# Drag of Car across frictionless surface.

1. Oct 3, 2008

### JosephLee

1. a 1400 kg car with a cross section of 1.8m wide and 1.46m high slides across a frictionless surface at 19 m/s.

The question is how long does it take (in seconds) to get to 13 m/s?

the 2nd part is how long it does it take to get to 10 m/s?

2. D = .5(A)v^2 where A is the area of cross section

and basic mechanics equation V = Vo + at

3. The real problem lies in the fact that drag changes over the change in velocity. I tried solving for it initially by plugging in different velocities for the drag equation but it doesn't come out right.

So far, there is a drag force going backwards and the momentum of the car going forwards. I'm so frustrated because i believe that im missing something very simple to solve this.

Last edited: Oct 3, 2008
2. Oct 3, 2008

### tiny-tim

Welcome to PF!

Hi Joseph! Welcome to PF!

Hint: use good ol' Newton's second law (force = rate of change of momentum) to make an equation involving t v and v/dt.