Finding velocity as a function of time

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Homework Help Overview

The problem involves analyzing the motion of a ball thrown upwards with an initial velocity, considering the effects of air resistance. Participants are tasked with deriving a differential equation for the velocity as a function of time, starting from the forces acting on the ball.

Discussion Character

  • Exploratory, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss rearranging the equation ƩF = ma to isolate terms for integration. There are attempts to clarify how to manipulate the differential equation to separate variables, with some questioning the method of finding inverses and the implications of integrating velocity with respect to time.

Discussion Status

The discussion is ongoing, with participants offering various suggestions for manipulating the equation. Some guidance has been provided on how to approach the integration, though there remains uncertainty about the methods being discussed, particularly regarding the concept of inversing both sides of the equation.

Contextual Notes

Participants note that they have not encountered this type of problem in their previous coursework, which may be affecting their confidence in applying the necessary mathematical techniques.

Mmarzex
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Okay so the problem is that we have the case of a ball being thrown up at initial velocity v° with air resistance expressed as F = -kv where k is a constant acting on it. We are suppose to find a differential equation for the velocity at a given point as a function of time. So I started with
ƩF = ma
- mg - kv = ma

Then I moved everything around to get
dv/dt = (-mg - kv)/m

Now we have never done a solution like this in my AP Physics class so I am rather at a lose. I know that I need to get it so that dv and v are on the same side but I'm not really sure how to go about that so that I can integrate the whole equation to get the solution.
 
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Try inversing both sides
 
Bread18 said:
Try inversing both sides

How would I find the inverse of dv/dt we have never done something like that before.
 
I know that I need to get it so that dv and v are on the same side
Take the kv term over to the other side. Multiply both sides by dt. Integrate.
You will need to remember that the integral of v*dt is distance. It works out nicely this way. I don't understand the inverse method.
 
You inverse both sides to get dt/dv = m/(-mg - kv) and then multiply both sides by dv
 

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