Car on a Trampoline: More Kicks With Kinetic Energy

Oh, sure, you’ve seen a watermelon dropped from a balcony onto a trampoline. But what happens when you drop a car from a high tower onto a trampoline? That’s a whole new level of physics fun, and it’s exactly what happens in this video from Mark Rober and the How Ridiculous guys.First they built their own monster trampoline with overlapping sheets of bulletproof kevlar for the pad, supported by a thick steel frame and 144 big old garage door springs. Then they tested it with a bunch of other things, dropping a whole sack of watermelons, 20 bowling balls and a 66-pound Atlas stone onto a bed of water balloons. The car drop happens near the end of the video, starting at 9:20.

Even if you don't think this is awesome (c’mon, it’s empirically proven to be awesome), it's still a great source for some physics problems you can work out at home, while we’re all doing this social distancing thing. I’m going to solve some of these for you—and I’ll pretend I’m doing them as examples. The truth? I can’t help myself; I just love physics.

1. How high is the drop?

Can you tell from the video how far the car falls before hitting the trampoline? This is the best question, and I'm going to spoil it by giving you the answer. So pause here if you want to try it on your own first.

Ready? If you know your physics, you realized that to find the distance, all you need to do is measure the free-fall time.

Let's start with the basics. Once an object leaves a person's hand, the only force acting on it is the downward gravitational force. The magnitude of this force is the product of its mass (m) and the gravitational field (g = 9.8 N/kg). Since the acceleration of an object also depends on the mass, all free falling objects have the same downward acceleration of 9.8 m/s2. But what’s the connection between fall time and height? I'm going to derive this—and no, I won't just say "Use a kinematic equation."The definition of acceleration in one dimension is the change in velocity (Δv) divided by a change in time (Δt). If I know the elapsed time (I can get that from the video), and I know the acceleration (because this is on Earth), then I can solve for the change in velocity. Note, I'm using negative g for the acceleration, since it's moving downward.