MAY THE NET FORCE BE WITH YOU

Student Lab Handout

(Page 1 of 3)

EQUIPMENT

Alabama Science in Motion – University of South Alabama

Revised: August 11, 2013

MAY THE NET FORCE BE WITH YOU

Student Lab Handout

(Page 1 of 3)

• Force Table Kit

• Balance

• Graph Paper

• Metric Ruler & Protractor

• Calculator, Pencil, Paper

Alabama Science in Motion – University of South Alabama

Revised: August 11, 2013

MAY THE NET FORCE BE WITH YOU

Student Lab Handout

(Page 1 of 3)

PRE-LAB PREPARATION

Before you begin your instructor will familiarize you with any necessary information about this experiment and how the equipment works.

Stop and answer each question as you come to it (they are italicized and indented).

Ask your instructor if you need further explanation or guidance.

1)Assume two equal forces acting on a solid, initially stationary object. Sketch a force diagram for each of the following situations, explain what the net force would be, and describe how you think the object would move.

  1. The two equal forces are acting in opposite directions on the object.
  2. The two equal forces are acting in the same direction.
  3. The forces act perpendicular to each other (say, at 0° and 90°).

IMPORTANT NOTE: In all cases throughout this experiment, the force is caused by a mass hanging over a pulley and is found using F = mg.

Assume the following forces acting on a point:

TRIAL 1

Force A (FA) = 60 grams at 0°; Force B (FB) = 60 grams at 90°

2)Sketch a diagram of the above forces acting on a point. Using your diagram, give an educated estimate for the magnitude and direction of the resultant force. Record your guess in a data table. In order to balance the resultant force, what magnitude and direction would the equilibrant force have to be? Record your guess in the data table.

PROCEDURE & ANALYSIS

PART A - EXPERIMENTAL METHOD

  1. Set up the force table on a flat, level surface by removing the three legs from the clips on the bottom of the force table and screwing them into the holes on the bottom of the disk (FIRMLY, BUT DONOTOVERTIGHTEN). Screw the center post up from the bottom until it stops, so that it sticks out above the top of the table.
  2. Attach three pulleys to the rim of the disk (the screw goes on the bottom of the table). Clamp two of the pulleys at the angles given above for Trial 1, and attach the third pulley at the angle that you guessed for the equilibrant force.
  3. Obtain the mass value for each of three mass hangers. Cut three pieces of string about 35 cm long. Tie one end of each string to a mass hanger and tie the other end onto the small plastic ring.

  1. Place the ring over the center post on top of the force table and drape the strings and mass hangers over the pulleys. Raise or lower the pulleys as necessary so that they are all at the same level and the strings are parallel to the surface of the force table (but not touching it). The mass hangers should hang unimpeded from the ends of the strings.
  2. Hang the given values for mass for Trial 1 over each pulley. Don't forget to include the mass of the hanger as part of the total mass! For the third pulley, hang the mass that you estimated for the equilibrant force. When the system is in equilibrium, the ring should be centered over the post. If necessary, by trial and error, adjust the mass and/or angle for the thirdpulleyonly so that it will balance the forces exerted on the ring by the other masses. You may also need to adjust the strings tied to the ring to assure that each string is aligned as precisely as possible with the angle markings on the force table.
  3. Record your observations in the data table. Remember that the third force in this case is called the equilibrant (FE) since it is the force that establishes equilibrium, and the equilibrant is the negative of the resultant force (FR).

3)Could there be more than one vector force that would balance the system? In other words, would it be possible to establish equilibrium by using a different amount of mass on the equilibrant force and adjusting the pulley to a different angle? Explain.

  1. Repeat the procedures for Part A for the following sets of forces, making estimates for each of the trials based on a sketched force diagram (as you did in Question 2) before trying it experimentally. NOTE: You will have to add another pulley to complete Trial 4.

TRIAL 2

Force A (FA) = 60 g at 0°; Force B (FB) = 110 g at 90°

TRIAL 3

Force A (FA) = 60 g at 20°; Force B (FB) = 155 g at 125°

TRIAL 4

Force A (FA) = 30 g at 40°; Force B (FB) = 110 g at 110°; Force C (FC) = 105 g at 250°

4)How close did your estimates for the equilibrant force vectors come to your experimental values? What types of conceptual errors, if any, did you make in determining your estimates?

PART B - GRAPHICAL METHOD

  1. Now, using a metric ruler, protractor, and graph paper, construct a tailtohead diagram of the vector addition of the forces for each of the trials, and calculate the resultant force (FR). You will need to organize your data, determine an appropriate scale, and draw a diagram. Explain your scale and show all of your work. Remember, the resultant force must have a magnitude and direction associated with it. Also determine the equilibrant force (FE), and record your results. If any of your results are very different from your experimental results, go back and check both methods.

PART C - COMPONENT METHOD

  1. Now calculate the resultant force and equilibrant force for each of the trials by using the component method and the appropriate trigonometric equations. Show all diagrams and calculations and record your results in your data table. If any of your results are very different from the graphical and experimental results, go back and check your methods.

5)How do the values for the magnitude and direction of the resultant and equilibrant forces compare between the experimental, graphical, and component method? If they are different, how far off were the values and what are some possible explanations for these differences?

6)If you had ignored the mass of the hangers in this experiment, explain how it would have affected your results (do this for Trial 1 only). Show any required calculations.

7)Explain the relationship between mass and force in this lab.

  1. When you have completed the experiment, disassemble the equipment and put it away.

Alabama Science in Motion – University of South Alabama

Revised: August 11, 2013