Notebook 2DC - 2D collisions

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Iowa State University *

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231L

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Physics

Date

Feb 20, 2024

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docx

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Lab 2DC - 2D collisions Equipment  A table with a glass surface on which two heavy, air-supported pucks can slide about with little friction.  A spark generator that produces sparks with a frequency of 60 Hz (or 60 sparks per second)  Carbon paper  21” × 21” paper  A set of normal pucks Lab 2DC – Page 1

Background Consider a puck that moves on a horizontal, frictionless table. “ The linear momentum of this puck is constant during this motion. In other words, the linear momentum of this puck is a conserved quantity.” True or false? Explain. Since there is no other external force acting on the pucks, this statement is true. Now assume that we have two such pucks on the frictionless table. The pucks collide around the center of the table, and then each comes out in a different direction. At any given moment, each puck has its own linear momentum: We can define the total linear momentum of the system of two pucks: Prediction Is the total linear momentum a conserved quantity during the collision? Explain. Even though they collide, the total linear momentum is a conserved quality because there’s no external forces acting on the system. Your goal today is to test your predictions about the conservation of total linear momentum using the record of a collision between two pucks on a low-friction table. Lab 2DC – Page 2

Experimental setup Before you proceed, make sure that:  The spark generator is turned off (to ensure you don’t get accidentally shocked)  A 21”  21” sheet of plain paper has been placed on top of the carbon paper. The paper does not need to be taped. To avoid abrasions and tears, the pucks should never be placed directly on the carbon paper. Then, use the figure below to identify all the components of your apparatus. Table leveling Use the spirit level and the leveling screws on the feet of the table to make sure that the surface of the table is horizontal. The results of this experiment will be affected by even small deviations, so make sure you are as precise as possible. Air cushion Air flows through the tubing to each of the pucks on provides a cushion of air, so the pucks can slide on the surface with negligible friction. Turn on the air (turn the knob 1 on the pressure regulator CW) to about 8 PSI (pounds per square inch) and check that both pucks can slide freely. 1 You should only need to adjust the knob that feeds air to a couple of tables, NOT one of two main valves in the room. That main valve should be only partially opened, because the maximum pressure in the system is 90 PSI! If the pressure is too high, the tubes will burst! Lab 2DC – Page 3

Spark generator When the spark generator is turned on AND the pedal is depressed, it periodically produces for a very short time a large electrical potential difference between the two pucks. This causes electric charge to flow from the center of the base of one puck, through the paper in the form of a spark, then along the carbon sheet, and back through the paper to the second puck. Each time this occurs, two small black marks are left on the bottom side of the plain paper, indicating the position of each puck at the time of the electrical pulse. When a puck is moving, a trail of black marks is left on the paper indicating the direction and speed of the moving puck. For today’s labs, the spark generator should be set to 60 Hz. What is the time between two successive sparks? 16.7 ms The spark marks are not a perfect record of the positions of the pucks. You will probably notice that the spark marks do not lie exactly along a smooth line, nor are they spaced by the exactly the same distance when the puck is traveling at constant speed. The source of these random errors is the changing direction of the sparks from the puck to the carbon paper, as illustrated in the figure to the right. To minimize these errors, the spark electrode is designed to be very close to the paper. These errors limit the accuracy with which you can determine the true position and speed of each puck. Safety tips While the spark apparatus is not dangerous, the sensation which it can produce is rather unpleasant. To avoid electric shock:  Do not depress the spark generator pedal until you are ready to take a data run.  Turn the power switch off as soon as that data run is finished.  Do not lift a puck from the table while the pedal is depressed.  Handle the pucks only by their insulated stems.  Do not touch the carbon or paper sheets while the pedal is depressed. Be gentle!  Handle the pucks gently and only by their plastic stems.  Handle the carbon paper gently. A carbon sheet should last for many classes. Tears may be repaired with ordinary transparent tape applied to the underside of the carbon paper (not the side that prints!) Lab 2DC – Page 4

Practice runs  Take some practice runs with spark recording. o Review the safety tips! o Holding the two pucks by their plastic stems, depress the foot pedal (to turn on the sparks) and give the pucks an initial push so they collide somewhere near the center of the table. o After the pucks have traveled well away from the collision point, release the foot pedal and turn OFF the power switch of the spark generator. o Inspect the spark record on the underside of the paper. Some of the marks may be faint. This is just fine as long as they are visible.  Don’t waste paper! Several test runs can be recorded on the same side of a data sheet, and both sides of the sheet can be used!  Practice giving the pucks speeds such that there is about 1 cm between spark marks. Lab 2DC – Page 5

Example of a good collision. Data collection Collision record Some collisions make the analysis a bit easier. In order to obtain good results, we recommend using a collision record such that: Each of the traces before and after the collision has several dots in a straight line. If the trajectories when the pucks are moving freely are not straight, think what can be the cause and correct it.  None of the pucks is moving too slowly or too fast at any given point.  It is not a head-on, 1D collision with almost the same directions before and after the collision. Draw arrows to record the direction of motion of the pucks along the traces. Once you have a nice record of a collision, turn off the equipment. Lab 2DC – Page 6

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