AP Physics - Experiment 8

Addition of Force Vectors

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Do force vectors really add the way the textbook says?


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.


force table

several metal washers

set of known masses*

* (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!)


  1. Be careful of the glass tubing. If you break a glass tube, tell your instructor - do NOT clean it up yourself.


  1. For each trial, you will need to:
    1. 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.)
    2. calculate x- and y-components for each vector, so you might as well record these in the data table, too.
  2. 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.
  3. The plastic ring goes over the central metal pin, and the glass tubing hangs over the edge of the table (to reduce friction).
  4. It may (or may not) help to level the table.

force table apparatus
closeup of ring & pin
The force table apparatus
A close-up of the ring and pin


  1. Arrange the three forces (glass tubes) in some convenient way on the table.
  2. 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.
  3. 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.
  4. Change the magnitude and direction of your force vectors and repeat for several trials.


  1. Calculate the x- and y-components of each vector.
  2. Add the components of the three vectors to find the components of the resultant.
  3. Find the magnitude and direction of the resultant vector.
  4. Calculate the precision of the resultant vector calculation.


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:

  1. How confident can you be in your results? Why do you think so?
  2. What measurement contributed the most uncertainty to your results? What could be done to improve it?

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last update July 12, 2003 by JL Stanbrough