get the brick moving (total hanging mass).
2. Record the tension forces on the spring scales. The horizontal force is FTx. The vertical force is
3. Draw a free body diagram for both the brick and the hanging mass. Label all forces. Include any
possible numerical values. Treat the hanging mass(es) as one object.
Part C: Atwood Machine
1. Set up apparatus as shown in the diagram. Tie spring scales into the system on both vertical
lengths of the string. Use different values for m1 and m2. Record both mass values.
2. Read the tension on both lengths of rope. Record the values as FT1 and FT2, respectively.
3. Draw free body diagrams for both masses. Label all forces. Include numerical values where
Part D: Equilibrium Statics
1. Set up apparatus as shown in diagram. Once set up, the system should be stationary.
2. Record the values for m, FT1, FT2, θ1, and θ2.
Part A
mass of brick 1 = ______________kg
mass of brick 2 = ______________kg
FBD Brick 1
FBD Brick 2
Part B
mass of brick = _______________kg
total hanging mass = ___________kg
FBD brick
FBD hanging mass
Part C
m1 = ______________kg
m2 = ______________kg
Fg1 = ______________N
Fg2 = ______________N
FT1 = ______________N
FT2 = ______________N
Part D
m = _______________kg
FT1 = ______________N
FT2 = ______________N
θ1 = _______________°
θ2 = _______________°
Part A
1. How does the tension in each length of rope compare? Are the values equal? Why or why not?
2. If you were to accelerate the system by increasing the force applied on the free string, how
would the acceleration of Brick 1 compare to the acceleration of Brick 2? Explain.
Part B
1. When the system is in equilibrium, how do the values for the horizontal and vertical tensions
compare to one another? Use data a proof of your answer.
2. If the system were to accelerate so that the hanging mass moved toward the floor, how would
the horizontal and vertical tensions compare? Explain. You may use numbers in your
Part C
1. How do the values for tension in the two sections of the rope compare when the system is in
equilibrium? Use numbers as proof.
2. How will the values for tension compare when the heavier mass is accelerating toward the
3. How will the value for acceleration compare to the value for acceleration due to gravity?
Part D
1. What is the net force acting in this system? How do you know?
2. What is the value for tension in the vertical rope supporting the mass? Explain.
Extensions and Applications
1. Ignoring friction, use your data from Part B to determine the vertical acceleration of the hanging
mass(es). Show your work.
2. Using your data from Part C, determine the value for acceleration of the heavier mass. Show
your work.
3. Mathematically verify your answer for analysis question 1 in Part D of the lab. Show all work.
Use your data.
Lab Write-Up Format
Coupled Motion Lab
Title (1pt)
Date (1pt)
Purpose (3pts) Full sentence.
Hypothesis (3pts) See lab sheet.
Materials (2pts) List all materials and equipment used.
Data (12pts) Include all data and free body diagrams.
Analysis (15pts) Completely and correctly answer all questions. Use data a proof.
Extensions and Applications (18pts) Show all work. Use data from lab.
Total: 60pts