We took a car and rolled it down a ramp into the wall. We then measured the amount that it bounced back off the wall. Next, we made a cushion that would increase the time it took for the collision to occur. We placed the cushion on the wall, rolled the car down the ramp again and then measured the bounce back with the cushion.
This is in an 8th grade middle school science class. We studied Newton's 3rd law, force-pairs, and we know that we need to increase the time in the implulse to decrease the force of the collision. Our goal is to have the car experience less force in the collision so it is safer for passengers. We related this to the real-world applications of airbags and crumple zones on cars.
The question is: for our cushion to be effective, do we want to have MORE bounce back or less bounce back?
I was assuming that having a cushion would reduce the bounce back by absorbing some of the momentum, but the examples given by the district show that the bumper car with more bounce back is safer than a car hitting a brick wall and coming to a complete stop really fast, so it seems like more bounce is better. When our test car bounced off the wall it bounced back an average of 85 cm. Most students are getting less bounce back with cushions than the control runs without.
I have also seen air drops where the delievered item is surrounded by bouncy material to protect it and it makes sense that more bounce is better than a crash landing. How can we talk about the amount of bounce back correctly in terms of Newton's 3rd law and collisions with and without a cushion.
Please help, I'm the teacher ;)
2 Answers
The goal for safety is to reduce the maximum acceleration and also the maximum change in acceleration. The key is to lengthen the time over which the collision occurs.
Imagine you're driving a car toward a brick wall and gently apply the brakes so that you just stop as the bumper touches the wall. Your total momentum change is the same as if you slammed into the wall and stopped (nearly) instantly, but the consequences are quite different. Since total momentum change is equal to the total impulse, the most important thing is to apply that impulse as slowly as possible, over a greater time and greater distance.
If there's a bounce, the total impulse is actually bigger, since you go from an initial positive momentum to a negative momentum, as opposed to going from positive to zero. So for the cushion, it's important to think about how it works. If it's something really springy, like hard rubber, it could actually be worse than the brick wall. If the cushion is a long, weak spring, the collision might occur over a long enough time/distance that the higher total impulse doesn't matter. On the other hand, a very thick feather pillow that causes the collision to happen over some distance but results in no bounce could be best of all. In fact, as far as I understand, the idea of crumple zones is precisely to increase the distance over which the collision takes places without causing a bounce.
It might also be useful to think about the air cushions used for stunt falls in movies. The stunt performers do not bounce when they hit them--the cushions collapse, slowing the person to a complete stop over some distance.
It's safer for the passengers if the distance over which the deceleration occurs is greatest. That leads to less force on them. Crumple zones in cars help.
The cushion will increase that distance, whether it then causes a 'bounce back' is not the main issue.
Air bags can increase the distance and also reduce the pressure on a person as the force is spread over a bigger area. Wide seat belts also increase the area.
All the best with your project!
