Example Lesson
Summary
We begin by reviewing the concepts of conservation of energy and collisions. We then have a short demo of inelastic collisions of basketballs bouncing. Next, we discuss what happens in a car crash, why that is bad for the humans inside. Once that is established, we try it out: a phone is strapped to the Collision Cart and is released down an incline into a rigid stack of books. After observing the velocity and peak acceleration felt inside the cart (measured by the Phyphox app), we have a discussion about what we could do to decrease that peak acceleration. Together we will have a discussion about crumple zones and why they might be effective. This segues nicely into a discussion about why seatbelts are important. I will have some crumple zones made out of various materials for our demo cart, and the class will engage in a discussion about their effectiveness. I will have the following prompts: "which will work the best/worst and why". We will do each of the crash experiments and then have a discussion about if our experiment matched our expectations. Finally, I have some photos and graphs of real-life crash tests and simulations to share with the class to give a sense of how realistic our experiment was.
Lesson plan
Part 1: Review of Momentum, Collisions, and Conservation of Energy
We begin with a brief lesson and review of conservation of energy along with elastic and inelastic collisions.
The change in energy in a system is equal to the work input into that system:
If the students have learned about ballistic pendulums, or calculations involving converting between gravitational potential energy and kinetic energy, I will remind them that this is another case where energy is always conserved, even though it is converting between types of energy.
Basketball Demo
We then demonstrate an example of an inelastic collision with basketballs at different levels of inflation. There will be a slow-motion video of the balls bouncing, such that the students will be able to see that the ball that was not inflated as much, deforms more. This is to imbue some insight into the idea that some of the kinetic energy is going into deformation and heat.
Basketballs of
different inflation rate
Collision demo with no crumple zone
We introduce the collision track and cart, explain how the measurements work with the phone strapped to the cart. Move the car around on the track to show that the v and acceleration values are working.
We launch the car down the track at the obstacle and observe the high acceleration it experiences (briefly) when impacting the stopper. This is to motivate the discussion about what is bad about car crashes.
The acceleration will cause a force on the human body, which will strain the parts of our body in different amounts, given the composition of most bodes some scientists have found that approx 300 m/s2 or 30 g forces can seriously injure someone. There are a lot of complicated systems in the body that mean different physical forces will have different effects, essentially our calculations will approximate the human body as a series beam that have a specific stress-strain curve that will thus break when subjected to adequate acceleration.
Leading a Discussion About How We Could Make the Cart Experience Less Acceleration
As a class we can discuss what happens in a car and how the occupants can experience a lower acceleration.
The impulse that the car experiences from the wall is J = F⋅Δt
F = ma, so we can lower this force, and thus the acceleration by increasing t,
Intuitively we are doing two things: Spreading out the forces absorbing some of the kinetic energy through the deformation and heat of the metals.
When we get to the idea of crumple zones, I have some great visuals to show how they work:
Figure 3 Explanation of Crumple zone and passenger cell from cartrade.com
Figure 4 Simulation of stress on vehicle frame from Vehicle Collision Dynamics Dario Vangi 2020
The image in Figure 4 Simulation of stress on vehicle frame from Vehicle Collision Dynamics Dario Vangi 2020, shows the stress that the beams in the vehicle will endure, it is instructive to view that it works as you might expect, there is a lot of complicated engineering that goes into the design, but there are some simple components too: they have thicker beams in the passenger cell. The car frame is as you might expect it.
Crumple Zones Could Work, but What Happens if We Do not Wear Seatbelts
As a review of the implications of Newtons first law: the person in the vehicle without a seatbelt will continue in motion inside of the car during a collision. Any of the dampening that happens via the crumple zone will not be distributed to the human body, and a new more damaging collision occurs between the human and the inside of the vehicle.
Figure 5 the speeds at which an unbelted passenger will hit the windshield Source: Exploring Engineering 5th Edition
Main Demonstration: experiment of deformable bumpers and their effectiveness
This is the part where the demonstration is mostly happening. I want to show the demonstration bumpers and ask the class to vote on which ones they think will be most effective at reducing peak acceleration. I will encourage them to discuss with their neighbour to come to a consensus.
We will follow the instructions in this document labeled Doing the Demo, noting the peak accelerations felt by the cart (with Phyphox)
We will calculate, using the mass and the height of the Collision Cart
The E.g. (Gravitational Potential energy),
KEat impact (kinetic energy right before impact)
and the expected impulse.
We will run the experiment again, a faster speed, with a similar discussion about how effective the bumpers are going to be.
remember that KE = 1/2 mv2, I want the students to consider that the vehicles will have 4x as much energy with 2x as much speed, so maybe the bumpers may work differently, or not as well.
After all of the data has been collected, we will talk about:
o Whether the results were as we expected
o Why do we think some crumple zones worked better than others?
Post Experiment Discussion
If there is time, I can pull up more detailed plots of the Phyphox data and compare it to some plots of actual crash test measured data. We can talk about what was similar and what was different.
I want to make a point that this process of having a theory about something and then trying it out is actually, pretty much how engineers design things. (Sure, they willl do a better job of manufacturing), but in general, you run a simulation, then you build a mock-up and try it out. Thus, this is not such a far-removed demo from real life.
Through design and experimentation Saab improved on their models of the safety cell over the course of 8 years
Figure 6 Saab structure comparison through the years source Vehicle Collision Dynamics Dario Vangi 2020