The Physics 500

Materials: Basic Household Materials

Time Allotment: 3 Class Days

Purpose:

The purpose of this activity is to express your understanding of average speed by calculating and comparing the average speed of various objects as they slide or roll down an inclined plane.

Procedure:

  1. Obtain a flat board or other surface that can be inclined by setting one end on either a chair or a couple of books. The board should be approximately 1-meter in length (or longer).
  2. Obtain various objects (soup cans, plastic cups, books, pet cats, pencils, toy cars, dead crickets, etc.) which are capable of sliding or rolling down the inclined plane.
  3. Roll (or slide) various objects down the inclined plane. Measure and record the distance traveled along the incline using a ruler or a meter stick. Measure and record the time to travel the entire length of the inclined plane using a stop watch (present on most digital wristwatches). Include the appropriate units for both of these measurements.
  4. Calculate the average speed for each of the objects which traveled down the incline.

Note: If you are unable to find a board which can be inclined, then the alternative procedure is to find a flat area over which you can slide objects a given distance by giving them an initial push. You can measure the distance traveled and the time of travel. Furthermore, if you do not have a stopwatch and you cannot find one from a friend, then you can measure time in "clicks." Have a friend or family member click on a table (or other flat surface) at a rapid (about 4x/sec) and periodic rate with a pen and measure time in units of clicks. Finally, if you do not have a measuring device to measure distance in inches, feet, cm, or m, then you will have to measure distance in units of paper clips, pretzel sticks, pencil widths, ear-rings, etc. The point here is that there is no excuse for not being able to perform this makeup lab.

 

Data:

Run a trial for at least four objects. Include units of measurements in the top row of the table.

Trial/Object

Distance (________)

Time (____________)

Ave. Speed (___________)

1.

_____________

_____________

_____________

2.

_____________

_____________

_____________

3.

_____________

_____________

_____________

4.

_____________

_____________

_____________

 

Post-Lab Questions:

  1. As each object rolled down the incline, was its average speed during its motion equal to its instantaneous speed when had rolled half-way down the incline? Explain your answer.

     

  2. Suppose that a soup can rolled down an incline and accelerated at a uniform rate. It started from rest (a speed of 0 m.s) and reached the bottom of the incline moving with a speed of 1.8 m/s. State the average speed of the soup can and explain your answer.

     

     

  3. Given the data in the above question (and the fact that the acceleration was uniform), determine the instantaneous speed at the one-fourth, one-half, and three-fourths time intervals (see diagram). Fill in the blanks on the diagram. Explain your answers.

     

     

  4. Suppose that your trip to school took 15 minutes and your odometer changed from 45 678.3 miles to 45686.5 miles. (Info: 1600 meters = 1 mile) Determine your average speed in ...
    1. miles/min. PSYW

       

    2. miles/hour. PSYW

       

    3. m/s. PSYW

       

  5. Does the speedometer of a car measure the car's average speed or the car's instantaneous speed?

 

 

Conclusion:

Summarize the meaning of the phrase average speed, state the equation used to calculate the average speed, and discuss the distinction between the average speed and the instantaneous speed.


 


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This page created by Tom Henderson and last updated on 11/16/98.