Introduction: PocketLab Crash Cushion Activity
Over 1.3 million people die per year in traffic fatalities on the road, according to the World Health Organization. One way to combat this problem is to make the roads themselves safer to drive on. The crash cushion activity serves to model a car crash, with the end goal of designing a structure or device that can mitigate the impact of the crash on the passengers of your model car. Whether you are a student, teacher, or parent looking for Summer projects for your kids, this activity is a great way to introduce physics concepts, such as force and acceleration, within the context of real-world engineering problems.
In this project, a small ramp will be used to accelerate a toy cart into a solid crash barrier. You will then need to design a crash cushion from common household supplies that can reduce the maximum force experienced by the cart as it crashes. A PocketLab sensor will be used to collect data on the acceleration of the car so that you can quantitatively compare the effectiveness of each crash cushion.
You will need:
- A small cart or toy car
- A ramp
- A wall or barrier to crash into
- A PocketLab sensor
- A PocketLab compatible mobile device or laptop
- The PocketLab app
- Materials with which to construct your crash cushion
- Common household items such as tape, scissors, or velcro painting hangers
Step 1: Getting Your Cart
When it comes to carts, there are definitely a lot of options available. If you've ordered the PocketLab Maker Kit (http://www.thepocketlab.com/store/#!/PocketLab-Maker-Kit/p/66296811/category=0), you can follow instructions to create a simple cart from the PocketLab box itself, which works perfectly for this activity. There are also other commercial carts available such as this dynamics cart: http://www.hand2mind.com/item/halls-car/4778. And of course, you can always use an old toy car from around the house or even build your own!
Whatever you choose to go with, just make sure that the cart isn't too light, since we want it to be able to create a satisfying crash.
Step 2: Connecting Your PocketLab Sensor
The PocketLab sensor is compatible with most iOS, Android, ChromeOS, and Windows10 devices. You simply need to download the PocketLab app from your respective app store and make sure that your device is Bluetooth enabled. Then, open the app and press the top button on your PocketLab sensor to link the two devices. The graph page should come on screen with data being collected from the PocketLab sensor. To disconnect the PocketLab sensor, just press the top button on the sensor again or close the app. If you need more help with setting up the PocketLab sensor, you can go to http://www.thepocketlab.com/users-guides.html.
Step 3: Attaching Your PocketLab Sensor to Your Cart
Now you will need to attach your PocketLab sensor to your cart so that it can send data about the cart's speed and acceleration to your device. Tape works fairly well for this purpose, but we find that an easy and convenient way to attach the PocketLab is with these velcro painting hanging strips. One goes onto the back of your PocketLab and the other goes onto your cart. Then you can easily attach and detach your PocketLab from your cart without having to fuss with tape.
When you attach your PocketLab, make sure you orient it so that it is easy to read the data that it sends. The front of the PocketLab shows the X, Y, and Z directions of the PocketLab, with arrows pointing in the positive direction. Align the PocketLab so that one of these arrows is pointing straight toward the front of your cart and you will be able to see an easy-to-read positive acceleration in the graphs later on.
Step 4: Building Your Ramp
A ramp of some kind will be needed to accelerate your cart to an adequate velocity before crashing it into the crash barrier. Again, there are a lot of options for this step of the project. We used a prefab Friction and Slope Ramp (http://www.hand2mind.com/store/handson-standards-stem-in-action-rons-ramp-adventure/1642#relatedproduct6120) since we had one lying around, but it's probably more expensive than what you'd like to spend on a ramp for this project. A pair of meter sticks can work well for smaller carts, especially since the markings will make it easier to define a starting line for your cart. The wheels of larger carts, however, might easily fall off the relatively thin meter sticks. Otherwise, you can simply use a wooden plank or even a large stiff piece of cardboard propped up by a stack of boxes or books.
Whatever you decide to construct your ramp out of, make sure that it's sturdy and rigid, about a meter long, and angled at about 30 degrees up from the floor. You'll also want to make sure that the bottom of the ramp is as flush as possible with the surface that it's sitting on, so as to avoid any violent bumps or jostling of the cart when it runs off the ramp and onto the floor.
Step 5: Building Your Collision Barrier
Next, your cart will need some kind of barrier to crash into. This is supposed to represent a solid wall or highway barrier, so you will need to make your collision barrier sturdy enough that the rolling cart won't shift or crush it. If you want this construction to be more portable, a heavy box or container will work well. You also simply position the ramp so that it faces a wall in your house.
Whatever type of collision barrier you decide to use, make sure that it is positioned about 20 centimeters way from the bottom of your ramp. This is so that the cart will have floor space to roll on and will be travelling horizontally when it crashes into the barrier.
Step 6: Finding Materials for Your Crash Cushion
All sorts of craft supplies and household items can be used in creating your crash cushion. Straws and chopsticks can be used to give your designs some structure, while tissue paper and bubble wrap provide a soft landing material for your cart. We recommend scouring your house for supplies before sitting down to build your crash cushion, as considering all these different types of tools and materials may give you some inspiration in terms of design. Most importantly, be creative in your sourcing of materials. You might find some unusual items being surprisingly useful or effective in the construction of a crash cushion.
Here are some ideas for potential crash cushion materials:
- Cardboard boxes
- Construction paper
- Plastic cups
- Pipe cleaners
- Rubber bands
- Paper clips
- Glue guns
Step 7: Making Your Crash Cushions
Now this is the part where you can really be creative! You need to design a crash cushion that can stop the speeding cart slowly, so that it and it's hypothetical passenger experiences the least force during the crash. Lower forces mean that the passenger is more likely to avoid violent motions that result in serious injury. In creating your crash cushion you'll probably want to start from simple designs first, like boxes of soft materials or structures designed to catch the runaway cart. From there, you can improve your original designs or work up to more complex constructions. You may also want to look at real world devices for inspiration for your crash cushion, such as an airbag that gently deflates as the cart collides with it or heavily textured surface that slows the cart down through friction.
In writing this instructable we came up with three different designs for a crash barrier: the Bubble Wrap Box, the Paper Cup Accordion, and the Craft Paper Net, as shown above respectively. Each design had its own flaws and benefits. The Bubble Wrap Box caught the cart easily, but did not give it a very soft landing. Although the Paper Cup Accordion stopped the cart nicely by slowly compressing, it was often hard to hit with the cart. The Craft Paper Net also caught the cart gently, but would then sometimes be too elastic, causing the cart to quickly rebound off of it. These are just a few ideas, as the possibilities for your crash cushions can be truly endless.
We encourage to you try many different types of designs, even if they seem weird or impractical at first. After all, the fun and learning of this activity is found in the exploration of the problem and discovering for yourself what works and what doesn't.
Step 8: Setting Up to Collect Data
When you begin testing your crash cushions, you need to make sure that for each trial the starting position and travel time of the cart are the same. By keeping these parameters constant, you can take consistent measurements of the effectiveness of each crash cushion. If you mark a line near the top of your ramp, you will be able to know exactly where to release the cart so that each trial starts at the same height. Additionally, you must keep the distance between the bottom of the ramp and the cart's point of impact the same. When placing the crash cushion between the ramp and the crash barrier, you will need to move the ramp or the crash barrier back so that the distance between the bottom of the ramp and the front of the crash cushion is the same as the original distance between the bottom of the ramp and the crash barrier. In the photos above you can see that the distances from the ramp to the wall and from the ramp to the Paper Cup Accordion crash cushion are both about 20 centimeters.
To start recording data about your cart crashes, first open the PocketLab app in whatever mobile or laptop device you are using and connect your PocketLab sensor. We recommend that you use the two graph setting with the Acceleration graph and Acceleration Scalar graph, which can be selected through the graph icon in the upper left-hand corner of the screen. The Acceleration Scalar graph will show you the net acceleration of the PocketLab sensor, while the Acceleration graph will show you the acceleration of the sensor in all three individual axes. The scalar acceleration will give you an overall picture of the acceleration, which is probably what a passenger in a car cares about most. The regular acceleration graph, however, will give you a more detailed description of the forces and accelerations acting on the cart.
To collect data, hold your cart at the starting point on your ramp and press the record button in the PocketLab app. Once data start to appear onscreen, release your cart. Once the cart has crashed into the crash barrier or crash cushion and has stopped moving, press the stop button to stop recording data. At this point you can interact with the graphs by scrolling through them or tapping on individual data points to see exact values. By pressing the share button, you can now export either a snapshot of what is currently on the screen or a full data table of all the values. Of course, you can always just write down on a piece of paper the important values for each trial.
Step 9: Collecting Your Data
As a proper scientific investigator, the first step in collecting data must always be to establish a control value. To run control trials, just have your cart crash straight into the crash barrier, without a cushion. This is so you can know how what the forces and accelerations look like without any kind of cushioning and so that you have values to compare your cushioned trials with. The first image above is a screenshot of a graph from our control trials, which shows a baseline value of about 4.30 g. The spikes in the graphs show points in time where the PocketLab sensor experiences a sharp change in acceleration, such as when the cart crashes. The spike showing the crash is usually the largest and found near the end of the timeline. The smaller spikes in acceleration found before it on the timeline are usually caused by bumps or jostling when the cart rolls off the ramp and onto the floor. In terms of decreasing injury in a car crash, the peak acceleration is probably what you want to be looking at.
Once few control trials recorded, you can begin to run test trials for each of your crash cushions. Do the same thing that you did for the control trials, but place the crash cushion you've designed in front of the crash barrier first. Remember, another important scientific habit is making to sure to run multiple trials for whatever you're testing. Collect data from each crash cushion at least five times so that you know you have accurate data. The three images underneath the control graph above show data taken from tests of the Bubble Wrap Box, Paper Cup Accordion, and Craft Paper Net cushions, respectively.
Step 10: Interpreting Your Data
Now, how do you interpret your data once you've collected it? Well, the goal of this activity is to improve crash safety by reducing the forces felt by car and its passengers during crash. And if you're familiar with Newton's second law of motion, you know that the force an object exerts is equal to its mass times the rate at which it is accelerating (F = ma). Because force is directly related to acceleration, the force increases when the acceleration increases. And when the acceleration decreases, the force is also decreases. If we want the force maximum force acting on the cart to be low, then we must make the maximum acceleration is experiences also low. In our tests, you can see that the Craft Paper Net performed the best, producing the lowest acceleration of 1.56 g.
By running many trials and comparing with control results, you can get a very precise idea of how effective each of your designs are. Once you have this information, you can go back to the drawing board and improve your designs to be even safer and more effective. If you need a challenge, you can restrict yourself to certain materials, a maximum size or weight, or the requirement that your crash cushion be easily reusable. Additionally, you can compete with friend to see who can build the best crash cushion.
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