Force and Motion – Prelab
1. Suppose you have a cart with very little friction and it is pulled with a constant force.
Sketch below your predictions for velocity, acceleration and force graphs below:
2. What are the 4 values for the masses you will use to fill in the table on page 4 of the
lab?
3. What is the formula for percent difference when comparing your results with an
accepted or theoretical value?
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CONSTANT FORCE AND MOTION
OBJECT: To study how to produce motion with constant acceleration; to investigate the
relationship between force and acceleration.
APPARATUS: Wireless force probe DLink bluetooth key, motion detector LabPro
interface, computer, Logger Pro software, scale, cart, track, lab jacks, pulley, string, blue
masking tape, masses.
PROCEDURE: Follow these steps to insure detection of wireless probe.
1. Turn on computer.
2. Turn on wireless force probe.
3. When desktop appears and the hourglass turns into an arrow, insert DLink
into one of the back USB ports.
4. Patiently wait for all the found new hardware messages to fly by
5. Then and only then, log on to Logger Pro.
6. Experiment menu, connect interface, wireless, scan for wireless (probably
have to repeat the scan once or twice.
7. 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 with the set screw. 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
You want the cart to speed up steadily (with a constant acceleration). Assume friction is
negligible.
Draw a freebody diagram of the cart, below:
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Sketch on the following graphs the velocity, acceleration and force graphs which you
would expect (ask for your prelab and attach, or rewrite).
Test your predictions.
Open Logger Pro and use the file Speeding Up Again(L03-2A2).
When using the force probe, zero it with no forces acting on it by clicking on Zero button in the
icon bar. For this experiment, It’s ok, but not necessary to zero the motion detector.
Did the shape of your graphs agree with your prediction? If not, why not?
Describe the motion, in terms of both velocity and acceleration, when a constant force is
applied to an object:
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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 your
graphs with the stats boxes visible, but not hiding the data.
Repeat the above experiment with 70, 100 and 120 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.
Part 2 - Acceleration vs. Force Graph
Predict the shape of this graph:
accel
(m/s/s)
force (N)
Load the file called Acceleration v. Force (L03A2-5) and enter your data from the table.
Use the software to perform a linear fit on your graph to find the slope and equation. If
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your data do not approximate a straight line, redo the part of the experiment with the bad
data points (make sure you replace graphs for the redo). 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? How can you rewrite F = ma so represents the equation of your
graph?
Question 2: What quantity is represented by the slope of the line? What are its units?
(HINT: Change Newtons into (kg m)/s2)
Question 2: How would you get the theoretical (or actual) value for the quantity
represented by the slope of your graph? Do it and report back.
Theoretical value with units: _________________ .
Question 3: What is the percent difference between the theoretical value and the
experimental value, which is determined from the slope of your line? This represents the
error in your data:
% difference = (|Theoretical value – Experimental Value|) x 100
Theoretical Value
your error: ______________ %
Completed lab with no other errors: Less than 5%: 20pts Less than 10% 18 pts
Less than 15% : 15 pts.
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Check your Understanding
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