Today I'm excited to introduce you to Christy and her simple catapult!

Christy is a new blogger at From Engineer to SAHM, and has some great ideas for little engineers--I love her optics and circuits tinker post!

Today she's sharing a catapult concept that is appropriate for any age and following it with math and physics for older kids (if you're interested in the quantitative analysis, just keep reading)! I love how similar this concept is to our pumpkin catapult -- they both fit

*so*well in a preschooler's hands!
So...here is Christy:

My oldest son is a super active kid, and my youngest two MUST have a nap. Otherwise the evenings are a series of tantrums and crying fits (by all). When he gets home from school I try to have some kind of activity to do with him while the babies sleep. One afternoon I decided we would run a little experiment. I wanted him to start exploring different kind of simple machines, and thought a catapult would be a fun machine to start with. And, besides, what kid doesn't love chunking things?

I researched several different types of catapults, but didn't want to have to go to the store for supplies. After finding some great ideas, I was able to some up with one on my own!

### Supply List:

2 Popsicle sticks

1 Cranberry

1 Small marshmellow (same size as the cranberry)

tape

1 Rubber band

1 Squishy ball (we used my toddler's "baseball")

Playdough

###
**Catapult Construction:**

**Step 1:**First my son took the ball of playdough and made two balls out of it: one large ball and one small ball

**Step 2:**Next he took the larger ball of playdough and stuck one of the popsicle sticks through it.

**Step 3:**He then took the second popsicle stick and stuck it into the larger ball of playdough, at about a 45º angle to the other popsicle stick.

**Step4:**Next he placed the squishy ball between the two popsicle sticks. He wrapped an elastic band around the ball and popsicle sticks to keep the ball in place. Just to make sure everything stayed in place, he taped the elastic band to the popsicle sticks.

**Step 5:**To complete the catapult, he stuck the top end of the top popsicle stick through the smaller ball of playdough. He then made an indention with his finger to make a launching pad for the projectiles!

### Experiment:

With construction complete, I needed to teach him some catapult terminology. I showed him the sketch below:

LEVER: The top popsicle stick with launching pad.

ANGLE: The distance between the lower popsicle stick and the lever.

Now it was time to experiment with the catapult! I placed a cranberry in one his hands and a small marshmellow (about the same size as the cranberry) in the other. I asked him which one was heavier, and he said the cranberry.

Then I told him,

**"Its time to see which one will go the farthest! We'll launch each one at two different angels, and see which one will go the farthest!"**
**

*Launching tip:*Press the large ball of playdough into the table and hold onto it when launching.**

**Step 1:**He placed the cranberry on the launching pad, pulled back the lever just a bit, and then sent the cranberry into the air. Well, just barely... The cranberry landed about 2 inches in front of the catapult. After we stopped laughing, my son placed a piece of tape where the cranberry had landed.

**Step 2:**He repeated step 1, but with the marshmellow. He also placed a piece of tape where the marshmellow landed, about 1 foot in front of the catapult!

**Step 3:**Next he untaped the elastic band to the lever. He moved the squishy ball further away from the large ball of playdough, which created a smaller angle. After placing the elastic band was around the ball again, he retaped it to the lever.

**Step 4:**He launched the cranberry, and placed a piece of tape where it had landed. He noticed the cranberry had gone further than it had before.

**Step 5:**Last, he launched the marshmellow at the smaller angle, and it flew into the air and landed off the table!

As we looked at the pieces of tape for the cranberry and marshmellow projectiles, it was easy to see the marshmellows had gone the farthest. So our conclusion?

*The smaller the weight, the farther the object being launched will go.*
I wanted to make sure our results matched the physics, though. In my post "Exploring Energy: How are Height and Distance Related?" (http://www.fromengineertosahm.com/exploring-energy-height-distance-related/ ), I wrote about my son exploring potential energy and kinetic energy. A catapult is doing the same thing. But instead of potential energy due to the height of an object, in a catapult the potential energy comes from the "spring" of the catapult. Potential Energy (PE for short) due a spring is expressed as:

where k = the spring constant (a specific value that is different for every spring)

x = how far the "spring" moves horizontally

x = how far the "spring" moves horizontally

Kinetic Energy (or KE for short) is the energy in an object when it is moving. Kinetic energy is expressed as:

where m=mass

v=velocity (speed) of the object

v=velocity (speed) of the object

According to the

*law of conservation of energy*,**energy can neither be created nor destroyed**. Energy can, however,*change forms*. In the case of my son’s catapult, the energy of the projectile was changing from potential to kinetic energy. When the object was sitting on the launch pad, its energy**was equal to**the energy the object had as it left the the launch pad and flew into the air. Which means, the potential energy of the object**was equal to**its kinetic energy:

Substituting in the equation for each forms of energy, you get:

In this case, we're looking for the speed (or velocity) of the object when it first takes of:

### Projectile motion

There are four equations that govern projectile motion, but since we're only interested in how far the projectile is going, we only need one of them:

where x = distance traveled

v = speed of object at take off

t = time

v = speed of object at take off

t = time

We found v (velocity) above, so

While the equation looks like a bunch of math terms jumbled together, I want to focus on two variables:

**x (distance traveled)**

and

**m (mass of the object)**

The two variables are inversely related, which means as one increases, the other will decrease.

In other words, as the

**weight of the projectile decreases, the distance it travels increases**, which is EXACTLY what we saw with our catapult experiment!
After working as an engineer for over 10 years, Christy finally made the leap to stay home to raise her 3 rambunctious boys. But once an engineer, always an engineer. Now she teaches her sons about the world through her eyes, the eyes of an engineer. You can visit her at From Engineer to SAHM and on Pinterest!

I may share at any of these parties!

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