Experiment 3: Newton's Second Law and the Atwood Machine
This experiment will demonstrate the mechanical laws of motion using a simple
assembly named the Atwood machine, similar to that used by Rev. George Atwood
in 1784, to verify Newton’s Second Law. Materials
Masking Tape
2 Paperclips
Pulley
5 N Spring scale Stopwatch
String
Tape measure
15 Washers Procedure
Part 1
1. Support the pulley so that objects
hanging from it can descend to the
floor. Do this by tying a short piece
of string to one of the pulley hooks.
Use a piece of masking tape to
secure the string to a table top or
door frame so that the pulley hangs
plumb (Figure 7).
Note: A higher pulley support will
produce longer time intervals which Figure 7: Pulley set up reference for Step 1.
are easier to measure.
2. Thread a piece of string through the pulley so that you can attach washers
to both ends of the string. The string should be long enough for one set of
washers to touch the ground with the other set near the pulley. (You may
attach the washers using a paperclip or by tying them on).
© 2014 eScience Labs, LLC.
All Rights Reserved 3. Use the spring scale to weigh the set of 15 washers. Divide the total mass
by 15 to find the average mass of a washer. Record the total mass of the
washers and average mass of one washer in Table 5.
4. Attach seven washers to each end of the string.
5. Observe how the washers on one side behave when you pull on the
washers on the other side. Answer Post-Lab Question 1 based on your
observations.
6. Add the remaining washer to one end of the string so one side of the string
has seven washers (M1), and the other has eight washers attached to it
(M2).
7. Determine the approximate mass of M1 and M2. Record their masses in
Table 6.
8. Place M1 on the floor. Use the tape measure to measure the height that M 2
is suspended while M1 is on the floor. Measure the distance M2 will fall to
the floor when you release the lighter set of washers. Record the distance
in Table 6.
9. Time how long it takes for M2 to reach the floor. Repeat Steps 7-8 four more
times (five times total), recording the values in Table 6. Calculate and record
the average time in Table 6.
10. Calculate the acceleration (assuming it is constant) from the average time
and the distance the washers moved.
Part 2
1. Transfer one washer, so that there are six on one end of the string (M 1) and
nine on the other (M2).
2. Determine the approximate mass on each end of the string. Record the
mass values in Table 7.
3. Repeat Steps 7 - 9 of Procedure 1. Record data in Table 7. Table 5: Motion Data
Mass of 15 Washers 40 Average Mass of 2.67 © 2014 eScience Labs, LLC.
All Rights Reserved (kg) Washer (kg) Table 6: Procedure 1 Motion Data
Mass of M1 (7 washers):
Mass of M2 (8 washers):
Height (m): 1.57
Trial Time (s) 1 2.56 2 2.72 3 2.66 4 2.47 5 2.72 Average 2.63 Average Acceleration (m/s2) 0.23 Table 7: Procedure 2 Motion Data
Mass of M1 (6 washers):
Mass of M2 (9 washers):
Height (m): 1.57
Trial Time (s) 1 1.28 2 1.32 3 1.54 4 1.44
© 2014 eScience Labs, LLC.
All Rights Reserved 5 1.29 Average 1.37 Average Acceleration (m/s2) 0.83 Post-Lab Questions
1. What do you observe about the motion of the washers when you give one
set a downward push compared to the other set? Does it stop before it
reaches the floor? Explain this behavior.
2. Draw a free body diagram for M1 and M2 in Procedure 1 and M1 and M2 in
Procedure 2. Draw force arrows for the force due to gravity acting on both
masses (Fg1 and Fg2) and the force of tension (FT). Also draw arrows
indicating the direction of acceleration, a.
3. Use Newton’s Second Law to formulate an equation for each of the free
body diagrams you drew in Post-Lab Question 2 (use the correct signs to
agree with your drawings). Solve these four equations for the force of
tension (FT). Record the four equations in variable form. 4. Set the two resulting expressions for the force of tension from Procedure 1
equal to one another (as long as the string does not stretch, the magnitude
of the acceleration in each equation is the same). Replace Fg1 and Fg2 with
M1 and M2, respectively. Solve the resulting equation for a. Then, go back to
Post-Lab Question 3 and solve for the FT. Repeat this with the resulting
expressions from Procedure 2. 5. Calculate the acceleration for the two sets of data you recorded and
compare these values to those obtained by measuring distance and time
© 2014 eScience Labs, LLC.
All Rights Reserved using percent error. Cites two factors that may cause discrepancies
between the two values. 6. Calculate the tension in the string for the falling washers. Show all
calculations. From these two values, and the one where the masses were
equal, identify a trend about the tension in the string as the acceleration
increases. © 2014 eScience Labs, LLC.
All Rights Reserved

 

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