HELP instantaneous force TO velocity

In summary, in a projectile motion problem, the initial velocity can be determined by using the initial acceleration (F/m) and taking into account the force applied, mass of the object, and angle above the horizontal. However, if the force is instantaneous, it will not affect the motion of the object. In order to solve the problem using MATLAB, the initial velocity must be calculated from the given inputs of force, mass, and angle.
  • #1
ginnerpip
4
0
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?

Please help!
 
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  • #2
The initial velocity will be 0.
 
  • #3
Integral said:
The initial velocity will be 0.

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
HallsofIvy said:
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.

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
arildno said:
All forces are "instantaneous" in their effect on an object, in that the acceleration of the object at a given instant is completely specified by the net force acting upon the object at that particular instant.
In particular, a force that acted on an object at an earlier (or later!) instant has no bearing/effect upon the instantenaous acceleration of an object.
When we say that a force is constant through time (as in gravity), it means that at every single instant, the instantaneous force acting upon the object is equal in magnitude and direction as every other instantaneous forces (acting upon the object at other instants).

If a single instantaneous force is to produce momentum change, i.e effecting, on its own, a non-zero impulse on an object,
(rather than that the (non-zero) impulse is the cumulative effect of the set of instantaneous forces acting on the object during a non-zero time interval),
then that force must be of infinite magnitude.

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
I agree. Message deleted.
 
  • #7
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 haven't figured out yet). all with inputs of initial agnle, force, and mass, (we can't 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
ginnerpip said:
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 haven't figured out yet). all with inputs of initial agnle, force, and mass, (we can't input the velocity)
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. :frown: )
 
  • #9
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.
 

What is meant by "HELP instantaneous force TO velocity"?

"HELP instantaneous force TO velocity" refers to the relationship between instantaneous force and velocity in physics. Instantaneous force is the force acting on an object at a particular moment in time, while velocity is the rate of change of an object's position with respect to time. This relationship helps us understand how an object's velocity changes when a force is applied to it.

What is the formula for calculating instantaneous force?

The formula for calculating instantaneous force is F = ma, where F is force, m is mass, and a is acceleration. This formula is known as Newton's second law of motion and describes how the net force acting on an object affects its acceleration.

How does instantaneous force affect an object's velocity?

Instantaneous force can change an object's velocity by causing it to accelerate. If the force applied is in the same direction as the object's motion, it will increase the velocity. If the force is in the opposite direction, it will decrease the velocity. The greater the force, the greater the change in velocity.

Can instantaneous force affect an object's direction of motion?

Yes, instantaneous force can change an object's direction of motion. This is known as centripetal force, which is a force that acts towards the center of rotation and causes an object to move in a circular path. For example, when a car goes around a curve, the centripetal force acts towards the center of the curve, causing the car to change its direction of motion.

Is there a difference between instantaneous force and average force?

Yes, there is a difference between instantaneous force and average force. Instantaneous force is the force acting on an object at a specific moment in time, while average force is the average of all the instantaneous forces acting on an object over a period of time. Instantaneous force is important for understanding how a force affects an object's motion at a specific point, while average force is useful for analyzing the overall effect of a force over a longer period of time.

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