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SOTM LAB: P 4
TEACHER NOTES & GUIDELINES
TITLE OF LAB
Forces and Motion (Atwood Pulley)
DEVELOPERS OF LAB
Kirk Reinhardt, JD738
OVERVIEW OF LAB
DESCRIPTION
In this exercise students will experiment with an Atwood Pulley to determine the
relationship between the applied forces acting on a system and its resultant motion.
Students will be asked to devise procedures that will allow them to correctly predict the
acceleration of the system.
CURRICULUM CONSIDERATIONS
This lab is intended as an expansion to Newton’s Second Law. Students should have
previously studied motion (specifically constant acceleration) and know how to represent
motions using graphs of position, velocity and acceleration vs. time. Students should
also have been introduced to the concept of force and its measurements. Vector addition
and free-body diagrams will be used.
SAFETY CONSIDERATIONS:
Students should be cautioned about allowing the masses to accelerate upwards and
collide with the smart pulley.
BACKGROUND INFORMATION:
A. SCIENTIFIC VIEWPOINT
The underlying concept involved in this lab is Newton’s Second Law (F = ma).
 F y = m ay
Students should also know how to determine the acceleration of an object from
its graph of velocity vs. time.
B. COMMON MISCONCEPTIONS:
1.When a force is applied to an object; it produces motion in the direction of the
force.
2.Under the influence of a constant force, the objects move with constant
velocity.
3. The velocity of an object is proportional to the magnitude of the applied force.
4. In the absence of a force, objects are either at rest or, if moving, are slowing
down.
OBJECTIVES:
After completing the lab, students will be able to:
1.Solve for the acceleration of the masses in the Atwood pulley machine.
a = g (m1 – m2 )/ (m1 + m2 ) m1 > m2
Plot a Vs. (m1 – m2)/ (m1 + m2).
Obtain g from the graph.
2.Describe the motion of an object acted on by a constant force.
3.State the relationship among the variables f, m and a (F = ma)
EQUIPMENT/MATERIALS
*Computer with PASCO 500/750 interface
spring scale reading in Newtons
*smart pulley, universal clamp and string
*mass set and hanger
*surge protector
*provided by SOTM
ADVANCED PREPARATION:
It would be helpful for the teacher to have assembled one set of apparatus and to have a
way of displaying the software to the whole class.
II. PRE-LAB
PRE-LAB EXERCISE TO
MISCONCEPTIONS
ELICIT STUDENTS’
PRIOR KNOWLEDGE AND
The teacher will set up and demonstrate the pulley system at rest
Discuss equilibrium and the sum of the forces = ma = 0.
Push one of the masses downward and have the students observe the motion of the
masses.
Ask the students to write answers to following questions:
1. What type of motion does the cart have?
2. What caused this type of motion?
3. Would the masses continue with this type of motion?
4. How would you graph displacement versus time?
5. How would you graph velocity versus time?
Pulley
T
Mass 2
Mass 1
T
M1
M1g
M2
M2g
DISCUSSION OF PRECONCEPTIONS
When the students have completed this, discuss the responses to the first three
questions as a class.
Important points:
The mass is moving with no unbalanced force acting on it.
The speed of the cart in nearly constant.
If the idea of friction comes up in the discussion, note that the friction is minimal
and acts in the opposite direction of velocity.
Second, have different students put a few of the graphs on the board and discuss
these bringing out the misconceptions about uniform motion versus acceleration.
(If you have the ability, show the graphs with PASCO)
III. EXPLORATION OF SCIENTIFIC PRINCIPLE
&
INTRODUCTION
OF
EXPERIMENTAL
PROTOCOL
PROBLEM
What will happen to motion of the masses if we apply an unbalanced force to the pulley
system?
EXPERIMENT AND TECHNICAL OPERATION OF EQUIPMENT
The teacher will run and describe the lab using the PASCO interface equipment and the
smart pulley. The teacher will take one of the smaller masses from one hanger to the
other (Keep the mass of the system constant) and let the larger mass move downward.
Ask the following questions:
What type of motion does the mass have?
What caused this type of motion?
Sketch the following graphs:
Displacement versus time
Velocity versus time.
Using the PASCO and laptop , show the students the three graphs( d Vs t, v Vs t
& a Vs t)
Discuss these graphs and make sure equipment and graphs are clear.
IV. ELABORATION OF SCIENTIFIC PRINCIPLE:
INQUIRY-BASED STUDENT INVESTIGATION
PROBLEM
What would happen to the motion of the masses if the amount of mass being exchanged
increased?
HYPOTHESIS OR PREDICTION
Could you predict the increase in velocity of the mass system by using the known masses
of the individual sets of hangers?
EXPERIMENTAL DESIGN
Can you list the variables that need to be considered.
How can you change the variables you have identified to predict the acceleration of the
system?
Set up a procedure for your lab and call me over when you would like me to look at it.
Smart Pulley
Clamp
Mass 2
Mass 1
Checkpoint (Teacher checks students’ experimental design for feasibility.)
tips: Check variables, how many remain constant or change?(m 1 + m2 is constant) ,
Why they chose these? How this design will answer their specific problem? Check to
see that both masses are being considered. What data are the students collecting?
PLAN FOR DATA COLLECTION & ANALYSIS
When you are setting up your data collection, keep these points in mind.
What kind of data will you need for your prediction?
What graphs will you need based upon your variables?
How will your graphs be able to show the relationship of your prediction?
When you believe that you have a scheme for data collection, call me over.
Checkpoint (Teacher checks students’ plan for feasibility.)
tips: Check number of trials, what are their expectations for their data, and how will
they manipulate their data ( i.e. y = m x + b for linear)?
example of data table:
a  m1 m2  m1 - m2  (m1 – m2)/ (m1 + m2).
CONDUCTING THE EXPERIMENT
Checkpoint (Teacher monitors students’ investigations in progress).
ANALYSIS OF DATA
Are students gathering data that will allow them to solve their problem?
Are they using correct units for their various quantities?
Are their graphs appropriate?
Do they understand what the graphs represent physically?
Checkpoint (Teacher checks students’ analysis.)
DISCUSSION OF RESULTS
COMPARE
What factors did change the acceleration of the cart?
Who was able to show their prediction?
Are there any groups, whose data did not provide enough information to conclude their
prediction?
How do our prediction that were shown compare with our misconceptions?
What misconceptions do we have confidence in dispelling?
PERSUADE
List predictions on the board that were shown by evidence in the lab.
Have students list their reasoning from data why their prediction was shown.
Come to a consensus on a prediction leading students to
a = g (m1 – m2 )/ (m1 + m2 ) m1 > m2
Have students in the class argue their point to other students for the correct prediction
based on their reasoning.
Can this lab lead to any further investigation about motion?(These might allow you to
lead this to other labs or upcoming concepts)
RELATE
Have students come with ways in the real world that the direct relationship between
force and acceleration is used.
More examples: Launch of the Space Shuttle, air bags, force to decelerate braking
systems in vehicles, car on a curved road & roller coasters
V. EVALUATION
POST-LAB SURVEY OF STUDENTS’ CONCEPTIONS
Have students retake the Pre-Lab Exercise. Compare pre-lab and post-lab responses.
TRADITIONAL
Questions: Set up additional questions to expand this concept.
A. Have the smaller mass on horizontal frictionless surface and the larger mass
hanging over the pulley.
1) Have the students draw the model.
2) Draw the free body diagram for both masses.
3) Have them solve for ay.
B. Have the larger mass on a frictionless incline plane.
1) Have the students draw the model.
2) Draw the free body diagram for both masses.
3) Given values for the masses. At what angle will give equilibrium.
ALTERNATIVE
You may provide an alternative assessment of the content, skill, and attitudinal
objectives for this lab with a scoring rubric.
Acknowledgment: This material is based upon work supported by the National Science Foundation under Grant No. ESI 9618936.
Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not
necessarily reflect the views of the National Science Foundation.