# Projectile motion experimental vs theoretical results

1. Jun 20, 2011

### stupif

in projectile motion, why the experimental results are different with theoretical result?
thank you

2. Jun 20, 2011

### chiro

Things like wind resistance effect real results.

You typically have to study the medium that objects are embedded in. The theoretical models are based on the assumption that the only force acting on the object is gravity, and based on this, the projectile motion equations are derived.

Usually the next step up is to consider something like drag or a co-effecient for wind resistance.

Just as a note for future learning, its a good idea to look at the assumptions used. They might be too few (like introductory projectile motion), or too many (like a model that takes into account fluid dynamics like navier stokes). Sometimes you need to be precise and the "too many" model is sometimes "not enough".

If you wanted to do the most accurate model you would identify all the types of forces and simulate them at a desired level. The math is going to be more complex, but its usually going to be more reflective of what happens.

3. Jun 20, 2011

### stupif

but i use steel ball as projectile.....the wind will affect the result??or other factors except wind....

4. Jun 20, 2011

### chiro

I'm not a physicist, so the short is answer I'm not sure. Can you describe your experiment? Since you mention your using a steel ball, you probably didn't throw it (correct me otherwise).

This is basically what happens in science. You do experiments and find that simplified models don't work, so you either accept their limited accuracy or you add to your model.

Asides from obvious things like wind and drag, I can't off the top of my head think of simple attributes that could be added to your model, without resorting to a first principles approach.

5. Jun 20, 2011

### silmaril89

The wind will definitely affect the result.

6. Jun 20, 2011

### phinds

You really need to quantify things in your questions. If you throw a steel ball of 1" radius straight up in the air as hard as you can with no wind, it is going to go straight up and come straight down. If the wind is 100mp the ball is NOT going to go straight up and straight down. A more intelligent question would be, if I throw the ball of radius x, with velocity y, and there is a wind of velocity v perpendicular to the flight path, would the wind affect the ball's landing point by more than z inches?

7. Jun 29, 2011

### Fun Value

It depends on what your theory is. If you have a set up in which the launcher and a penny are at practically the same location, and you theorize the penny will strike the floor first, the experimental result may not support your hypothesis.

8. Jun 29, 2011

### ZapperZ

Staff Emeritus
This is VERY vague. What "results" exactly are different? And how much do they differ? If your experimental accuracy is +/- 0.01 and the result differ by 0.001, do you consider this as different? You need to put in SOME effort in present as clear and as complete of a description as possible.

And if this is part of a lab/schoolwork/etc., it should have been posted in the HW/Coursework forum.

Zz.