Newton's Laws
Apparatus
Cart with flag
Track or airtrack
Weight hanger & pulley
Mass set
String
Two photogates
LabPro interface attached to a PC
II. Procedure
You will determine the acceleration of the cart from measurements of
the instantaneous velocity of the cart after it has accelerated various times
and distances along the air track. Two photogates will be used with the LabPro interface, the computer, and the LoggerPro
program to measure the time that it takes to the cart to travel a certain
distance, between the photogates, and the velocity at each gate. The photogate will
read a narrow cardboard (PASCO) or metal (airtrack) flag which protrudes from the top of
the cart and passes through the photogate. Think about what the size of this flag
has to do with the accuracy of this experiment.
You will use a cart on a track attached to a hanging weight
which is strung over a pulley (like the pre-lab). The
tracks we will use are extremely low in friction, so that we can compare our results to
the ideal, frictionless situation.
Preliminary operations
- Level the track by means of the adjusting screw on the single leg. The track may not be
flat at all points, so try to make it as level as possible. Perfection is not attainable -
do your best. Then you must consider the effect of any remaining unlevelness on your
results. If you are using an airtrack, you will want to turn on the air blower while
leveling.
- The air track carts should have four small masses attached to them, while the PASCO
carts should have the slotted wooden block on top.
- Weigh the cart (with its attached masses) and the weight hanger (we are not
going to add any mass to the weight hanger, so it should be about 50 g) on the balance and
record these numbers. Make one measurement and use as your uncertainty your best
estimate of how well the measuring device can be read.
- Use a string long enough so that, when the cart is at the far end of the track, the
weight hanger just barely clears the pulley. The string attaches to the cart so that the
segment of the string between the cart and the pulley is horizontal. Make sure that the
pulley is aligned.
Experiment
In this section, you will use the computer to take data from two
photogates positioned a distance away from each other along the track. You will measure the
time required for the cart to move a measured distance between the two points, as well as the instantaneous
velocities of the glider at those two point.
- You want to measure the length of the flag which passes through the gate and supply this
information to the LoggerPro program so that it can calculate the velocity of the cart.
The photogate allows us to read the time that the beam is blocked by the flag, and if the
length is divided by this time, we get the approximate instantaneous velocity as the cart
passes by the photogate.
- Setup the photogates with the
LabPro interface and
LoggerPro program as described at
this link
- Release the system from rest, and be sure the cart passes between the two
gates before the hanging mass hits the floor (so the acceleration is
constant.) Look at the data to see if it is reasonable before continuing at different gate
separations. You should take at least two readings at each distance to see
how consistent your readings are. And then take data at multiple distances.
Since the distance between the photogates (or the time between the
photogates) is the independent variable that we will be using for best fit
line plots, you need to make sure you use enough different distances (i.e.
at least five) and a good spread.
- Be sure to copy ALL Logger Pro data to an Excel spreadsheet, you may
need to go back to it to figure things out.
- Do not average any data or calculate any accelerations directly from the
numbers (data). The straight line fits in the analysis section are used to
determine the acceleration.
Department
of Physics
