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 Return to Honors Physics Labs Overview  (revised 07-10-02)

Lab 3:  Center of Gravity

  Purpose:       (Hint:  What physical aspect of what specified object do you want to determine?  Using what principle?) 

Materials:      (List them)

Procedure: (Group of three)

1.      Collect five metal clamps.  Place a small piece of masking tape on four of them, labeling the first clamp “F1”, the second clamp “F2”, and so on.  Determine the mass of the four clamps and the metric stick, separately, recording these values in the data table you have constructed.  You do not have to determine the mass of the fifth clamp, which is acting as the pivot.

2.   Record the identifying number written on the edge of the metric stick.

3.   Slide the pivot clamp onto the metric stick exactly to the 45 cm mark and secure it by tightening the screw.  Some clamps have an arrow that helps to align the clamp to the designated mark.  The tightening screw must be oriented downward (Figure 1).

4.   Balance the metric stick on the pivot stand, allowing the right side to rest on the table.

5.   Trial 1:  Slide F1 onto the left side of the metric stick, adjusting its position until the metric stick becomes perfectly balanced in a horizontal position (Figure 1).  Check for equilibrium using the metric ruler to measure from the table surface up to each end or the metric stick. 

6.   Measure the distance of F1 from the pivot.  Use the metric ruler, or subtract its position on the metric stick from 45 cm.  Record this distance as “D1”.   Each distance (D) is measured from the center of each load clamp to the center of the pivot clamp. 

7.   Trial 2:  Slide clamp F2 onto the left side of the metric stick with F1 still attached (Figure 2).   Loosen F1’s screw and slide it closer to the pivot.  Adjust both clamps until the metric stick becomes balanced, making sure the ends of the metric stick are equidistant from the tabletop.  This chore can be made easier if you choose a location for F2, then VERY gently slide F1 until equilibrium is established.  Record D1 and D2.

8.   Trial 3:  Slide F4 onto the RIGHT side of the metric stick, adjusting all three clamps until equilibrium is established (Figure 3).  This task can be made tolerable if you allow F1 to slide all the way to the pivot, adjusting F2 and F4 until the metric stick just tips to the right (F2’s torque is dominant), THEN adjust F1 carefully to the point of equilibrium.  Record D1, D2, and D4.

9.   Trial 4:  Finally, slide F3 onto the LEFT side of the metric stick, adjusting all four clamps until equilibrium is established (Figure 4).  Allow the inner clamps F1 and F2 to slide to the pivot, adjusting F3 and F4 so that the metric stick tips heavily to the right.  Adjust F2, then F1 until equilibrium is established.  Record D1, D2, D3, and D4.

10. Use the principles associated with the equilibrium of torques to mathematically determine the location of the metric stick’s center of gravity, DM.S., for each of the four trials.

                  Do not remove clamps or metric stick from your lab station until all four trials have been completed, in addition to all required math calculations.

Labelled Diagrams:  (the following diagrams must be included as part of this lab:  Figure 1 through Figure 4)

Sample Calculations:     (Show all math steps for EACH of the four trials, including formulas and units for the determination of DM.S.)

 

Data Table:  

            Metric Stick Number: ______                                                                                    

 Counter Clockwise

         Clockwise

TRIAL

F1

D1

 F2 D2 F3 D3   FM.S. DM.S. F4 D4 Pivot C of G
   1                          
   2                          
   3                          
   4                          

                                                                                                                                                   Average Center of Gravity Location:

  

                                                                                       

Conclusion:

                  1.      What physical principle learned in this unit did you use to establish the center of gravity of the metric stick?   Write the mathematical formula representing this principle.

2.      State the center of gravity average value for your metric stick.

3.      Why is 50.00 cm not the location of your metric stick’s center of gravity?  Explain.

4.      What type of equilibrium did the pivot stand provide?  Explain.

                  5.   What affect, specifically, does the force of friction have on your experimental value for the meter stick's center of gravity?  Explain thoroughly.

 

Sources of Potential Lab Error:     (State three technical sources, including the specific variable(s) affected and specific data as substantiation)

                 

Practice Problems:

1.      A 280 kg wooden telephone pole is 7.5 m long.  The narrow end of the pole can be lifted off the ground with a force of 1200 Nt when the other end remains on the ground.

(A)     How far from the heavy end is the center of gravity of the pole?

(B)     What force would be required to lift the wider end of the pole, leaving the narrow end on the ground?

2.      A uniform board 4.0 m long and weighing 350 Nt rests on a pier with 1.2 m of the board extended out over the water.  How far from the pier can a 480 Nt man walk out on the board without getting wet?

                   3.      The center of gravity of a 3.2 m fishing pole that weighs 7.0 Nt is 0.8 m from the end of the handle.  If you pivot the pole on its handle with your right hand while holding your left hand 1.2m from the pivot, how large a force must you exert to lift a 48 Nt fish above the water?  Assume the rod is horizontal.