The Physics of the Girl Who Got Hit In the Face With a Watermelon
Most of you have seen it by now, but for those who haven’t:
Aren’t you surprised she didn’t die? Or even get a bloody nose? In fact, she seemed perfectly fine (aside from an understandable headache).
So how could that be possible? Well, like all things, it can all be explained by simple physics.
Lets start with the slingshot. An elastic material can be pulled and stretched. When you stretch an elastic material, you can feel that it wants to go back to its original (equilibrium) position. This is called a “restoring force” because it is restoring the elastic to its original shape. How big the restoring force is depends on the elasticity of the material, as well as how much you stretch it
So when this courageous young lady stretches the slingshot, it snaps back as soon as she lets it go because of the restoring force. Unfortunately for her, the pocket which contained the watermelon got turned around and the whole process started again. The watermelon and slingshot travelled to the opposite side, stretched once again, and a restoring force pulling the slingshot back towards her face was created, and then POW! Right in the kisser!
But how come she was nearly unhurt? That watermelon was moving at a pretty good clip, and you can plainly see it broke apart quite spectacularly.
In fact, it is the breaking apart of the watermelon that saved her. Permit me to explain.
Newton’s second law of motion states that the force experienced by an object is equal to its mass multiplied by its acceleration. The mass of the watermelon is roughly 2 kg (or 5 lbs for my American readers). Acceleration is simply the rate of change of velocity or how quickly the speed of the watermelon is changing.
When the watermelon hits her it begins to slow down. This is because there is a force acting on the watermelon causing it to slow down. Because of Newton’s third law of motion (that for every action there is an equal and opposite reaction) her face also experiences a force, as she can certainly attest to.
Now, when the watermelon experiences this force it simply is not strong enough to withstand it and breaks apart. This causes two things to happen. First, the mass of the watermelon is now decreasing. Because all the little bits of watermelon are flying in all directions, there is less mass hitting the girl in the face and consequently she experiences a smaller force.
Second, once the hard outer shell of the watermelon is broken, it is the softer insides of the watermelon hitting the young lady. Since this material is much softer, the watermelon slows down over a larger distance, its acceleration is smaller and therefore she experiences a smaller force.
Think of it like this: If I drive a car into a brick wall at 100 mph, I come to a stop pretty fast. The faster I come to a stop, the greater the force I experience, and in this case I would probably get thrown straight through the windshield!
Now imagine I drive into a giant sponge. The sponge has some “give” to it; its squishy. This means that I don’t slow down as quickly. It takes a bit of time for me to come to a complete stop, therefore I experience less force and I DON’T go crashing through the windshield.
The same thing is happening with the watermelon. Softer insides, more distance for it to slow down and less force.
Ever see a NASCAR crash when the car gets flipped a few dozen times and yet the driver is completely unhurt? Same sort of idea.
The car may be flipping, but it is slowing down over a large distance, so the driver does not experience too large a force, and can walk safely away from the crash.
You gotta love Physics! The watermelon girl sure does.