# Catapult Lab Help: Calculating Projectile Motion and Energy | Physics Lab Tips

• alli66
In summary: Use vi from #3.The ball had a PE of .8266 at the peak height. It was moving at a speed of 3.15 meters per second at the peak height. This work was done by gravity. The spring constant was .9 and the ball had an elastic P.E of 1.
alli66
[Note from mentor: This was originally posted in a non-homework forum, therefore it does not use the homework template.]

we did a lab on catapult but I'm having trouble figuring out the math? i calculated the hangtime, but all i did was take the original time and divide it by 2, is that right?
QUESTIONS:
1) how long did it take to reach peak height?
2) what is the projectiles initial vel in the x and y direction?
3) what was the initial vel.?
4) what was the peak height?
5) how much PE did the ball have at the peak height?use h from #4
6) what's the KE of the ball at the launch? use vi from #3
7) how fast is the ball moving at the peak height?use conservation of energy
8 )what was the elastic P.E of the catapult?
9) what was the spring constant?
10) how much work was done by gravity on the ball?

DATA:
- catapult launch angle 70 degrees
- length of armature = 28cm
- elastic band length before= .875in
- elastic band length after= 2.75 in
- mass of ball= 18.75 g

launch distance: trial 1: 12.85m, trial 2: 3.09m, trial 3: 3.15m, avg of the 3= 3.03 m
hang time: original time divided by 2 to get hang time? that's what i did but i think its wrong: trial 1: 1.60s/2 =.8 sec, trial 2: 1.65 sec/2= .825, trial 3: 1.71/2=.855s,, avg of 3 hangtimes= .8266secs

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In your handwritten data sheet it is 2.85 meters on trial#1, not 12.85 as you typed. I was wondering how you came up with that average.
The definition of hang time, is the total amount of time in the air ( https://en.wikipedia.org/wiki/Hang_time ) That is from time it leaves the catapult until it touches the ground. So hang time would be the total time. But did your teacher define it differently? What do you mean by original time?

With no air resistance, and if launched from ground level, the projectile will spend half of the time going up, and half going down, so your time to peak (in that situation) is half of the total time. So there is going to be air resistance, but those effects may be minimal if the projectile is smooth shaped and has "enough" mass, and the air is calm. So was it launched from ground level, where the distance up equals the distance back down? If the catapult is "short enough" then you could approximate that it was launched from ground level. That may be what you have to do for this particular lab.

## 1. How do I construct a catapult for the lab?

To construct a catapult, you will need a base, an arm, a lever, and a projectile. The base can be made of wood or cardboard, and the arm can be made of a sturdy material like popsicle sticks or dowel rods. The lever can be a spoon or a wooden stick. Use hot glue or tape to secure the pieces together. For the projectile, you can use a small ball of clay or a pom-pom.

## 2. What is the purpose of this lab?

The purpose of this lab is to demonstrate the principles of projectile motion and how they relate to the design and function of a catapult. By building and testing a catapult, you will gain a better understanding of concepts such as force, distance, and angle.

## 3. How do I measure the distance of the projectile?

You can measure the distance of the projectile by marking the starting point and the landing point on the ground, then measuring the distance between them using a ruler or measuring tape. Alternatively, you can use a measuring wheel or a smartphone app designed for measuring distances.

## 4. What factors can affect the distance of the projectile?

The distance of the projectile can be affected by several factors, including the angle of the catapult, the force applied, the weight of the projectile, and air resistance. Additionally, the surface on which the projectile lands can also impact the distance.

## 5. How can I improve the accuracy of my catapult?

To improve the accuracy of your catapult, you can experiment with different angles and forces, adjust the placement of the lever, and make sure the arm is securely attached to the base. You can also try using different projectiles and keeping them consistent in weight and size. Additionally, practicing and making small adjustments can help improve the accuracy over time.

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