Objective:	To examine the acceleration of a frictionless air track cart under an applied force from a bottle rocket engine and if the data recorded conforms to Newton’s Second Law, F = ma.

Procedure:	Download and view the video which shows the following:

1.	A bottle rocket construction
2.	The air track experiment
3.	Evaluation of force from rocket engine using a laboratory scale.

A bottle rocket offers the physics experimenter a good source of propulsion force for motion experiments.  Its small mass and dramatic operation can be utilized in an exciting investigation of Newton’s Second Law.  In this lab we will use an air track  mechanism which provides a frictionless surface for movement and propulsion experiments.  Airtracks work by a cushion of air forced between the gliding surfaces.  Look up air tracks on line to better understand their operation.  Hovercrafts work on the same principle as airtracks.

Most college students have seen bottle rockets, especially during the Fourth of July celebrations.  They are designed to propel themselves into the air at heights of up to 100 feet and then ignite a firecracker explosive attached at the nose cone of the rocket resulting in a loud bang.  The propulsion comes as a result of a solid fuel engine rapidly releasing hot gases from the back engine opening – thus moving the device along by the resulting thrust. This thrust is measured in Newtons and is generally less than 0.5 Newtons in magnitude.  Since the rest of the rocket is lightweight, the subsequent acceleration of the rocket into the air can be somewhat spectacular and rapid.  By utilizing this thrust mechanism we can adapt the small rocket engine to provide a horizontal thrust and move a tiny metal cart along a frictionless surface (on the air track).   If we increase the number of bottle rockets attached to the cart, we should be able to increase the thrust and therefore increase the acceleration.  By charting the acceleration VS number of engines, and knowing the mass of the cart, we can determine the value of the rocket thrust.

To complete the scientific investigation (and close the circle so to speak), we will place the rocket engine on a small laboratory balance, nose down, and upon ignition display the actual thrust of the engine as it quickly burns.  The thrust of the engine determined from the carts movement using the air track should be equal to that displayed on the laboratory balance.

Procedure:	The mass of a small air track metal cart is determined.  In addition the mass of a bottle rocket engine (minus the guidance stick) is also determined.  An air track is setup in the horizontal mode with optical sensing gates placed a distance of 0.30 meters apart.  The starting optical gate is positioned in such a way that the metal cart with the rocket engine will break the optical “electric eye” beam as soon as it starts to move down the track.  When the beam is broken by the cart, a computerized timing program is initialized which will yield the total time for the cart to travel from the start to the finish, a distance of 0.30 meters.  The total time for this travel is read directly on the computer screen.  After the airtrack is setup, the following steps are undertaken:



1.	Record the time for the cart to travel the 0.30 meters down the frictionless track being propelled by the burning rocket engine.

			2.	Repeat the trial again.

3.	Place two engines on the cart – record the time for the cart to travel the 0.30 meters down the track.  Using two engines should result in less time for the same distance traveled because the force of thrust and hence the acceleration is greater.

4.	Repeat the above with the two engines.

5.	Try three engines twice.

6.	Try four engines twice

		Determine the average time for all engine configurations.  Next, using this time value, and making the assumption that the acceleration was constant over the 0.30 meters of travel, determine the acceleration of the cart using the D = ½ a t ^ 2  equation of accelerated motion.  We will also make the assumption that the initial velocity of the cart is zero.  Using the mass data (and remember to increase the mass each time an engine is added), determine the thrust force for each configuration.  Plot the thrust force as a function of the number of rocket engines used put N on the X axis where N is the number of engines used. Place the thrust force on the Y axis. Place the TRENDLINE equation of this line on your chart and incorporate this chart in you lab report.   From the equation of this relationship, determine the slope of the line.  This will indicate the thrust force per engine. The units should be Newtons.

Next, to actually check this out and determine the thrust force in another manner (and see if the two thrust magnitudes are reasonably close)  place a bottle rocket engine nose down on a laboratory scale.  Ignite the engine and record the average thrust.  The balance display will indicate grams (most chemistry scales do) so a conversion to kilograms and then to Newtons of thrust will give a second method of thrust force determination in a completely different way from the acceleration cart technique.  Compare both numbers in your report – commenting on which technique is more accurate.

Write-up:	Your lab report  must contain the following (along with a title):

			1.	Introduction	(5 points)
			2.	Procedure	(5 points)
			3.	Observation	(10 points)
			4.	Chart showing the thrust force per rocket engine used (30pts)
			5.	Determine the thrust force per engine from the chart (20 pts)
			6.	Determination of the thrust force from the lab scale method. (20pt)
			7.	In your Discussion, make a comparison of both techniques.
				Comment on whether or not Newton’s Second Law of motion
				seems to be correct.  (10 points)