PURPOSE:
To expose the student to new equipment and lab procedures. And to calculate the momentum and kinetic energy of moving air carts.

INTRODUCTION:
A collision is an event where momentum or kinetic energy is transferred from one object to another. Momentum (P) is the product of mass and velocity (P = mv). A large truck massing 10,000 kg and moving at 2 meters/sec has the same momentum as a 1,000 kg compact car moving at 20 meters/sec; they both have P = 20,000 kg m/sec. The other quantity that can be transferred in a collision is kinetic energy. Kinetic energy is the energy of motion; it is defined as KE = 1/2 m v². The relationship between kinetic energy and mass is linear, which means that a vehicle massing twice as much has twice as much kinetic energy. The relationship between kinetic energy and velocity is exponential, which means that as you increase your speed, kinetic energy increases dramatically.

There are two general types of collisions in physics: elastic and inelastic. An inelastic collision occurs when two objects collide and do not bounce away from each other. Momentum is conserved, because the total momentum of both objects before and after the collision is the same. However, kinetic energy is not conserved. Some of the kinetic energy is converted into sound, heat, and deformation of the objects. A high speed car collision is an inelastic collision. In the above example, if you calculated the momentum of the cars before the collision and added it together, it would be equal to the momentum after the collision when the two cars are stuck together. However, if you calculated the kinetic energy before and after the collision, you would find some of it had been converted to other forms of energy.

An elastic collision occurs when the two objects "bounce" apart when they collide. Two rubber balls are a good example. In an elastic collision, both momentum and kinetic energy are conserved. Almost no energy is lost to sound, heat, or deformation. The first rubber ball deforms, but then quickly bounces back to its former shape, and transfers almost all the kinetic energy to the second ball.

During this lab you will test the principle that momentum and kinetic energy are conserved under certain conditions.

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A standard air track will provide a setting that is almost friction free to test elastic and inelastic collisions.

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A pair of "photo-gates" and monitor are provided to time the passage of the carts on the air track.

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If you get into trouble and perform some procedure that causes the lab to fail(lab equipment will no longer operate), you can press the "Reset" button and the simulation will return to the starting position.

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PROCEDURE:

Inelastic collisions:

1) You can adjust the background shading by clicking on the "Special" button to the right and selecting "Background". Click on the "Special" button and select "Print Blank Report" to obtain a web page that can be printed and used as a lab report. (the program will not be interrupted)

2) Click and hold on to cart 1 and at the same time press the "p" key. This will allow you to pick up cart 1 and move it over to the electronic balance on the shelf. Place the cart on the balance and record its mass. Place the cart back on the air track and record the mass of cart 2 in the same manner. Pick up the ruler on the table and move it over to the top of cart 1. A close up of the ruler and card atop the cart will be seen. Record the length of the card. Caution: the length should be recorded in meters and not cm as shown.

The card on top of the cart has a reflective surface that will "set off" the photogate timers, so in effect the card represents the length of the cart.

3) The air track normally needs to be leveled before each use. On the left leg of the air track is an adjustment wheel. Clicking on the right side of the wheel makes the left end of the air track go up and the left side of the wheel makes it go down. Click on this wheel until the bubble in the green level (next to the words "air track") is in the center.

4) Click on the button located on the left side of the base of photogate 1. The light emitter should start to glow in the top part of the photogate. Move cart 1 over next to photogate 1. Lower the top part of the photogate until the light emitter is aligned with the top card of the cart. If the photogate emitter is not aligned, then no readings will be sent to the monitor. Follow this same procedure with photogate 2. Turn on the photogate monitor, right shelf, by clicking on the On/Off button.

5) Extending from the right end of the air track is a spring launcher. Drag the gray ball of the launcher to the right and cock the mechanism. Move cart 1 all the way to the right on the air track. Place cart 2 in the middle between the two photogates.

6) Turn on the air blower canister, on the table, by clicking on its green button. You should hear the air being produced by the device. Air is forced into the hollow air track and is released through the holes that line the track. Each cart covers these holes with a flat plate on each side of the track. The pressure of the air lifts the cart off of the track and allows it to move without any friction (actually so small we can ignore it).

7) Track leveled, photogates and monitor on, carts ready, blower on and spring cocked.... ready to launch! Click on the red lever to the right to launch cart 1. You will see cart 1 travel to the left and finally collide with cart 2. The ends of each cart are covered with velcro which causes them to collide in an inelastic manner.

8) As cart 1 passes the photogates, the monitor will display the time it takes for the card atop the cart to pass the photogate light emitter. Record the two times. Move to the next room.

Elastic collisions:

9) The main difference in this room is that the carts are equipped with springs on each end. When the carts collide, they bounce off in a nearly elastic collision. Each cart needs to be timed separately thus each has a photogate card on their top. NOTE: these cards are the same size and the same size as was measured in the previous room.

10) Mass each cart as before and record their values. Set the experiment up in the same manner as the last room and launch cart 1. This time there will be three readings on the photogate monitor. The first value will be the time of cart 1 passing photogate 1. The next two values can vary based on your particular mass and initial velocity. Watch the experiment to determine if value two belongs to cart 1 or cart 2. You may have to repeat the run to make sure. Record the three times.

11) Calculate the requested values asked for on the lab sheet and any given by your teacher. For help on these values click on the "Special" button and select "View Data & Hints". Select "File Report" to send a copy to be viewed by your teacher.