# Catapult Help Needed

1. Nov 10, 2005

### mcoy

I need to make a catapult in less than 2 weeks... it has to be able to throw a 5 cent coin (aust coin) on exactly 2 metres.... its not much but i cant figure out what design i'm gonna make, since i need to be able to do calculations beforehand and make the catapult using my calcs...
so now i need a good and easy-to-make design... i.e. making a catapult that doesnt need any gears... i was thinking of a counter-weight type but that would be too much for a 5 cent coin... i think...

somebody help me plz??

2. Nov 10, 2005

### Diane_

Doesn't that rather depend on the size of the counterweight, the weight of the arm, and the moment arms involved?

3. Nov 10, 2005

### Andrew Mason

Calculations will only give you an approximate result. I would suggest that you use an energy approach.

You have to determine the angle at which the coin leaves the catapult. Then determine how high the coin has to go. Calculate the change in energy of the catapult arm and counterweight between initial and final positions. Assume the energy loss of the arm is gained by the coin.

If you use a spring or elastic band, you have to measure the spring constant and measure the load distance to find the energy stored in the spring. You also have to take into account the kinetic energy of the arm after the coin is released.

AM

4. Nov 10, 2005

### daniel_i_l

Once I was really in to building catapults with counter-weights. they are called trebuchets. you can see a movie of one of the big ones (6-feet+) fireing at my site. I also made small ones out of lego and cardboard that shot for small distances (though if you plan it right I know from experience that you can make a 1-foot trebuchet that can shatter a marble on a wall from over 10-feet) a rough approximation for the distace that a trebuchet can shoot is:
$$Range = 2 * h * (M_{cw}/m_{p})$$ were h is the distance that the CW falls. You can start with that and then make small changes untill it works just right. You can learn a lot about building them on the web.

5. Nov 14, 2005

### mcoy

yeah but firing a coin doesnt need a trebuchet setup... i've done some calculations, but i just need the design, and to callibrate it... i dont have any idea how create and calibrate the initial velocity of the arm (the throwing bit) of the catapult...

6. Dec 3, 2005

### colombo

Pls help me Calculation on distance

I'm doing a Assignment is to conststruce a captapult that will launch a marshmallow(Large size) at target on the floor at the other end of the classroom.
I designed catapult. the rules saing the energy;; to launch the marshmallow cannot come directly from the motion of a human being.so i used elastic band .
but, they saing to calculate the distance.
so, i use an enegy approach. so I determine the angle at which the mashmallow leaves the catapult.then I assume then energy loss of the arm is gained by the marshmallow.but I ued elastic band, but, i dont know how to measure the load distance to find the energy stored in the elaastic band.how to i take into this measure ments...coz, I dont know the formula of the elasticband energy stored.
just i used this formula to find the velocity.
Mgh = 1/2*M*V^2

but I dont consider the elastice band energy
please any one could me help to solve this probelm
I need the full foumula how to fine the calculation...
pls sent me to desitamil@gmail.com
Thanks for look through my post.