IMPORTANT ANNOUNCEMENTS

dropbox DROPBOX! Prepare to safely archive your data and prepare to share files with lab partners. See Archive Data link.

        MOMENTUM LAB

PostLab Scenario

You are a professional witness in a lawsuit. What do you tell the judge? What do you tell the Physicist (Read the following scenario.) You are on an RTD Stretch Bus (the long busses with the accordion in the middle. (This is called an articulating bus.)) The bus is fully loaded with fuel and people. You are traveling at 30 [mph] when a mini-Van in front of the bus pulls in front of the bus as it travels in a similar direction; the van is traveling 20 [mph]. The bus driver slams on the brakes, but there is black ice so the bus gracefully slides right into the van sending the van flying ahead. Three passengers riding the public transit bus, upon realizing their opportunity to make some quick cash, start crying of back and neck pain. Mass of bus + passengers + fuel is about 66,600 [lb] or 30,200 [kg]. Mass of van is about 2,000 [kg]. Please Note: A few students have contacted me and said that I made an error converting from pounds to kilograms. Here are my conversion notes: 66600*4.4492454 /9.8 = 30236.709 #[lb] * 4.4492454[N] / 1 [lb] / 9.8[m/s/s]---> kg 66600*0.454 = 30236.400 #[lb] * 0.454...[kg] / 1 [lb] ---> kg See Wolfram Alpha for more information. 1 pound is about 4.45 Newton. Forensics estimates from the bending of the light-pole that the van was moving at an uninhibited speed of about 35 [mph] when it hit the pole after sliding on ice. You are a professional witness; what is your statement to the judge and why? (Three other bus passengers tell the judge they didn't even feel the collision. They heard it and saw the van fly forward; but they claim they felt nothing.) You are a professional witness; what is your statement to the Physicist and why? Your paper needs to address both the Judge (jury) and the Physicist. Treat each separately. 1) The Judge needs terms that are easy to understand. You need to include some "real world" examples to drive home your answer. The Judge needs to see concepts of momentum exchange and a final answer for change in speed for the bus in mph. 2) The Physicist needs to see calculations for momentum exchange and a final answer for change in velocity for the bus in m/s. Some students will want to address peak force (not required, but interesting.) In order to find peak force we need to know the time of the collision. Do some research to find some values for the time of collisions and calculate max and min force based upon your time range. I called RTD and spoke with a trainer. He told me the Articulator holds 163 passengers, 183 gallons of fuel and weighs in at 66,600[lb] when fully loaded. I (Douglas Howey) was on a bus when this happened. We were traveling westward on East Colfax just west of Peoria Street. The van flew ahead of us and crashed through a bunch of fence posts. People on the bus did make claims that they were seriously injured. These types of claims are common on RTD busses.

PostLab Instructions

In a formal, word-processed paper*, answer as a discussion with full thoughts and numerical calculations the discussion question listed above about the RTD bus**. Hand this in on paper (with a proper header including what is below) at beginning of next week's class. Proper Header: Momentum PostLab Name(s) Lab Time: (Day and start time, W8 would be a Wednesday class that starts at 8am) The Header should be in upper left of paper. *The post lab cannot be more than one sheet of paper. You may print two-sided. **Your discussion needs to have two major emphases: Your paper needs to address both the Judge (jury) and the Physicist. Treat each separately. 1) The Judge needs terms that are easy to understand. You need to include some "real world" examples to drive home your answer. The Judge needs to see concepts of momentum exchange and a final answer for change in speed for the bus in mph. 2) The Physicist needs to see calculations for momentum exchange and a final answer for change in velocity for the bus in m/s. Some students will want to address peak force (not required, but interesting.) In order to find peak force we need to know the time of the collision. Do some research to find some values for the time of collisions and calculate max and min force based upon your time range.

Begin Q&A Forum for "MOMENTUM LAB"

How is it possible to calculate the velocity at the end of a ramp in a ramp problem when you are not given a mass magnitude?Anonymous
Tue. 06, Jul 2010, 00:02
I would need to know more of the context, but you can use Kinematics if mass is not known. v = v_o + a*t; where a = g*sin(theta); t=time; v_o=initial velocity.Douglas
Tue. 06, Jul 2010, 05:54

Hello Douglas,
How long did the collision occur? It would be helpful to know what the acceleration of the bus was in order to make a helpful translation of the events. Thanks!
Davis, Christal
Sat. 16, Oct 2010, 12:15
crdavis8384@gmail.com
Excellent question! That's why you are being paid the big bucks to be the expert witness! :)
You are correct; to find the peak in impulse force we need to have an idea of the order of magnitude for the time of collision.
We don't know time of impact. But again, you are the expert here. In finding an estimate two things are important: possible range of values and how that range of values impacts your situation (no pun intended.)
Part of your analysis might be to show a range for common time values of a collision. You might start by looking at your data to see how long the collision took in lab. But the carts should travel in the same direction. Simulation would reveal an approximate value.

Airbag research might lead you to collision data found by car manufacturers. An airbag is designed to deploy on the order of tens of milliseconds (60-80 [ms] according this article: https://en.wikipedia.org/wiki/Airbag ).
You are probably safe to say for a "same direction" collision the time is between 1 and 0.1 [second]. The extreme case might be 0.01 [s]. But I'm not the authority on collisions; and you are the expert witness, so do not quote me for your assignment. :)
Douglas
Sun. 17, Oct 2010, 04:14

Hello Douglas,
I am a little confused on the part that says “moving at an uninhibited speed of about 35 [mph] when it hit the pole after sliding on ice.” Does this mean the bus was going at 35 mph after the collision?
Anonymous
Fri. 09, Mar 2012, 11:30
"... the van was moving at an uninhibited speed of about 35 [mph]" (after the collision)Douglas
Fri. 09, Mar 2012, 11:59

Copyright 1997-2013 by Douglas W Howey.
No content may be edited, copied, nor distributed
in part or in whole without the express written permission of Douglas W Howey

© Copyright 1997-2013 Douglas W Howey
This page was last Modified: Sun. 20, Aug 2017, 17:59 MT and has had 14158 visits since Jan 2013.
Generated by onthefly.cgi      contact web manager