# Golf Ball Catapult (HS Physics)

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## Introduction: Golf Ball Catapult (HS Physics)

For our high school physics class at Verrado High School in Buckeye, Arizona, we made golf-ball catapults.
They are designed to send golf balls flying at insane speeds reaching up to or over 40 yards in distance.
Depending on your own construction, the catapult explained here was recorded to have an acceleration of 235 m/s/s, and could probably more if built more effectively then ours!

Construction is simple and requires only basic tools that are likely already somewhere around your house.

This took several days, but could be accomplished in a few hours, most likely, if you were dedicated enough and had all supplies ahead of time.
We did this mainly because of our study of motion and its related laws, including the law of gravity and Newton's Second Law.

## Step 1: Materials

To build this catapult, you will need some of the following materials. Keep in mind these can be variable depending on what you have available (i.e. arms to pivot on, drill sizes, etc).

Five 2-foot planks of wood(pref. 2x4)
Two 3-foot planks of wood(pref. 2x4)
Some form of pivot arm(can be vaired in width; must be over 2 feet long.
Wood or metal arm which can be used to throw golf-ball
Power-drill and drill bits
Golf ball
Metal braces
Screws
Two trampoline springs
Duct tape
Two 'circle' screws (i.e. pieces of metal which can be screwed into wood, the opposite end of which has a circular head which the springs can latch onto.) (See picture below if you don't understand.)

## Step 2: Building the Base

First, take one of the three feet pieces of wood and lay it out straight.

At this point you'd have two options; you can either connect the pieces of wood directly with screws, or you can connect them with braces, again using screws.
We recommend using braces as it is likely to make your catapult more stable. However, we did not and the catapult is still structurally stable. It's up to you.

Lay one of the two feet pieces of wood out, aligning the bottom of it with the side of the three-foot piece. Then, align the side of the two foot piece of wood with the top of the three foot piece. You are essentially stacking the two foot piece on top of the three foot piece, and aligning their corners.
Proceed to safely screw the pieces of wood together(with or without metal braces), ensuring that after you have finished that the pieces of wood do not move even slightly.
Then, place the other three foot piece of wood aligned with the opposite corner of the two-foot piece. Screw and/or brace those together as well, and ensure they do not move.

Proceed to connect the other two-foot piece of wood to the opposite ends of the three-feet pieces of wood. As you will notice in the attached picture, this piece is not at the very end of the three-feet pieces; this is because, we determined it would be too difficult and probably dangerous to stretch the springs that far. If you wish, you can experiment with different posistions. We used about 2.5 inches from the end of the three-feet pieces.
MAKE SURE THAT THIS PIECE OF WOOD IS 100% SECURE, AS IT IS GOING TO BE TAKING ALL OF THE FORCE OF THE 'LOADED' SPRINGS!

## Step 3: Building the Arm House

The "arm house" is the part of the catapult that holds the pivot arm(a metal pole in our case) as well as the "stopper" which is essentially the most important part of a catapult, as it transfers energy from the arm to the ball ( think of inertia and moving cars, what happens when you get into an accident?!).

To start with, you're going to need to place the third two-foot piece of wood standing straight up off of one of the three-foot pieces of wood, as can be seen in the attached picture (yes, it's a duplicate!)
Place it approximately in the middle of the three-foot piece of wood.
Screws will not work here, you NEED metal braces for this point; screws would either be too long and protrude from the bottom, or would be too short and be ripped form the wood upon impact of the arm.
Do the same for the other three-foot piece of wood with the fourth two-foot piece of wood. Make sure they are alligned, otherwise your pivot arm will not go through both of them!

Next, you're going to need to drill holes into your standing pieces of wood. Make sure to align those as well, and to make the hole large or small enough to perfectly fit your pivot arm. Remember the pivot arm does NOT need to rotate, so it can be a tight fit. The tighter the better; they are less likely to shift or fall out during use.

After drilling your holes, make sure your pivot arm can safely and securely be placed from one side to the other. If it cannot, then you need to change something.

Finally, connect the last piece of two-feet long wood from one standing piece of wood to the other, as is again seen in the picture (the piece of wood the arm is 'stopped' against).
It is VERY LIKELY that you will NEED metal braces for this arm as it will take quite a beating (our first piece actually broke in half due to improper construction). Be CAREFUL!

## Step 4: Constructing the Arm and the Spring Holders

Depending on what you chose to use for an arm, this step may differ slightly or completely for you.

To build a replica of our catapult, keep reading.

You're going to need another two-foot piece of wood to use as the arm. Around the top of this piece of wood, on the skinny side, drill a hole which can hold a golf ball securely, or use some object which can hold it.

Towards the bottom, drill a hole through the middle of the longer side of the wood, which will be where the arm rotates about the pivot arm. For this reason, make sure to make the hole slightly larger then the arm in diameter, otherwise friction will be your enemy.

Next, drill a hole straight through the the skinny side BELOW the hole on which the arm pivots. This will be where your circular spring-holding-screw will go. Make sure your circular-screw goes all the way through and is secured with a nut and washer on the other side, or it will most definitely come dislodged form the force of the springs pulling on it.

Lastly, drill a hole on your two-foot piece of wood which you had the ability to modify the distance on, make sure to align this hole with the hole you drilled previously into your arm. Take your other rounded-head screw and secure it through that piece of wood. Again, washers and nuts are essential, the speed of the arm is likely to rip it out if it is not properly secured. The net force on said screw would simply be too much, if it was not nutted down!

## Step 5: Tighten, Seal, and Finish

Make sure you go around and ensure that no pieces of your catapult move too easily. You can tighten or slightly modify in places where you need to, just make sure everything is stable!

Next, you're going to want some form of arm-stopper-protection. As you can see, we use duct tape. While this is definitely not the best solution, it should work for a few attempts. The centripetal force generated by rotating about the axis will almost definitely break your arm-stopper, though, if you don't have some kind of real protection.

Then, obviously, connect the springs. Be VERY careful in doing so; these things have some serious instant speed, though they slow down very quickly! Point is, if one slips, someone can lose an eye!

Lastly, make sure you have a golf ball ready to go!

## Step 6: Launch Day!

Now that everything is connected, secured, and finished up, you are able to launch your golf balls. Your numbers will almost certainly be different because you will likely be unable to produce the exact same catapult as we did (nor would you probably want to!)

The velocity, while obviously in a forward motion, will be something like 235 m/s.

Things like friction will definitely play against you, as the sheer weight of everything and the transfer of energy will result in some part of the metal being heated. So be careful when using your catapult, but make sure to enjoy responsibly! (Don't hurt anyone or anything, we'd never want that =) )

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## Questions

how do you launch the catapult like to let the arm go

i was just wondering im doing this for a physics project but then we can only use rubber bands as our "spring" i was just wondering will this design work with rubber bands as the springs and if so around how far will it travel?

how does it work

235 m/s is 2/3 the speed of sound.

3 replies

As jschmadeke replied, we are sure of the data. Whether it is entirely accurate over time, is questionable. But initial acceleration is indeed 235 m/s/s

You've confused acceleration with velocity. You should correct that.

L

that did confuse me.

Yep, 235m/s according to our triple axis accelerometer!  I saw the data :)

Let me try this again.  This group's data was 235 m/s/s (if left off the other second in my comment above)

Don't confuse 235 m/s (speed) with 235 m/s/s (acceleration - change in speed over time, hence the second "s")

Students attached digital 25g and triple axis accelerometers to their catapult arms in a crude attempt to document the greatest acceleration of the arm.  The question posed to them was "What is the greatest change in speed over a said period of time?  Remember, acceleration can be negative and positive!

To find the velocity or speed of the arm, students would have to use our laser photogates to determine the time it took to travel a predetermined distance.  Since the class is an introductory course, I thought they could just investigate the acceleration aspect of the arm.

And... the greatest acceleration of the arm would be classified as negative acceleration - when the arm smacked it's stopping point (angle of release) and almost instantaneously stopped moving.  Kids may have confused it as being positive b/c of their perspective of the data being collected.

Here' s an example of some of the equipment and data collecting software they used:  http://www.vernier.com/pkgs/physics.html

Hope this helps ;)
Jonathan