# Can I use physics to solve this?

Ok, I just want to know if this is solvable by physics. If it can, it would also be helpful if you could supply an equation or something, but you don't have to. I just want to know if I can figure it out with physics. So here's the question:
Suppose there is a person that is 80 ft away. I have a sling shot and one water balloon. If I am supposed to hit the person with a water ballon from 80 ft away, how far would I have to pull back the sling shot and at what angle would the sling shot have to be tilted at.
Although it may sound silly, it is a serious question. I would really appreciate it if you could help me out. Thx.

There are too many unknown variables here. The only constant given in your description is the distance.

Well, lets say you know the variables. Is still it solvable?

Pengwuino
Gold Member
Actually there are no variables I can think of that will make this impossible. What you would need to find however, and I'm sure its certainly possible, is the equivalent spring constant of the sling shot and of course, the mass of the water balloon.

Thx a lot guys! I really appreciate your help.

pervect
Staff Emeritus
chanome said:
Ok, I just want to know if this is solvable by physics. If it can, it would also be helpful if you could supply an equation or something, but you don't have to. I just want to know if I can figure it out with physics. So here's the question:
Suppose there is a person that is 80 ft away. I have a sling shot and one water balloon. If I am supposed to hit the person with a water ballon from 80 ft away, how far would I have to pull back the sling shot and at what angle would the sling shot have to be tilted at.
Although it may sound silly, it is a serious question. I would really appreciate it if you could help me out. Thx.

It rather depends on what you mean by "solve it". You would have to calibrate the slingshot, there are several possible conceptual ways you could do this, one of them would be to get a spring scale and measure the force it took to pull the sling back a distance of 'x'.

You would also have to know the aerodynamic properties of the water baloon. This would probably be harder to model accurately, though you could certainly crudely approximate it as a sphere, and knowing the density of water you could figure out it's weight from it's volume (or vica-versa).

It is an open question as to how accurate your predictions would be. The drag on the water baloon probably depends on its exact angle if it has a baloon shape. The angle of attack of the water baloon probably varies in flight, meaning in practice that no two will land in the exact same spot.

Chronos
Gold Member
Like pervect and others said, you need to quantify the variables. It is obviously doable because certain missiles [e.g. cruise missiles] can be launched from relatively great distances and delivered with amazing accuracy. A good exercise would be to quantify the accuracy required to qualify as a 'hit', itemize all relevant variables, and quantify your technological capability to limit the margin of error for each variable. You then have a valid basis to deduce the statistical probability of a 'hit'. Theory is easy compared to application [an engineering thing].

40 degree angle

Pengwuino
Gold Member
But cruise missiles have radar-enabled computerized terrain following systems :P

Chronos
Gold Member
Of course, it's just a matter of some inconsequential engineering details to make that happen

But artillery can still hit a target up to 40kms away. They use computers to plot and aim the trajectory, but they can't control the round once its been fired.

The easiest thing to do would be to pull the slingshot back to a certain point, and then launch the ballon and note how far it went and the angle at which it was launched.

Using this information, it is a relatively simple physics problem to find the angle which would lead to striking your target 80ft away, if you pull the slongshot back to the same point.

Pengwuino
Gold Member
For a longer range... attach rocket.

Well, im starting to think this isnt going to be possible. The reason i want to know is because my physics teacher is letting us try and hit him from 80 ft away using a sling shot and a water ballon. And the thing is, i really want to hit him (he's really boring and gives really hard tests, so i mean he deserves it). The problem is, i only get two balloons to try with and the sling shot is supplied by my teacher. So i dont think i can do any of these tests. Is there any way just to assume a lot of it?

Pengwuino
Gold Member
No way to assume. The slingshot can be as ... springy as an atv's shocks or as springy as a slinky. If you don't know how much energy can be stored up in that sling shot, no chance of doing any calculations.

I advise you to 'feel' out the slingshot and on the first shot, try to fire it farther then you want it to go. That provides a better calibration then going short the first time in my opinion.

are you planning on making a mechanical device to hit someones house...
learn some projectile motion and then calculate the force and angle needed to make that projectil motion.

I guess this teacher has either done some calculations himself, or, more likely, he has just tried it and found it impossible to throw a balloon 80 ft. with his slingshot (I mean, it's unlikely he would allow to do that if he were not sure he wouldn't get hit...). Thus, the solution is to bring a water hose along, and then tell him you calculated that the task of making him wet cannot be solved with a water balloon and a slingshot :)

Dhl said:
I guess this teacher has either done some calculations himself, or, more likely, he has just tried it and found it impossible to throw a balloon 80 ft. with his slingshot (I mean, it's unlikely he would allow to do that if he were not sure he wouldn't get hit...). Thus, the solution is to bring a water hose along, and then tell him you calculated that the task of making him wet cannot be solved with a water balloon and a slingshot :)