# Homework Help: How can i make a graph of velocity vs time taking into account air resistance?

1. Nov 7, 2009

### nnis13

How can i sketch a graph of velocity vs time for an object falling from rest taking into account air resistance.

Will it be a curve ?

How can i do it?

2. Nov 7, 2009

### Andrew Mason

At low speeds the air resistance is a linear function of speed and at higher speeds it is proportional to the square of the speed. Eventually the resistance builds up so that it is equal to the body's weight. What happens to the speed then?

AM

3. Nov 7, 2009

### nnis13

I am new to physics so i don't understand very much so can you please explain in a bit more detail or provide a link that shows a graph of this type please ???

4. Nov 7, 2009

### Andrew Mason

To being, draw the velocity time graph of a body falling due to gravity without resistance. That will be a line v = at where a = g to begin. As velocity increases, resistance increases so a gets smaller. So what happens to the graph? Does the graph get steeper or flatter? At some point, the resitance is equal to the force of gravity. What happens to the graph at that point?

AM

5. Nov 7, 2009

### nnis13

At a point where resitance is equal to the force of gravity i think that it will be 0 acceleration but constant speed but how can i draw this ??

Will it look like this > http://www.juniata.edu/faculty/roth/QM/Images/incConcUpLabel.gif

or like this http://www.juniata.edu/faculty/roth/QM/Images/decConcDownLabel.gif

By the way thanks a lot for answering

6. Nov 7, 2009

### Andrew Mason

Let's say at t=15 seconds the acceleration is 0 and its speed,v, is 60 m/s. Plot that point on the graph. Now at t=16 seconds, what is v? At t=17, 18, 19, 20...? Connect those points. What does the graph look like?

AM

7. Nov 7, 2009

### nnis13

from 1-15 it will be a curve abd then from 16s until it hits the ground will be a straight line parallel to the x-axis ???

i am not sure about that but thats how i understood it

8. Nov 8, 2009

### nnis13

So is that right ?

That's what will look like ??

9. Nov 8, 2009

### ideasrule

Yes, that's right. However, note that in theory, acceleration is never zero; the speed of an object approaches a certain speed (called terminal velocity) as the force of air resistance becomes closer and closer to the force of gravity, but never quite reaches it. So draw your graph as a smooth curve; don't include any jumps or pointy vertices.

10. Nov 8, 2009

### nnis13

Ok thanks for answering but i have one more question
Wil my curve be above the x-axis or below like this http://www.juniata.edu/faculty/roth/QM/Images/decConcDownLabel.gif

11. Nov 8, 2009

### ideasrule

I suppose you mean the t-axis, since this is a v vs. t graph. Think about it. Should velocity change faster as time passes, or should it change slower?

12. Nov 8, 2009

### nnis13

Yes the t-axis velocity changes slower i think