### 042 Marble momentum redesigned twice

Laboratory four was redesigned twice this term. Fall 2013 Liberation Day wiped out the banana leaf marble ramp demonstration of the square root mathematical model for gravitational potential energy and kinetic energy. I opted to attempt to run the demonstration as a laboratory, but was not not encouraged by the result, "The laboratory went moderately well but ultimately was confusing and certainly not discovery oriented. I see no compelling reason to swap out the momentum laboratory which leads naturally to the groups presenting their results."

Laboratory four has in the past gone well and has fallen flat. The laboratory feels inconsistent term-on-term. This term I redesigned the laboratory, and then redesigned the laboratory again.

On Monday I quantified the RipStik gravitational potential to kinetic energy conversion demonstration and generated a homework in ergs. This led much more naturally into the banana leaf marble ramp demonstration on Wednesday.

In the 8:00 class the class set up banana leaf marble ramps and collided a single inbound marble with a line of five marbles on the leaf. The speed of the single inbound marble and the single outbound marble was calculated from distance and time measurements. Then the momentum was calculated.

Ryan and Terrance with the basic set-up. The single duck marble (about five grams) was released from different heights. The ultimate goal was an xy scatter graph of momentum in versus momentum out.

As seen above, the inbound momentum was to be doubled to account for acceleration on the ramp. Bear in mind that the inbound velocity was measured on the slope. The average speed was expected to be half of the speed at ramp bottom. The factor of two, however, was hard to explain to the students and left the inbound momentum far larger than the outbound.

As can be seen on the data sheets, the factor of two was not multiplied into the inbound side, the result was larger outbound momentum by a factor of 1.1 to 1.3.

I then redesigned the lab for 11:00 to completely eliminate the slope issue, laying the leaves flat between two tables. An eight o'clock lab session leaf can be seen in the background, the 11:00 format in the foreground.

Gister and Lilly set up their leaf.

Jessica and Nikita tape down their leaf. Leveling the groove took some finessing.

Note the petiole and midrib between the tables.

Marleen is timing the inbound marble, Johnnyboyd times the outbound marble. Afilina is the marble shooter, Shra Ringlen records data. A fifth member catches the marble. Groups were four to five students. Dual timers was a key feature. Each timed a distance on the banana track. The shooter would call 3, 2, 1, go.

Gister and Lilly show how meter sticks were set up to track distances. The distance had to vary with the marble speed.

The lab in action. The yellow is hitting as the orange is departing in this unusually timed shot.

Afilina, Shra, and Johnnyboyd get ready to make a run at speed.

The laboratory went moderately well, with momentum losses generally being seen of around 20%. Some data points were spurious - timing remains difficult as times are usually under one second. Although obtaining the leaves before class involved getting soaking wet this term, the lab is worth rerunning. The discovery oriented nature of the lab is lost, but a linear regression is gained. Not sure the loss is worth the gain, discovery is good, but this class proved particularly weak on the pretest, hence my attempt to hammer home linears into every lab. Six will use time versus temperature, five is already linear, and seven will be meters per minute, linear version. Eight will be our first break from xy scattergraphing.

Post-script: After a week of cogitating on what might be done differently, I think the solution is to have a single timer with a single stop watch time the inbound and outbound marble. There are two reaction time errors in the above double timer layout. One timer starts at "3-2-1-go!", hits the lap button at the moment of collision, and then hits the stop button at the predetermined outbound distance. Right button-left-button-right button. Even the basic stopwatches I now have will handle this single lap time. The time difference between the inbound marble hitting the line of five and the outbound marble heading out is so far under human reaction time that these times can be taken to be equivalent. The force imparted must move at something like the speed of sound in the marble, minus jumps between marbles if there are gaps.