Name:__________________
Force and Motion – Prelab
1. What must be done to the force probe before taking measurements?
2. What are the 6 values for the masses you will use to fill in the table on page 4 of the
lab?
3. In part 2, you are required to make an Acceleration vs. Force graph, which should be
linear. If the slope had a magnitude of 2 and the a-intercept had a magnitude of 0, write
the appropriate equation using the appropriate variables:
4. What is the formula for percent error when comparing experimental
results with an accepted or theoretical value?
1
Lab Group Members:
Date:
CONSTANT FORCE AND MOTION
OBJECTIVE: To create a graph for Acceleration vs. Force and to use Newton’s 2nd Law
to determine an experimental mass for the cart.
APPARATUS: Wireless force probe, motion detector, interface, computer, Logger Pro
software, scale, cart, track, lab jacks, pulley, string, hanging masses, including large mass
to hold cart in place.
PART ONE: FORCE PROBE INTRODUCTION
The force probe measures the force interaction between itself and any object pushing or pulling
on the hook. In this case, it will be measuring a tension force, between the hook and the falling
mass. Unlike the motion detector, you must zero the force probe before using it. When using the
force probe, a zero button will appear in the tool bar of Logger Pro. Zero the probe frequently -before taking measurements -- with nothing attached to the hook, always holding it in the
orientation in which it will be used. Check that the zero was successful by checking the
measurement value in the upper left corner of the screen.
PROCEDURE: Follow these steps to insure detection of wireless probe.
1. Turn on wireless force probe.
2. Log on to Logger Pro.
3. Experiment menu, connect interface, wireless, scan for wireless (probably
have to repeat the scan once or twice.
4. Confirm that your force probe is the one detected (name on side of probe).
Set-up the ramp and cart system as indicated. Record the total mass of the cart, with
force probe attached, in the space below. The motion detector is at the far end of the
track and the force probe is secured to the cart. Use the lab jacks to give you > 1m above
the floor. Make sure your track is level (unlike picture below!). Start with a hanging
mass of 50 g.
Cart mass:__________kg
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You want the cart to speed up steadily (with a constant acceleration). Assume friction is
negligible. Draw a freebody diagram of the cart, below:
Make predictions for the motion described above (ignore numbers, just shape):
Test your predictions for a mass of 50g.
Open Logger Pro and use the file Speeding Up Again(L03-2A2).
When using the force probe, zero it with no forces acting on the hook, by clicking on Zero
button in the icon bar. For this experiment, it is not necessary to zero the motion detector.
Did the shape of your graphs agree with your prediction? If not, how did they differ?
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Describe the motion, in terms of both velocity and acceleration, when a constant force is
applied to an object:
Measure average force and average acceleration during an interval when both are
relatively constant. It should be the same interval for each sensor and should include at
least 10 data points (stats function will tell you the number of data points included in
selection).
Record your values in the first row of the table below:
mass of hanging
mass (g)
Average force (N) – Average acceleration
measured by FP
(m/s2) – measured by MD
Put a title on your graph that indicates the value of the mass used, and PRINT each of
your graphs with the stats boxes visible, but not hiding the data.
Repeat the above experiment with 70, 100 and 120, 150 and 200 g (you may have to use
two masses together) to obtain 3 more values for acceleration. When you get a good data
run, enter your data in the table, and PRINT your graphs with appropriate titles, and stats
boxes visible but not hiding the data.
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PART TWO: ACCELERATION VS. FORCE GRAPH
Predict the shape of this graph:
a
(m/s/s)
F (N)
Load the file called Acceleration v. Force (L03A2-5) and enter your data from the table.
The resulting graph should be linear, with a value of the y-intercept close to zero. If this
is not the case, repeat some or all of the measurements. Use the software to perform a
linear fit on your graph to find the equation of the best fit line.
Write out the equation here, using the variables that correspond to the names of the axes,
instead of x and y.
_____________________________________________________________
Print your a vs. F graph when satisfied, making sure the linear fit box displays the
equation, but doesn’t hide the data.
Question 1: Newton’s 2nd Law is often quoted as F = ma. How is this different from the
equation of your graph? Is it possible to Newton’s 2nd Law to be the equation of the a
vs F graph? If so write it here:
_____________________________________________________________
Use the graph to determine the experimental value for cart mass: _______ kg.
“Theoretical”(aka true, for this case) value for cart mass with units: ______ kg .
Question 2: What is the percent difference between the theoretical value and the
experimental value, which is determined from the graph? This represents the error in
your data:
% error = (|Theoretical value – Experimental Value|) x 100
Theoretical Value
your error: ______________ %
(must be within 5% for full credit)
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PART THREE: MORE QUESTIONS
13. Construct a free-body diagram for each of the sketches below. Object of interest is
in bold. Use standard subscripts e.g. (FE on B) to label the force of the Earth on the boy.
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Sketches
1.
Car, no friction
Freebody Diagram
2. Static traffic light
3. truck, no friction
5.Truck, slowing
down due to
friction
7.
Ball falling, no air
resistance
9. Ball tied to a rope and
pulled straight up
Freebody Diagram
Sketches
4. Man on
scale
6. Truck, wheels locked,
sliding with friction
8.
Box on the bed of
accelerating truck,
(friction is present)
10. falling mass attached
to block
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