Newton’s Second Law Lab
Objective:
To investigate if F = ma is true by accelerating a cart with a mass through pulleys. We will compare our
predicted acceleration (using F = ma) to the actual by measuring it by the sonic motion detectors.
Background:
Newton’s second law of motion states that the acceleration of a body is directly proportional to the net force
on it. An object will accelerate if it has a net force acting on it, and the acceleration will be in the direction of
the force. In this experiment, a lab cart will be accelerated by a known force, and its acceleration will be
measured by a vernier motion detector. The product of the total mass accelerated and its acceleration
equals the force causing the acceleration.
In order to calculate the net force acting on the lab cart, friction forces that oppose the motion must be
accounted for. If the cart moves at a constant velocity, then the net force acting on the cart is zero because
the acceleration is zero. The friction force can be accounted for by providing enough small masses at the
end of the string to make the cart move forward at a constant velocity.
Procedure:
Have a computer operator skip to procedure number 5 and 6 on the next page. The rest of the group should
start at 1 below.
1. Set up a table, 2 metal ramps, pulleys, and motion detector as shown below. You will need 5 m of string
for this bad boy.
Figure A
2. Determine the mass of your cart. Also determine the mass of your ‘mass hanger’ if you’re using one; the
flat circular piece of metal with a hook to hang the masses on. Record those values below.
Mass of Cart _____________
Weight of Cart _____________
3. Assemble the cart, pulley, and string, as shown in figure A.
4. Give the cart a small push toward the pulley. It probably will stop due to friction. Attach a tiny amount of
mass (you may have to be creative and use things lying around because there isn’t enough tiny masses to
go around. Tape whatever you can find to the string if there isn’t enough weights. )a little at a time until you
can give the cart a small nudge and it will move at a CONSTANT velocity. If it stops, you need to give it
more mass, if it accelerates you need to remove some. The force of friction is your mass times 9.8.
Force of Friction_____________
5. Open Logger pro from the Math and Science folder. If it doesn’t find the calibration folder or opens
directly to a Voltage vs. Time graph, follow the directions on the laminated sheet.
6
Click on.
Adjust your experiment length to 2.5 seconds:
Adjust the samples per second to 30.
Set your zero by clicking on Experiment / Zero. If you get an error just do it again.
Click View… Graph Layout… 2 Panes
Adjust your max and min values by double clicking on each graph.
Position min: 0 max: 1.0 m
Velocity min: 0 max: 1.0 m/s
You may adjust these at any time to get a better view of your data as needed.
7. Hang up to 75 grams on the mass hanger and get ready to collect your first round of data. Hold the cart
at your zero mark. Start collecting sonic data and release your cart. Be sure to catch the cart before it
collides with the pulley or crashes to the floor. Do this step as many times as is necessary to get a good
run. The cart can’t hit the sides of the track and you need to have a good ‘chunk’ of data.
8. Once you have a good run,
Paste your graphs into MS Word
9. Make sure your names are on
The MS Word document. Print it to
Buisness Ed. DO NOT GO GET IT.
We will wait until class is over to
get it.
9. Calculate the acceleration from the printed graphs by finding the slope of the Velocity vs. Time graph.
Remember that slope is rise over run. Be as accurate as you can in your estimates of where the coordinates
are.
Measured Acceleration_________
10. Calculate what your mathematical prediction of the acceleration should be. F = ma. The mass is the
mass of the cart and string. The Force is the WEIGHT (w=mg) of your hanging mass minus the WEIGHT it
took to overcome friction.
Calculated Acceleration_____________
11. What is the absolute error and relative error for this run of the experiment?
Absolute Error = |Calculated Value – Measured Value|
Relative Error (%) =
Calculated  Measured
Calculated
%
Absolute Error____________________
Relative Error____________________
13. Give two reasons why our calculated and actual acceleration don’t agree 100%. Then give two ideas
you have to make this experiment more accurate.