# HELP instantaneous force TO velocity

1. May 3, 2004

### ginnerpip

HELP!! instantaneous force TO velocity

I have big problem,

In a projectile motion problem, we are given a variables for force applied (instantaneous not continuious), mass of object, and angle above horizontial.

For my Matlab code (program like visual basic) i need to get the initial velocity, BUT HOW!!!

I know the initial acceleration of the object (F/m), but because the force is instantaneous, does this convert to velocity, or not??????

2. May 4, 2004

### Integral

Staff Emeritus
The initial velocity will be 0.

3. May 4, 2004

### HallsofIvy

Staff Emeritus
And just how do you know that?

I was once watching an AP math class on television in which they were doing a problem where an object was dropped from a certain height and the problem was to find when it hit the ground. At one point the teacher said "Since we are not told the initial speed we will take that to be 0".

I almost threw a brick through the t.v.!! You NEVER take a value to be 0 simply because you are not told what it was. In the t.v. problem it happened to be correct because the problem said "dropped" as opposed to thrown up or down.

It's very likely that the object in this problem is starting from rest but that had better be stated somewhere in the problem.

I am concerned about the force being "instantaneous not continuous". If, by that, you mean that the force is applied only for an infinitesmal time, then it will not affect the motion of the object at all.

4. May 4, 2004

### ZapperZ

Staff Emeritus
Whoa! We need to get you into an anger-management class! :)

I don't know about you two, but I find the original question rather vague and confusing. In a typical projectile motion problem, we never care about "instantaneous" force. There is only one force in a typical projectile motion problem - gravity! This is a constant throughout the motion. The only possible instantaneous force is the impulse given to the projectile at the beginning of its motion. This never comes into play as far as the parameters of the projectile problem is concerned. Only the initial velocity is typically the relevant parameter that is the result of such impulse.

So unless this is a different type of problem that is specifically including the impulse at the beginning of the motion, then I have no idea what "instantaneous" force we're dealing with here.

... just don't throw a brick at me. :)

Zz.

5. May 4, 2004

### ZapperZ

Staff Emeritus
I hate to argue about semantics here, especially since the originator of this question hasn't clarified the exact problem. However, I hate to think that we teach students at this level using such complicated and, frankly, confusing terminology. An "instantaneous" force implies, at least to me, something that operates only over a very short time scale. That is why I brought up the example of an Impulse, something that students at the high-school/intro level can understand and have come across.

However, to equate that with the situation F(t)=constant because at any "instant" t1, F(t1) acts "instantaneously" on the object is confusing, at least pedagogically. You and I know what this actually means. But does that justify conveying the muddle picture to someone at an intro level, for example? I can think of a whole other scenarios where one can argue that since time is continuous (let's not bring in discrete time and space in a classical physics problem), F(t) is also continuous, so that an "instantaneous" value is simply due to our measurement window, not the actual situation.

The issue here is that we can complicate this seemingly elementary problem ad nauseum. If this is a "typical" projectile motion problem, then I suggest we find the solution via the "typical" approach unless we're willing to spend time formulating and explaining our own methodology.

Zz.

6. May 4, 2004

### arildno

I agree. Message deleted.

7. May 8, 2004

### ginnerpip

Ummm, thanks for the help, but the specific question i was askin for was that i am needed to write a program in matlab to calculate the max height, range, velocivty of impact and GRAPH it (which i havent figured out yet). all with inputs of initial agnle, force, and mass, (we cant input the velocity)

So i was wondering how i can change that force and mass into a useable initial velocity that i can use in the formula, (help with the MATLAB code would also be much appreciated).

Thanks
Alex.

8. May 8, 2004

### Staff: Mentor

I think everyone is eager to help, but it's not clear what is meant by "force applied (instantaneous not continuous)"? (See ZapperZ's comments.) I presume that some impulse is imparted to the mass to give it an initial speed? If a force is given, but not the time of application, then you can't calculate the speed attained.

Can you state the problem exactly as given? (Unless that was exactly as given. )

9. May 8, 2004

### Gokul43201

Staff Emeritus
Yes, what is the exact wording of the problem ?

An 'instantaneous' force needs to be actually non-instantaneous to produce some initial velocity, i.e. there needs to be some short impulse time, unless the force is infinitely large.