# Force due to wind and rolling friction

1. Feb 27, 2007

### fruitl00p

1. The problem statement, all variables and given/known data
The graph shows the speed as a function of time for a minivan, coasting on neutral along a straight, level road, loaded with windsurfing equipment and towing a boat. The total mass is 2500 kg, weight= 5512lbs. Find the size of the force due to wind and rolling friction when the van speed is 44.17 mph. (1 mph=0.447m/s)

2. Relevant equations
F=ma

3. The attempt at a solution
I don't think anyone needs to see the graph to understand my problem. Well I hope.

I understand that a= delta v/delta t. But whenever I use the equation I get an acceleration that does not work. Then I thought that since the equation wants the force at 44.17 mph, that I need to find the instantaneous acceleration.

That's my problem: I don't know how to get the instantaneous acceleration. At 44.17 mph the time is at 19 s. Should I create a derivative. If so, how do I go about it?

The answer for this problem is 764 N

Please tell me why my approach is incorrect.

2. Feb 27, 2007

### da_willem

That's right, calculate the instantaneous acceleration by finding the derivative of the graph at the given time. You must be able to find a way to do that (draw a tangtent).

Good luck!

3. Feb 27, 2007

### fruitl00p

da_willem, that is my problem. I do not know how to find the derivative of the graph. If I am given a derivative, I can do it; but to look at a graph and create one I just don't know.

4. Mar 2, 2007

### da_willem

In the graph you draw a line tangential to a point where you want to know the derivative (a 'tangent line'). The derivative in that point is now the 'slope' of this line, i.e. a vertical interval ([itex] \Delta v[/tex]) divided by a corresponding horizontal interval ([itex] \Delta t[/tex]): ([itex] \Delta v /\Delta t[/tex]).

5. Mar 6, 2007

### fruitl00p

I got it now. Thank you da_willem