Trebucheting Tennis Balls at 124 MPH

A trebuchet is one of the older machines of war. It’s basically a sling on a frame, with a weight that you can lift up high and which pulls the sling arm over on release. Making one opens up the doors to backyard mayhem, but optimizing one opens up the wonders of physics.

[Tom Stanton] covers just about everything you need to know about trebuchet building in his four-part video series. Indeed, he sums it up in video two: you’ve got some potential energy in the weight, and you want to transfer as much of that as possible to the ball. This implies that the optimal path for the weight would be straight down, but then there’s the axle in the way.  The rest, as they say, is mechanical engineering.

Video three was the most interesting for us. [Tom] already had some strange arm design that intends to get the weight partially around the axle, but he’s still getting low efficiencies, so he builds a trebuchet on wheels — the classic solution. Along the way, he takes a ton of measurements with Physlets Tracker, which does video analysis to extract physical measurements. That tip alone is worth the price of admission, but when the ball tops out at 124 mph, you gotta cheer.

In video four, [Tom] plays around with the weight of the projectile and discovers that he’s putting spin on his tennis ball, making it curve in flight. Who knew?

Anyway, all four videos are embedded below. You can probably skip video one if you already know what a trebuchet is, or aren’t interested in [Tom] learning that paying extra money for a good CNC mill bit is worth it. Video two and three are must-watch trebucheting.

We’re a sad to report that we couldn’t find any good trebuchet links on Hackaday to dish up. You’re going to have to settle for a decade-old catapult post or this sweet beer-pong-playing robotic arm. You can help. Submit your trebuchet tips.

Thanks [DC] for this one!

18 thoughts on “Trebucheting Tennis Balls at 124 MPH

  1. The optimal path isn’t straight down, but a curving spiral that gets closer to the pivot in the end and stays there.

    That’s because of conservation of angular momentum. If the weight drops straight down, the arm swings faster at first but then starts to decelerate towards the end of the swing as the weight recedes from the pivot point. Moving the weight away from the turning hub means the angular velocity of the system is lost in having the weight track a wider arc.

    But imagine if the weight gets closer and closer to the pivot as it drops and finally wraps completely around the hub. Then the swing gets faster and faster like a whiplash.

    1. Thinking out loud, in spherical cow mode: If the weight starts off at height h above the ground, the most energy you’re going to get out of it is m*g*h. If you have it fall only as far as the pivot, and the pivot is “p” off the ground, you’ve only got m*g*(h-p) to work with. If the weight is moving at all when the ball is thrown, you’ve got 1/2 mv^2 still wrapped up in the weight, and not in the ball.

      So ideally, to get the maximum juice out of the weight, you want it as high up as possible and travelling as slowly as possible when the ball is released.

      Spiraling the weight in to the axle uses the kinetic energy of the weight well, but loses you the height of the pivot off the top. Dropping the weight straight down dumps a lot of the kinetic energy into the ground, which can’t be good either.

      So yeah. I spoke/wrote too soon. I’m gonna go read that PDF linked above.

  2. As kids on a farm we built one with what we had lying around. Some scaffolding for the frame ( 2 meters in height) . A metal pole for an arm and a 150kg metal block as a weight. A chain to connect the weight, and some rope for the sling. A couple of washers created a release point and a nail was used as a release pin. No maths, nothing optimised, we could fling a half brick about 70 meters. The scary thing was to watch the scaffolding violently shake back and forth as the weight dropped near vertically. A shot lesson in impedance matching followed, we decided to try a full brick… 70 meters.

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