Newton’s Second Law How does a cart change its motion when you push and pull on it? You might think that the harder
you push on a cart, the faster it goes. Is the cart’s velocity related to the force you apply? Or does
the force just change the velocity? Also, what does the mass of the cart have to do with how the
motion changes? We know that it takes a much harder push to get a heavy cart moving than a
A Force Sensor and an Accelerometer will let you measure the force on a cart simultaneously
with the cart’s acceleration. The total mass of the cart is easy to vary by adding masses. Using
these tools, you can determine how the net force on the cart, its mass, and its acceleration are
related. This relationship is Newton’s second law of motion.
? Collect force and acceleration data for a cart as it accelerates on a horizontal surface.
? Compare force vs. time and acceleration vs. time graphs.
? Analyze a graph of force vs. acceleration.
? Determine the relationship between force, mass, and acceleration.
Power Macintosh or Windows PC Logger Pro
LabPro or Universal Lab Interface low-friction dynamics cart
Vernier Force Sensor kg mass sets
Vernier Motion Detector
Physics with Computers 9 - 1
PRELIMINARY QUESTIONS (THINK ABOUT YOUR RESPONSES)
1. When you push on an object, how does the magnitude of the force affect its motion? If you
push harder, is the change in motion smaller or larger? Do you think this is a direct or inverse
2. Assume that you have a bowling ball and a baseball, each suspended from a different rope. If
you hit each of these balls with a full swing of a baseball bat, which ball will change its motion
by the greater amount?
3. In the absence of friction and other forces, if you exert a force, F, on a mass, m, the mass will
accelerate. If you exert the same force on a mass of 2m, would you expect the resulting
acceleration to be twice as large or half as large? Is this a direct or inverse relationship?
nd1. Open the Logger Pro Program. The experiment file ( Newton’s 2 Law) is located at
2. Connect a Dual-Range
Force Sensor to Channel 1
on the LabPro Connect the
Motion Probe to Dig/Sonic
The Logger Pro Program
may not automatically
detect the force probe.
Click CONNECT and
make sure the sensor name
channel is correct.
3. To get the best
numerical results in the
experiment, you will
calibrate the force sensor
a. Choose Calibrate
from the Experiment
b. Click the Force icon that appears in Channel 1.
c. Click the button.
d. Remove all weight from the Force Sensor and hold it vertically with the hook pointed
e. Type 0 in the Value 1 edit box.
Keepf. When the displayed voltage reading for Input 1 stabilizes, click .
g. Add the 0.500 kg (4.9-N) mass to the hook of the Force Sensor.
h. Type 4.9 in the Value 2 edit box.
Keepi. When the displayed voltage reading for Input 1 stabilizes, click .
Keepa. When the displayed voltage reading for Input 2 stabilizes, click , then click .
9 - 2 Physics with Computers
Newton’s Second Law
You should check the calibration by measuring the weight of different masses. Find the mass
of the cart with the Force Sensor. Record the mass in the data table.
4. Attach the Force Sensor to a dynamics cart so you can apply a horizontal force to the hook,
directed along the sensitive axis of your particular Force Sensor. Next, attach the force sensor
to its holder on the dynamics cart. Attach a string to the force Sensor and be sure it is long
enough to connect to a slotted mass holder which should be hung over a pulley as shown in
6. Place the cart on a level surface. Make sure the cart is not moving and click , then click
7. You are now ready to collect force and motion data. Vary the FORCE acting on the cart
(weight in the mass holder). Do not use too much force or the cart will move too quickly and
the force calibration will not apply to your range. Protect the floor will a thin object ( an
English or math text would do nicely-not your wonderful physics text)
8. Note the shape of the force vs. time and velocity vs. time graphs. Determine the acceleration
of the cart. Determine the avg force during the interval of the acceleration. Highlight section
and press “STAT”. Record the Force and acceleration in the data table.
9. Repeat to determine average Force and acceleration. You should have 3 values of each to
10. Use a different force (weight) and repeat. You should have at least 4 different forces
11. Open Graphical Analysis. Make a graph of Force(y axis) vs Acceleration (x axis).
Determine if a relationship can be determined.
Data using cart Mass of Cart =_____kg
kg Force Acceleration
Hanging Trial 1 Trial 2 Trial 3 Average Average trial 1 trial 2 trial 3 weight (N) (N) (N) Force (N) acceleration 2222(m/s) (m/s) (m/s) (N) (m/s)
1. What are the units of the slope of the force vs. acceleration graph? Simplify the units of the
slope to fundamental units (m, kg, s).
Physics with Computers 9 - 3
2. What physical quantity does the slope of your Force vs Acceleration graph represent?
3. Describe the relationship between force and acceleration by looking at the GRAPH and
explaining the line you decided on. How can the line be extrapolated to include VERY
LARGE Forces (weights)?
4. Write the equation to your graph and whether there is a limitation to it’s useful range.
5. Consider your answer to question 2. Was the physical quantity kept constant? What was
wrong with the lab procedure here? Was something controlled? Explain.
6. Is there a discrepancy with what Newton’s 2nd law predicts? Explain why or why not.
Lab report should include one sample of Force vs time and velocity vs time graphs from Logger
Pro showing average Force “STAT” and acceleration from “SLOPE”. All sections of the LAB REPORT FORMAT should be followed except for the Discussion of
Results and conclusions. Be sure to copy and paste the questions into your answer document. Be
sure you answers to the questions above are complete.
9 - 4 Physics with Computers