Newton’s Laws of Motion
Phys 1401
Purpose
In this lab you will investigate applications of Newton’s Laws of Motion.
Materials
Collision cart, PASCO track, force sensors (2), motion sensor, super pulley, photogate pulley
and rod, small clamp, (ME-9348) mass and hanger set, string
Static equilibrium preliminaries
1. Set up the PASCO track with the ends
overhanging the edges of the table as
indicated in the figure at right. Make
sure the track is level: the collision
cart should remain stationary when
placed on it. Attach the super pulley
on one end and attach the photogate
pulley on the other end with the small
clamp. Ensure that both are solidly
affixed.
2. Place one force sensor in the top of the collision cart, with the sensor pointing toward
the super pulley. Add a second force sensor to the top of the stack, with the sensor
pointing toward the photogate pulley
3. Cut one piece of string about 60 cm in length. Tie a
loop on each end of the string. Attach one loop to the
uppermost Force Sensor and slide the other loop over the
post (NOT the pulley) of the photogate pulley.
4. Cut a second piece of string about 1.2 m in length. Tie
loops in both ends and connect one to the lower Force
Sensor. Loop the string over the super pulley; hang the
mass hanger from the other loop.
!
Make sure that the strings are horizontal when under
tension. (Use a small piece of tape to hold the loop over
the post in place.)
The simplest knot to use for tying loops is the overhand on a bight,
illustrated at left. Double over about 7 cm of the end of the string.
(Bight is the term for where a line is doubled.) Make a loop and push
the doubled end of the string (darkened section) through the loop.
This is a modestly strong knot but difficult to untie after it has been
tightened.
c 2007 The University of Texas-Pan American
Version 2.2 °
Phys 1401
Newton’s Laws of Motion
2
5. Connect the topmost Force Sensor to the PASCO interface in Channel A. Connect the
lower Force Sensor to the PASCO interface in Channel B. Open Data Studio and create
experiment. Click Channel A on the interface and select Force sensor; the Force sensor
icon should appear. Select Force for the quantity to be measured. Click Channel B and
again select Force sensor; a second Force Sensor icon should appear. Select Force for
the quantity to be measured. Close the Experiment Setup window.
6. Double click each of the Force icons in the Data window and set the Numeric output to
record four decimal places. Turn off the Scientific Notation Threshold. Drag each icon
to the Graph icon. (Create two graphs.)
!
The force sensors are sensitive to shocks and the force measurements will drift. To
resolve this issue, the force sensor must be tared before use. Remove any load from the
sensor BEFORE pushing the Tare button.
7. Set the suspended mass hanger on the table to release the tension in the strings. Click
Start to check the force sensors. Dots should be plotted along the y=0 axis. (Don’t
forget to expand the y-scale so that you can see what is happening.) Press the Tare
button to verify that the force measurements have been properly zeroed. Click Stop
and delete the trial runs.
!
Note that the force sensor gives a negative reading if it is pulled and a positive reading
when pushed.
Static equilibrium
1. Download the workbook for this experiment to your My Documents directory from the
course web site and open the workbook in Excel. Go to the Static worksheet.
2. Add a 5 g mass to the mass hanger. The mass hanger has a mass of 5 g, so this is a
total suspended mass of 10 g.
!
When making measurements, do not allow the suspended mass to swing. It must be
stationary.
3. Click the Start button and record five seconds of data. Click Stop. Then support the
mass in your hand so that there is no tension in either string. (You may need to push
the cart in the direction of the post to relieve the tension on that side.) Click Start and
record another five seconds of data. Click Stop. Enter the mean value and standard
deviation of each of these runs in the Excel worksheet. The second step, where the
tension in the strings has been released, is called the zero-point measurement in the
worksheet. We perform this additional measurement because the force sensor is quite
sensitive to shocks and the zero-point of the sensor can shift.
4. Complete the Static worksheet by measuring the additional masses indicated. (Click on
the mass values in the Post tension table to see what masses are required.) Note that
Graph 1 contains the information for the Post tension table and Graph 2 contains the
information for the Hanging mass tension table.
Phys 1401
Newton’s Laws of Motion
3
Format the results of the Static worksheet. (Pay careful attention to significant figures.)
Cut and paste into a Word document for submission with your lab handout.
?
For each of the trials that you performed, how does the measured weight compare with
the product of the mass and the acceleration due to gravity? Justify your answer.
?
From your experimental results, how does the tension between the cart and the post
compare with the force exerted by the suspended mass? Justify your answer.
?
Draw a free body diagram for the cart. What is the net force acting on the cart? Justify
your answer.
?
How would the tension in each string change if we were to replace the post with another
pulley and suspend a second mass equal to the first? (You will verify your prediction
in the next section.)
Dynamic equilibrium
1. Cut a new piece of string about 1.2 m in length and tie loops on both ends. Remove
the short string from the topmost force sensor and replace it with the new string. Loop
the new string over the photogate pulley and suspend a second mass hanger. Reset the
tare on the force sensors.
2. Place 20 g masses on each of the two mass hangers.
3. Click Start and record five seconds of data. Click Stop. Rest both masses on the
tabletop, releasing the tension in both strings. Click Start and again record five seconds
of data. Record your results in the Dynamic worksheet of the workbook.
?
What are the forces on the cart due to the suspended masses?
?
What is the behavior of the cart? (Is it moving? If so, how?)
Phys 1401
?
Newton’s Laws of Motion
4
How do the results of this experiment compare with your prediction from the previous
section?
Force and Acceleration
In this section of the laboratory, you will investigate what happens when the forces acting
on the cart are unbalanced. We’ll use the Acceleration worksheet from the Excel workbook.
You can clear the data from previous runs.
1. Place the motion sensor on the track next to the
photogate pulley. You may need to adjust the photogate pulley so that the string does not touch the
top of the motion sensor but remains as close to horizontal as possible. On the Experiment Setup menu,
add the motion sensor to Channel 1 and select Velocity as the quantity to be measured. Unselect the
other outputs. In the Data window, drag the Velocity icon to the Graph icon. Check if the motion
sensor is working properly and then delete your trial
runs.
2. Hold the dynamic cart at about the 80 cm mark. Add an additional 10 g to the weight
hanger at the super pulley end of the track (total suspended mass=35 g). Rest both
masses on the table top and record zero values and standard deviations for both force
sensors in the worksheet.
3. Gently lower the masses back into position. Click the Start button and release the cart.
Click Stop just before the mass hanger hits the floor.
4. Using the selection tool, measure the mean values and standard deviations of the force
while the cart was in motion. (Refer to the velocity vs. time graph to identify the
appropriate time range.) Fit a line to the velocity data over that same time interval.
Record the slope and standard deviation on the acceleration worksheet.
5. Repeat steps 2–4 using the different hanging masses indicated in the acceleration worksheet.
6. Measure the mass of the force sensors and the dynamics cart. (Don’t include the masses
of the force sensor cables.) Record this on the acceleration worksheet.
Clean up the tables on the Acceleration worksheet and submit with your lab handout.
?
How does the ratio of the net force and acceleration compare with the system (collision
cart + force sensors) mass? Justify your answer.
Phys 1401
?
Newton’s Laws of Motion
5
Based on your experiments, how are the force, mass and acceleration related to each
other?
Making Connections
Do you agree or disagree with the following statement: ”A force is required to keep an
object moving”. Justify your answer.
This is tile mason apprentice Romeo’s first day on
the job and he is eager to impress his colleagues.
The 165 lb Romeo has just loaded four 50 lb sacks of
mortar mix into a bucket and is preparing to lift them
up to his coworkers. What will be Romeo’s first lesson
on the job?