Due in an HOUR Early Physics problem relating drag, weight, and speed.

[jwoods1101]

Due in an HOUR! Early Physics problem.. relating drag, weight, and speed.

Homework Statement

A 1.74 m wide, 1.48 m high, 1500 kg car hits a very slick patch of ice while going 21.0 m/s. Air resistance is not negligible. If friction is neglected, how long will it take until the car's speed drops to 18.0 m/s?

Homework Equations

I think this one would be useful:
final velocity = initial velocity + acceleration x change in time

The Attempt at a Solution

My attempts are WAAYY off. I have two pages of work done and an hour of my time and nothing to show for it. I tried to calculate drag as part of the acceleration = net force / mass equation, but I don't know if that was right. Please hurry! It was the last question on my online set of homework due in an hour.

 

[flyingpig]

air resistance is NOT negligible?

 

[jwoods1101]

yes. that's correct. which pretty much means calculate the drag I assume.

 

[jwoods1101]

bump. anyone??

 

[Fredrik]

You should have asked three days ago. Now it's too late, since we're not allowed to give you the complete solution. Since you didn't include any formulas that can be used to calculate the drag, I'm going to have to assume that you haven't been taught any. (I don't recall any such formulas myself, but it's been a long time since I had to solve this type of problems). Without any such formulas, the numbers you were given can't be relevant, and all you can do is to realize that the drag force depends on the speed and try to figure out how. If you can't find an exact answer, how about an approximate one? Try to write down an approximate formula for the drag force at velocity v. It will obviously contain some constant whose value you can't know. Then use Newton's 2nd.

Also, a tip for next time. You need to show some of your work. If you have no idea where to begin, it's OK to just say so, but then we will only tell you where to begin.

final velocity = initial velocity + acceleration x change in time

This only works when the acceleration is constant. In this case it's not. The drag force obviously depends on the speed, since speed 0 implies force 0.