Stopping Distances, Work = F*d = Change in E

Exam Friday!  Energy, Work, Power, Chapters 5 & 6  See yesterday's post for review ideas.  more review here 

In class today, students were asked to predict how far a car would take to stop if they were traveling at 70 mph and needed to stop suddenly to avoid an accident.  They have a small piece of paper with their name and a tiny piece of tape on it; they are asked to walk down the hall and place the paper on the wall where their prediction is.  

We then return to class and do calculations; converting mph to meters per second, figuring out how reaction time plays a part, and then how Work (force times distance) is done by the brakes to dissipate Kinetic Energy (and convert it to thermal energy).  

Once we have a distance, there are often arguments; we’ll look up some of the BEST cars’ stopping distances to verify that our numbers are right, then go back to the hallway and count it out – all the kids are usually way off (underestimating) and the closest guess gets bonus points.  We discuss how changing velocity relates to this (slower and faster) and how increasing speed significantly increases stopping distance (and risk of injury).

I make sure all the papers are collected and then we return to class; discussing the significance of this and time when not being attentive (TEXTING!) to the road on stopping distance and safety, and that in the real world, we have other issues (non-perfect brakes, slope of road, less friction due to wet/snow, non anti-lock brakes, etc.)

Tomorrow we'll do some WORK  (Force * distance)  :)  with machines and have some notes to write regarding work, power, and efficiency.