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DSHS AP1 Physics Mathy
Ch 4 Forces
Lab #5 Atwood Demonstrator
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Uniformly Accelerated Motion: The Atwood Machine
Newton's first law of motion states that objects at rest remain at rest unless an unbalanced force is
applied. The second law of motion describes what happens if the resultant force is different from zero. If
the acceleration is constant, the body is said to be moving with uniformly accelerated motion. The
purpose of this experiment is to measure the acceleration of a given mass produced by a given force and
compare it with that calculated from Newton's second law of motion.
The Atwood machine consists of two weights connected by a light, flexible string which passes
over a light pulley. The pulley should be nearly frictionless. The machine is used to measure the
acceleration produced by an arbitrarily chosen force acting on a given mass. Using F = ma, the
acceleration can be determined.
Materials
Base stand
Small Radius Pulley
String
Stopwatch
Hooked masses
Equations
F applied = ma
(m2-m1)g = (m1+ m2) a + f
a= 2x / t2
y
Procedures
1. Make sure you have at least 1.25m of string. Your masses must eventually touch the lab floor.
2. Using a total mass of 1000 g, put 500g on one side and put 500g ( 400g with five 20g masses) on the
other side. Determine the force of friction in the machine by transferring masses from the ascending side to
the descending side until the mass on the descending side moves downward with uniform velocity when
given a very slight push. Record the mass on the descending side, the mass on the ascending side, and the
force of friction. Don’t forget…. Arrange the masses so as to have five 20-gram masses on the ascending
side when the machine is in the balanced condition.
3. Transfer one 20g mass from the ascending side to the descending, and thus determine the acceleration
produced by the net force. Begin the observations when the ascending mass is on the floor, starting the
stop watch at the instant when you let the weights go, and stopping it at the instant when the other mass
strikes the floor. Record four independent observations.
4. Measure the traversed distance.
5. Repeat procedure (3) using accelerating forces of 40, 60, 80, and 100 grams by transferring additional
masses each time. Make four independent observations for each accelerating force.
Data
Suggested Data Tables
Descending
mass
Ascending
mass
m1+m2=_____________kg
m2-m1(kg)
1
2
x=____________m
Time
3
4
Average time
Calculated Friction Force=____________
Applied Force
(m1-m2)g (N)
Experimental
Acceleration
(m/s2)
Theoretical
Acceleration
(m/s2)
% of error
Calculations
1. Compute the average time taken with each accelerating force.
2. From the known distance and time taken in each case, compute the acceleration of the system caused by the mass
differences for each Trial
3. Calculate the applied force (m2-m1)g for each mass difference and record in the data table. Use 9.81m/s 2 for g.
4. Compute the theoretical value for the acceleration from Newton's second law of motion (via Atwoods equation).
Assuming this to be the correct value, compute the percent error of the observed value for each case.
Questions for Discussion
1. Plot a graph of your experiments Applied Force vs acceleration . What does the slope represent? Describe the
relationship between force and acceleration observed in this experiment.
2. According to Atwood’s equation, the applied force (m2-m1)g should be proportional to acceleration a with the
total masses m1+m1 as the constant and frictional force f as the intercept. According to your graph’s linear
regression equation : find the slope of your graph and discuss what it should have equalled and discuss the y
intercept and what it should have been.
3. Derive the Atwood equation and show its application to this lab!
Conclusion