Experiment 4: Newton’s Laws of Motion
You will experimentally verify Newton’s first and second laws. A little sled is supported by air
blowing out of holes in the track it sits on. Since it doesn’t touch the track, the only friction is air
drag, which is practically zero. To check the first law, just observe that once in motion, it stays in
motion. For the second law, you will separately find F and ma, and see if they match. The force
comes from a known weight hanging from the end of the track which is connected to the glider by a
ribbon. To find acceleration, the glider's position is recorded periodically by sending out a pulse of
ultrasound and measuring the time for the echo to return. From how the glider's position changes
with time, you could calculate velocity and acceleration as you did in the freefall lab. The sensor is
connected to a computer which does it for you instead.
Procedure:
Place the air track with its "air pulley" hanging over the end of the table, so the counterweight can
drop, as shown below. Check that the screw at the bottom of the "air pulley," which serves as a
valve, is open about one turn.
Level the air track by turning on the air and adjusting the screw under one end until the glider
doesn’t coast downhill. (It always moves a little after you let go, but, there should be no consistent
direction when released from various points.) Then, do not move the air track, or you might unlevel it. (If a weight is attached to the glider, remove it until part 2.)
Check that the glider moves freely, without friction.
Part 1: Newton's First Law
Give the glider a nudge to start it slowly moving. Observe its behavior: Once in motion, what does
the glider do until it hits the other end of the track? In your conclusion, comment on whether
Newton's first law seems to be working.
Part 2: Newton's Second Law
Turn off the air.
glider’s mass.
Measure the
Put the glider at the “release from
here” position. With scotch tape,
attach a piece of audio tape to the
top of the glider and run it to the
counterweight (between 5 and 20
grams) hanging just below the
"air pulley."
Connect the interface to the computer with the USB cable. Connect the motion sensor to the
interface, yellow to channel 1 and black to channel 2. Turn both the computer and the interface on.
(The button at the upper left of the interface should turn blue.) Open PASCO Capstone on the
computer, then
a. Click Hardware Setup at the upper left. Click the yellow circle by Input 1 on the upper left of
the picture. Click Motion Sensor II. Click Hardware Setup again to hide that window.
b. At the top on the far right of the screen, double click Graph. Click <Select measurement> by
the vertical axis and select Position (m). Adjust the vertical scale so that it goes from 0 to
around 1.5 m: To move the graph up or down, click on it and drag it. To change the scale, click
on a number by one of the axes and drag it toward or away from the origin.
Aim the motion sensor:
a. It should be in line with the air track, a little higher than the top of the track, aimed slightly
downward at the back of the glider, which is in its release position. (The sensor can’t “see”
anything closer than 40 cm.)
b. Click Record at the lower left. Dots should appear on the graph at what looks like the correct
distance. (Check with a meterstick if unsure.) If it’s not picking up the glider, slightly twist the
sensor up or down until it does.
c. Slide the glider farther down the track to see if the sensor still picks it up. Click Stop.
Collect data: Click Position (m) by the vertical axis and select Velocity (m/s). Hold the glider at
the release position. Start the air. Click Record, release the glider, click Stop at the other end of the
track. Keep anything which might reflect ultrasound, such as your hands, away. Turn the air off.
The graph should have a diagonal part where the glider was picking up speed. If not, ask for help
aiming the sensor.
Change the scale of the graph to get a better look at the part you are interested in by dragging the
numbers by the axes.
Once you have good data, do a best fit line: On the toolbar at the top, click the triangle in
Select Linear: mt + b and a line should appear. Occasionally it doesn’t;
do a control-click on a data point and try again. Right now, it's trying to
fit this line to every point in the data set. Click
. A box
.
will appear on the graph. By dragging the sides of the box, get it around
the part of the graph where the glider was gaining speed, and it will fit
the line to just these points. When in doubt, it's better to leave out some
good points than to include some bad ones. Record the slope, m, written on the graph. (Do not
confuse m for slope with m for mass.)
Ask the instructor if your results look right before investing time in using them. You do not need to
attach a hard copy of the graph to your report if the instructor looks at it on the computer screen
instead. In case something needs to be rechecked, do not delete the file until you are completely
done and the instructor has approved your paper.
Include the following calculations and conclusions in your write-up:
1. What does the shape of the graph show about how velocity and acceleration vary with time?
2. Compute ma, with its uncertainty. Express the result in SI units (based on METERS,
KILOGRAMS, and SECONDS). m is the total mass which is accelerating, glider plus
counterweight, and has an uncertainty of only about .2 gram. Remember, when multiplying
numbers, you add percents of uncertainty.
3. Separately, determine the magnitude of the net force on the system, in
SI units. (Forces other than the one in this diagram cancel each other out.)
Assume it has a 1% uncertainty.
If F does not match ma, double check your results. (Good science is reproducible.) Use a new
piece of recording tape; they sometimes get rough causing them to drag.
4. In writing the discussion: As usual, you should describe the basic idea of the equipment and
what you did. Step-by-step details are not the point. You don’t need to mention checking the screw
at the bottom of the air pulley or what you clicked on in the software. Rather, how does the air track
work? How does the motion sensor work? Mention the graph on the computer, the information
you got from it and how that told you ma.
For the conclusion at the end, state whether F agrees with ma. (Don’t be surprised by a very slight
difference beyond the range of measurement uncertainty. In spite of your best efforts, the track
might not have been perfectly level or the audio tape might have dragged over the end of the track.)
Please remove any tape you put on the weight or glider.
Also, shut down the computer. (It consumes power if you just log off.) Put it back in the cart, with
its charger plugged in.
PHY 121
Experiment 4: Newton's Laws
Part 1:
Observations: Once in motion, what does the glider do then?
Part 2:
mglider = __________________
mcounterweight = ___________________
Slope of best fit line = _______________ + _______________
Answer questions 1-3: