PHYSICS 3200:  Heat, Light & Sound

 

Activity  3:   Velocity and Acceleration.  (Thursday & Friday, March 13 & 14)

        In this activity I want you to measure the velocity of an object by recording it position at different times.  The object will be a tire on a shaft that rolls along a track.  You will have to mark the position of the tire every time a beep sounds.  Usually we set the beeps to go every second or so, I have used one beep every 1.5s in the example below.  The setup is shown below.

The track consists of two pieces of metal tubing with tape attached to one of the tubes as shown.  The distance, x, between two successive marks is the change in position for the tire during that time interval.  Its speed will be that change in position divided by the time interval between beeps, T, or

        Speed = (change in position)/(time interval)

        You should have a series of marks on your tape.  When you are finished with a run you can measure the distances between each pair of successive marks and their distances from the first mark.  Make your first mark after you’ve released the tire.

        You will do this for three different situations.  One will be with the track level and two with it inclined; one going up and one going down.  Analyze the data for each run as I have illustrated below.  Label each table indicating whether the track was level, or slanted up (the tire rolling up the slant), or slanted down (the tire rolling down the track).

 

Make a table like the one below.

 

interval

Position

(at end of interval)

Change in Position

Time

Time interval

Velocity

0th

0

 

 

 

 

1st

5cm

5cm

1.5s

1.5s

3.33cm/s

2nd

12cm

7cm

3.0s

1.5s

4.67cm/s

3rd

21cm

9cm

4.5s

1.5s

6.0cm/s

etc.

 

 

 

 

 

Notice that the first mark is your origin, i.e. the position is 0.  I’ve called this the 0th interval.  This is the first mark after you’ve released the tire.  (I’ve just made up the numbers; for this example, they do not correspond to an actual experiment.) 

Answer questions 1-5 for each run.

1.     Is your velocity approximately constant or does it vary?  If it varies appreciably (by more than about 3cm/s), does it increase, decrease or vary randomly? 

2.     Given the slope of the track, or lack of it, what would you expect the velocity to do?  Why?

        If the speed is changing, the object is accelerating.  One can calculate the acceleration from the velocitiess by calculating how much the velocity changes from one interval to the next and dividing by the time between the two intervals.  However, a better way is to make a plot of velocity vs. time from the table for each run.  The slope of that line is the acceleration.  Make such a plot, draw a trendline, and calculate its slope.  Plot each run on a different sheet of graph paper. 

3.     Does the plot for each run look like a straight line? 

4.     What is the acceleration?

5.     Are your accelerations consistent with what you would expect from the slant of your track in each of the three situations?

 

 

Do this for three situations.

1.    Make the measurements when the track is level.  (Also make a graph of position vs. time for the level track only, draw a trendline and calculate its slope.  What does this slope represent?)

2.    Make the measurements when the track is sloped downwards.  The upper end should be about 8” higher than the lower end.  (You should not have to push the tire in this case.)  Here you will plot velocity vs. time and find the acceleration from the slope.

3.    Make the measurements when the track is sloped upward.  The upper end should be about 5 to 6” higher than the lower end.  (You will have to push the tire in this case.)  Here you will plot velocity vs. time and find the acceleration from the slope.