AP Physics - Experiment 8
Addition of Force Vectors
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Purpose:
Do force vectors really add the way the textbook says?
Discussion:
First, remember from Physics 1 that if an object is in equilibrium
- that is, not accelerating - then the net force on the object must
be zero. This is a consequence of Newton's First Law. Also remember
that the net force is the sum, or resultant, of all of the forces
that act on an object.
In this lab, you will apply three forces to a small, plastic ring.
If the ring is in equilibrium, then the three forces on it must add
to zero. Do they? You can measure the magnitude and direction of each
vector and add them analytically to check.
Equipment:
force table
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several metal washers
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set of known masses*
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* (Note: The apparatus was designed to use washers
as weights to supply force, but you might find that you can get more
precision using calibrated masses instead. Try it both ways and
decide - don't mix washers and weights!)
Safety:
- Be careful of the glass tubing. If you break a glass
tube, tell your instructor - do NOT
clean it up yourself.
Setup:
- For each trial, you will need to:
- record the magnitude and direction for each of the
3 vectors. (Note: You might as well just measure the
vector magnitude in "washers" or "grams". Since the magnitude
of the force is the weight of whatever you place on the weight
hanger, nothing would be gained by converting the washer count
or mass to force units.)
- calculate x- and y-components for each vector, so you might
as well record these in the data table, too.
- If the force table is not assembled, you need to screw the
three legs into the base, and push the central metal pin up from
the bottom.
- The plastic ring goes over the central metal pin, and the
glass tubing hangs over the edge of the table (to reduce
friction).
- It may (or may not) help to level the table.
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The force table apparatus
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A close-up of the ring and pin
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Procedure:
- Arrange the three forces (glass tubes) in some convenient way
on the table.
- Place identical washers or known masses (See the note above.)
on the weight hangers and make adjustments until the plastic ring
remains in the center of the force table without touching
the central pin.
- While you have the ring in equilibrium, spend a little time
investigating the precision of this experimental setup. How much
can you vary the magnitude and direction of a vector and still
keep the ring centered? Record this data in your data table.
- Change the magnitude and direction of your force vectors and
repeat for several trials.
Results:
- Calculate the x- and y-components of each vector.
- Add the components of the three vectors to find the components
of the resultant.
- Find the magnitude and direction of the resultant vector.
- Calculate the precision of the resultant vector
calculation.
Conclusions:
Go back and reread your statement of purpose for this lab. Now,
answer the question. Here are some additional points you might want
to cover:
- How confident can you be in your results? Why do you think
so?
- What measurement contributed the most uncertainty to your
results? What could be done to improve it?
[Lab
Index]
BHS
-> Staff
-> Mr. Stanbrough
-> AP
Physics-> AP Labs-> this
page
last update July 12, 2003 by JL
Stanbrough