Bumper Design

Impulse is the change in momentum, as expressed in the equation: I = Δp = pf - pi. Because of this, the unit for impulse is the same as the unit for momentum: kg-m/s. Impulse can also be defined as the product of a force and the time over which the force was applied, i.e., I = F * t.

We learned that in a collision where the impulse (or change in momentum) is the same, increasing the time over which the collision occurs will decrease the force experienced by the colliding objects. For example, a car going at a given velocity crashing into a brick wall will have the same impulse as the same car at same velocity crashing into a large air bag. However, the change in momentum will be over a longer time, which means the force experienced is less in the case of the air bag: I = Δp = F * t = F’ * t’; if F * t = F’ * t’ and t’ > t, then F’ < F.

In class, we demonstrated this principle by constructing bumpers for little carts, and measuring the forces experienced by the cart. For this experiment, we were given a plastic bag, tape, and a few sheets of paper. Because the force and time are inversely related in the case of impulse, we knew that we had to maximize the time over which the cart went from moving (non-zero momentum) to stop (zero momentum).

After some thought, we determined that the best way to achieve this was through maximizing the ability for the bumper to compress slowly. We knew that, with our given materials, the best way would have been to just crumple paper in a way that leaves a good volume of air between the folds, and then stick it in the bag. However, we decided to try a different approach which involved rolling strips of paper into little tubes. This created a “spring” effect. These springs were then wrapped in paper, which was wrapped in a plastic bag.

The idea was that the springs would provide enough resistance to not have the bumper collapse completely, but they would also be malleable enough to collapse when the cart collided. Also, because the tubes were separate from each other, we had hoped that the tubes would slide past each other, which would also increase the ability of the bumper to collapse.

TRIAL ONE

image of cart in Trial One

video of cart in Trial One

graph of Trial One

Trial One resulted in a force of approx. 1.49 N being experienced by the cart. This was pretty good, however, we knew that it could be better. We realized that our bumper did not collapse as much as we would have liked. For our redesign, we slightly unrolled the tubes to make the diameters larger, which resulted in an overall weaker spring effect. We also removed a few tubes to allow for more air/movement between the tubes.

TRIAL TWO

image of cart in Trial Two

video of cart in Trial Two

graph of Trial Two

Our redesign slightly improved our results, as the experienced force dropped to approx. 1.23 N. If we were to redesign again, we would have decreased the rigidity of the tubes even further.