The Science of Soft Landings
Imagine someone tossing an egg at you, and you must catch the egg without causing it to break. Naturally, you would want to move your hands back, along the motion of the egg as you catch the it, because you know that the egg is fragile and will break if enough force is applied to it. The reason for this action of moving your hands back as you catch an egg comes from the principle of impulse.
Impulse Equation
Impulse is another word for change in momentum, and is defined as force multiplied by time. Let’s consider two cases with the egg toss example:
- You catch the egg with your arms and hands rigid (which means they do not move as the egg is caught). The egg’s motion comes to a complete stop when it hits your hands (and the insides of the egg are probably all over your hands as a result).
- The egg is tossed at you at the same speed as in Case 1, and you catch the egg while moving your hands back along the motion of the egg, until the egg comes to a complete stop.
In both cases, the change in momentum of the egg is the same, because the egg’s initial speed and final speed (coming to a halt) are the same. What’s different between the two cases are the force applied to the egg, and the amount of time it takes to bring the egg’s motion to a stop.
In Case 1, the time that the egg took to come to a stop is extremely short: it was flying in the air at the speed at which it was thrown, and a split second later it came to a stop as it hit your rigid hands. Since the impulse of the egg remains constant, this means that the force acting on the egg during this split second is extremely large, which inevitably brings the egg to its doom and causes it to break.
On the other hand, in Case 2 the time in which the egg comes to a stop from it’s initial flying-in-the-air speed is much longer, because you were smart and moved your arms and hands with the egg as you caught it to bring it to a stop. By the same reasoning that the impulse remains constant, a long time implies that the force acting on the egg during that time is small. This significantly smaller force acting on the egg keeps it from breaking (if the force is small enough).
This principle of impulse to protect objects from breaking can be seen in many everyday devices. Some examples include airbags in automobiles, landing pads on the ground for gymnasts, and boxing gloves. The main idea behind all of these devices is that they deform when something hits them, which effectively increases the time required for a change in momentum, and therefore reduces the force acting on the incident object (in these cases, a person) onto the deforming safety device.
I first learned about this concept using the boxing analogy mentioned in this article. My high school physics teacher told me that someone receiving a punch would want to essentially go with the flow rather than stand rigid and taken the full force of the punch.
Following through with the person throwing the punch increases the time of the impulse, lessening the received force, and lessening the severity of the black eye.
We made a mistake. I updated the impulse equation. I had it equal to power before. Thanks for catching this error.