SPH3U
Ms. Kueh
Lab: Newton’s 2nd Law
/21 I
/11 C
Name: _______________________
Purpose: To verify that the gravitational field strength on Earth’s surface is 𝑔, 9.80 𝑁/𝑘𝑔
Background: In this lab, you will be using Newton’s 2nd Law to help you to determine 𝑔 and
compare it to the expected value. You will be measuring the acceleration of a dynamics cart being
pulled by falling masses. NOTE: A pulley changes the direction of a force, but NOT the
magnitude!
Materials:
 Dynamics cart
 A LabPro with a Motion detector
 Pulley and string
 Several masses
 Mass Balance
Procedure:
1) Setup your equipment on the edge of the lab bench as shown in Fig 4.1, pg 149. We will
be using the motion detector in place of the ticker-tape timer.
2) Put approximately 0.50 𝑘𝑔 on the end of the string.
Record the actual mass in your
observations.
3) Using the mass balance, determine the mass of your dynamics cart. Record it in your data
table.
4) Release the cart from rest while capturing the motion on the LabPro.
5) IF the graphs match your expectations, copy the VT graph into your observation table (the
section showing the relevant motion). Be sure to include labels and a scale on your graph.
If the graphs DO NOT match your expectations, re-run the trial.
6) Using what ever method you think is most appropriate, determine the acceleration of the
cart while it is being pulled by the falling mass.
7) Add a mass to your dynamics cart and repeat steps 3 – 6 above. Repeat for a total of 4
trials.
Observations (5I):
Mass of falling weights:
_____________________
𝑘𝑔
Mass of empty cart:
_____________________
𝑘𝑔
Trial
Mass added
Total Mass of
on Cart [𝑘𝑔]
System [𝑘𝑔]
VT Graph
1
2
3
4
2 of 4
𝑎 [𝑚/𝑠 2 ]
Analysis:
1) Create TWO (2) separate graphs of your data. Graph 1 should plot acceleration versus
total system mass and graph 2 should plot acceleration versus 1/total system mass. Don’t
forget things like labels, titles, units, etc. Include a line-of-best-fit if appropriate. (10C)
2) Which graph is more useful? Explain in as much detail as possible. (3I)
3) For the graph you identified above, determine the slope of the line-of-best-fit. Include
units. Show that the slope of the line should represent 𝐹𝑛𝑒𝑡 , the net force acting on the
mass-cart system (ignoring friction and air resistance). (3I)
4) Draw a FBD for the dynamics cart while it is accelerating including all forces. You may
ignore friction and air resistance in this case. (2I)
3 of 4
5) Examine the equation for Newton’s 2nd Law, the FBD you drew, and the slope you
calculated above. From this information, calculate a value of 𝑔. Hint: Where does the net
force on the cart come from, and how is it calculated? SHOW YOUR WORK! (5I)
Conclusions: (Conclude your experiment by stating the value of 𝑔 that you measured) (1C)
Discussion: (Calculate the percent difference between your measured and expected value for 𝑔,
comment on your value, and account for any sources of error) (3I)
%difference 
4 of 4
Measured _ Value  Accepted _ Value
100%
Accepted _ Value