Catapult Science

How can you make your catapult shoot further?

Well, to do that we need to understand some of the science behind what happens when it shoots an object (like a pebble)!

When the catapult shoots an object, it gives it both a vertical speed and a horizontal speed. How much of each depends on the angle the catapult shoots from. If the catapult tilts straight up, all the speed is in the vertical direction, the pebble does not go very far (just high). If the catapult tilts perfectly sideways, gravity (g) will pull the pebble to the ground too quickly!

To shoot a far distance (sideways) you need to the pebble sideways and up. By doing this, you give the pebble some time to hit the ground while still moving sideways (so it can travel a longer distance):

You can adjust the Shooting Angle by placing the catapult on a small notebook and adjusting the tilt (see picture below):

The second thing that affects how far the catapult shoots is the shooting speed. This can also be adjusted by tightening of loosening the rubber band.

So, how does the shooting speed and angle affect the distance that the catapult can shoot? Try a few different angles and tensions in the rubber band and see if you can notice any effect.  If you make the rubber band tighter, do you increase the shooting speed? What does that do to the shooting range?


What if you make the shooting angle larger of smaller? What is the best shooting angle?

Using Physics and Mathematics you can estimate the distance the catapult can shoot.  If you have experience in these topics you can go here to see how the equations are formed.

If not, that’s okay!  We’ll just put the final equation here:

In the equation there are four variables, each representing something. The range r is how far the catapult shoots. This is what you want to know. The shooting speed v is related to how tight your rubber band is (tighter increases the shooting speed). The gravity g is something that is called a constant – we assume it does not change. Last, the shooting angle (theta) is how much you tilt the catapult when shooting.

The sine (sin) is a special function that changes value depending on the angle. All you need to know is that it is a maximum when 2*theta is equal to 90 degrees (or theta is 45 degrees, halfway between completely vertical and flat).

So, looking at the equation what can you find out? Well, two things: First, increasing the rubber band tension increases the speed, which increases the range. Second, the shooting angle should be at 45 degrees for maximum range.

Now that you know these pieces of knowledge play with your catapult some more. See if you can modify the design for maximum range.  How far can you shoot?  Tell us!!


2 responses to “Catapult Science

  1. William Jelbert

    Say whaaaaaat…?

  2. MY son would like to cite this article in a research project. Can you give us more info?

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