Download Lab 2, Activity 1(final)

Lab 2: Newton’s Laws and the Force of Friction
Activity 1: Modified Atwood’s Machine
Pre-Lab Activity
1. Read through the entire lab document and put a question mark next to any step
that you find confusing. Highlight or underline steps in which you must record
data so that they will be easy to find when you do the lab.
2. We will solve for the acceleration of the modified Atwood’s machine as a
function of the mass of the cart, mC, and the hanging mass, mA.
a. Assuming that friction plays a negligible role in this experiment, draw
the force diagram below by drawing and labeling all of the forces in the
figure below. Create a free body diagram for the cart and the hanging
mass.
Force Diagram
Free-Body Diagrams
b. The acceleration of the cart aC and the acceleration of the hanging mass
aA are also not known. Write an equation of constraint relating aA to aC.
c. There are two known forces (the weights of each object) and three
unknown forces (the normal force on the cart, the tension force on the
cart, and the tension force on the weight). Write an equation of
constraint that relates the two tension forces that assumes a massless
cord. Write down the relationship between the normal force on the cart
and the weight of the cart.
d. Newton’s 2nd law states that the acceleration of an object is proportional
to the net force on the object and inversely proportional to the mass of
the object. Write Newton’s second law for the two objects.
e. You should now have enough equations to solve for all the unknown
quantities. Find the normal force, the tension force, and the acceleration
of each object in terms of mC and mA.
3. One source of error in this experiment is the presence of friction in the wheels of
the cart. How would you include the force of friction in your force diagram and
free-body diagrams? How do you expect friction to effect the acceleration?
Force Diagram
Free-Body Diagrams
4. We will use a motion sensor to collect data in the same way we did for the cart in
demonstrations. We will find the acceleration using the technique that we used in
the acceleration due to gravity lab. Explain how you will find the acceleration of
the cart from the data recorded by the motion sensor.
Activity Instructions
George Atwood was an English mathematician, living near the
turn of the 17th century, who invented a machine for illustrating
the law of uniformly accelerated motion. Two masses are
suspended from a pulley as shown to the right. The system is
accelerated by the different in the force of gravity on the two
objects. In this lab, we will examine the modified Atwood
machine where one of the masses moves horizontally along a
frictionless surface.
Experimental Procedure
1. Setup: A light string will connect your hanging masses to the cart. The string
runs over a frictionless pulley whose height is adjusted so that the string runs
horizontal to the table. On the opposite side of the table is a motion sensor to
measure the velocity of the block when acceleration occurs. Connect the motion
sensor to the first digital port on the LabPro.
2. Record the mass of the cart, the mass of weights to be added to the cart, and the
values of any hanging masses to be used.
Cart Mass
Cart Added
Mass A
Cart Added
Mass B
Hanging
Mass A
Hanging
Mass B
Hanging
Mass C
3. Open LoggerPro. Open the file P:\HHS\PasquiniT\Physics\Lab2Activity1.xmbl.
Connect on COM1. Confirm that the motion sensor is operational.
4. Attach ~100g to the hanging end of the string, run the cord over the pulley and
hold the cart in place approximately 0.5m from the motion sensor. Begin
collection with the computer. When data appears on the graph, release the cart
and allow it to accelerate. Do not allow the cart to collide with the pulley. The
pulleys are both fragile and expensive.
5. Record the acceleration of the cart in the table. Repeat the experiment to obtain a
second trial.
6. Repeat for the combinations indicated in the data table.
Masses Used
Cart Only
Hanging Mass A
Cart Only
Hanging Mass A,B
Cart Only
Hanging Mass A,B,C
Cart and Mass A
Hanging Mass A,B
Cart and Mass A,B
Hanging Mass A,B
Total Cart
Mass (kg)
Total Hanging
Mass (kg)
Total System
Mass (kg)
Acceleration
Trial 1
(m/s2)
Acceleration
Trial 2
(m/s2)
Average
Acceleration
(m/s2)
Data and Analysis
7. For each combination of masses, use your pre-lab activity to calculate the
predicted acceleration for the system. Use the table below to compare your
experimental acceleration with the predicted acceleration.
Masses Used
Cart Only
Hanging Mass A
Cart Only
Hanging Mass A,B
Cart Only
Hanging Mass A,B,C
Cart and Mass A
Hanging Mass A,B
Cart and Mass A,B
Hanging Mass A,B
Predicted
Acceleration
(m/s2)
Average
Acceleration
(m/s2)
Percent Difference
Analysis Questions
Describe the change in acceleration of the system as the hanging mass is increased.
How does Newton’s 2nd Law apply?
Describe the change in the acceleration of the system as the total mass of the cart is
increased. How does Newton’s 2nd Law apply?