Experiment 3: 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 above. 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.
Check that the glider moves freely, without friction.
Part 1: Newton's First Law
To check the first law, give the glider a nudge to start it moving and then observe its behavior. (We
will not take time for exact measurement.) In your conclusion, comment on whether Newton's first
law seems to be working.
Part 2: Newton's Second Law
Turn off the air. Measure the glider’s mass.
With scotch tape, attach a piece of audio tape to the top of the glider, run it over the "air pulley" on
the far end of the track and hang a counterweight, between 5 and 20 grams, on the end.
Connect the interface to the computer with the USB cable. Connect the motion sensor to the digital
inputs on 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. In the column on the far right, double click Graph, which is at the top. Click <Select
measurement> by the vertical axis and select Position (m).
Aim the motion sensor at the glider:
a. Put the glider where the counterweight is all the way up. Place the sensor about 45 or 50 cm
from there, in line with the air track and a little higher than the top of the track. (It can’t “see”
anything closer than 40 cm.) Measure how far from the sensor to the back of the glider with a
meterstick.
b. Click Record at the lower left of the screen. Dots should start appearing on the graph at the
distance you just measured. If it’s not picking up the glider, slightly twist the sensor up or down
until it does. Make sure the sensor still picks it up if the glider is in different spots. Click Stop.
Collect data: Click <Select measurement> by the vertical axis again and select Velocity (m/s).
Hold the glider where the counterweight is all the way up. 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, out of the area.
Change the scale of the graph, to get a better look at the part you are interested in. To do this, click on
a number by one of the axes and drag that number toward or away from the origin.
The line on the graph should be horizontal before you released the glider, then diagonal as it picked
up speed, then drop off or jump around when the glider got out of range. This “line” won’t be very
straight, but that should be the general pattern. If you've got something ugly instead, it's usually the
aim of the sensor. Adjust it further or ask for help.
Once you have good data, do a best fit line: On the toolbar at the top, click the triangle in
.
Select Linear: mx + b and a line will appear. Right now, it's trying to fit this line to every point in
the data set. Click
. A box will appear on the graph. Get this box around the part of the
graph where the glider was gaining speed, and it will fit to just these points instead. Do this by
dragging the sides of the box. 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, and its uncertainty. (Do not
confuse m for slope with m for mass.)
To put error bars on the graph: Right click on the box that told you the slope. Put a check mark by
Show Root Mean Square Error. You will use the number which appears for the uncertainty in v.
Click
at the top of the Graph. In the Properties window, click on “Active Data
Appearance.” Scroll down to “Show Vertical Error Bars” and click the check box next to this.
For Error Bar Type, select Fixed Range. Enter the RMSE value in both the “above” and “below”
boxes.
Ask the instructor if your results look right before investing time in using them. Once approved,
print a copy of the graph by connecting the laptop to the printer on the cart. One copy per group is
enough. Click on "file", at the upper left, then "print active display," then ok.
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. m is the total mass which is accelerating, glider plus
counterweight, and has an uncertainty of only about .2 gram. Express the result in SI units (based
on meters, kilograms, and seconds).
3. Separately, determine the net force on the system:
a. First identify the force causing the system to accelerate. For example, the weight of the
glider and the normal force on it cancel each other. What is the name of the one force on this
system which is not balanced out by something else?
b. Calculate the magnitude of this force, in SI units. 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.
Please remove any tape you put on the weight or glider.
PHY 131
Experiment 3: Newton's Laws
Part 1:
Observations: Once in motion, what does the glider do then?
Part 2:
mglider = __________________
mcounterweight = ___________________
Slope of best fit line = _______________ + _______________
Name of person whose paper has your group's graph: ____________________
Answer questions 1-3: