Download Lab Instructions

NEWTON’S SECOND LAW LAB
Overview
The purpose of this lab is to demonstrate Newton’s Second Law. A cart will be accelerated by various net forces while
keeping the mass of the entire system constant. The net force is controlled by the force of gravity on a set of weights. The weight
will then pull a cart. The cart will be attached to a ticker tape timer in order to calculate the acceleration of the cart in each situation.
We will then compare the force pulled and the acceleration of the cart in order to get the mass of the cart. Neatness counts and this
lab is a major grade.
Procedure
1.
Load a cart with the following masses: a 50 g hanger, a 50 g mass, a 100 g mass, and a 200 g mass. Attach one end of the
string to the cart. Tie a paper clip to the other end of the string and pass it over a pulley on the end of the table. Hang metal
washers on the paper clip until the cart moves at a constant rate. The washers are compensating for friction. Leave them
on the paper clip for the entire experiment.
2.
Prepare a piece of ticker tape as long as the table is high. Attach it to the cart. Move the 50 g hangar off the cart and attach
it to the end of the string.
3.
Start the timer and release the cart. DO NOT LET THE CART CRASH ONTO TH FLOOR OR THE PULLEY. Remove the tape and
label it the 50 g trial.
4.
Remove the 50 g mass from the cart and place it on the hanger at the end of the string. So, there should be 100 g on the
end of the string. Turn on the timer and release the cart. Label the ticker tape the 100 g trial.
5.
Put the 50 g mass back on the cart and place the 100 g mass on the hanger. You should now have 150 g on the end of the
string. Repeat the experiment.
6.
Continue to do the experiment until you have 6 trials, a 50 g, a 100 g, a 150 g, a 200 g, a 250 g trial, and a 300 g trial.
7.
Find the mass of everything used in the experiment. Put the cart, string, washers, paper clip, and masses on the scale to find
the total mass and record below.
Data
The timer strikes the tape 60 times per second. Therefore the space between each dot is 1/60 of a second. For simplicity
and accuracy we will count every 3rd space and make up an arbitrary time of a tock. Count every three spaces for the ticker tape and
measure the displacement of each section and record in cm on the table on the next page. Each measurement in cm is actually the
cart’s speed in cm/tk. Record the data for up to 20 data points. If you don’t get 20 data points, and you won’t on some, that is fine.
Record the data for all 6 trials.
Graph
Construct a full page graph with speed in cm/ tk on the y-axis and time in tocks on the x-axis. Draw a best fit line. Put all 6
sets of data on the same graph. Use colored pencils and a key to distinguish between the different trials.
Analysis
1.
Determine the acceleration of the cart during each trial by finding the slope of the best fit line on each trial. The units for
the slope will be cm/tk2. Then fill them in on DATA TABLE 2.
A50g = slope = ∆v/∆t = _________________________________________
A100g = slope = ∆v/∆t = _________________________________________
A150g = slope = ∆v/∆t = _________________________________________
A200g = slope = ∆v/∆t = _________________________________________
A250g = slope = ∆v/∆t = _________________________________________
A300g = slope = ∆v/∆t = _________________________________________
2.
Now calculate the net force on the cart for each trial. Do this by finding the weight on the end of the string of each trial.
Show the 50 g trial calculation below and repeat or the rest of the trials. Then fill them in on DATA TABLE 2.
F net 50g = ______________________________________ = __________N
3.
Now convert the acceleration from the slopes above to m/s2. To do this divide the slope by 100 (100 cm = 1 m) and then
divide it by (.05)2, because 1 tock = .05 s. See the explanation in the DATA section. Please show a sample calculation below.
Then fill them in on DATA TABLE 2.
a 50g = ___________cm/tk2 _________________________________________________m/s2
DATA TABLE 2
Trial
a (cm/tk2)
Net Force (N)
a (m/s2)
50g
100g
150g
200g
250g
300g
Final Graph
Use the data above to construct a new graph. Make a full size F vs. a graph. The Force is on the y-axis and acceleration (in
m/s ) is on the x-axis. Draw a best fit line and calculate the slope. Show the slope calculation below.
2
Slope of the F vs. a graph = ∆F/∆a = ______________________________________________________________
Questions
1.
How do you know that the cart underwent constant acceleration?
2.
What evidence is there that force is directly proportional to acceleration according to Newton’s 2nd Law.
3.
What is the significance of the slope on the F vs. a graph? Does this support of refute Newton’s 2 nd Law?
4.
Calculate the percent error. The mass you measured on the scale is the actual value, and the slope of the graph is the
experimental.
5.
Look at the y-intercept of the F vs. a. If it is not at the origin, what could have caused that?
Discussion: Evidence for systematic error or random error? After researching technical and scientific resources on
“random” and “systematic” error, write a conclusion that compares the two types and argues which kind of error was
evident in your Newton’s 2nd Law Lab. Be sure to support your position with evidence from the lab data.