# Force due to wind and rolling friction

1. Oct 19, 2007

### bulldog23

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 2450kg (weight = 5402lbs). Find the size of the force due to wind and rolling friction when the van speed is 49.27mph (1mph= 0.447m/s).

2. Relevant equations
delta v/delta t=a
F=ma

3. The attempt at a solution
I converted the mph to m/s, and I drew a line tangent to the point on the graph where the speed is 49.27 mph. I just can't seem to get the numbers right for some reason. Can someone help me out?

2. Oct 19, 2007

### PhanthomJay

When you drew the tangent, what did you conclude about the acceleration at that point?

3. Oct 19, 2007

### bulldog23

The acceleration is negative, right. So doesn't that make the force negative? The van reaches 49.27 mph at 11 sec. So what other point should I use to find the slope?

4. Oct 19, 2007

### PhanthomJay

Draw the tangent as best you can, acros the whole graph, and see what the slope is by choosing any 2 convenient points to determine the approximate acceleration ( change in v/change in t) between those points. Yes, the force will be negative.

5. Oct 19, 2007

### bulldog23

Then once I get the acceleration, I just multiply by 2450 kg?

6. Oct 19, 2007

### bulldog23

alright, I got it, thanks!

7. Oct 19, 2007

### PhanthomJay

Yes, that should do it. Your answer will be approximate because you're just scaling from the graph, unless you were given the exact equation for the graph.