This is the problem I was given: The force needed to keep a car from skidding on a curve varies jointly as the weight of the car and the square of the car's speed, and inversely as the radius of the curve. If a force of 3600 pounds is needed to keep an 1800 pound car traveling at 20mph from skidding on a curve of radius 600 feet what force would be required to keep the same car from skidding on a curve of radius 570 feet at 50mph? round to the nearest pound of force.
The equation comes out to be F=k[(ws^2)/r]
F=force, W=weight, R=radius, what does k stand for?

Question

This is the problem I was given: The force needed to keep a car from skidding on a curve varies jointly as the weight of the car and the square of the car's speed, and inversely as the radius of the curve. If a force of 3600 pounds is needed to keep an 1800 pound car traveling at 20mph from skidding on a curve of radius 600 feet what force would be required to keep the same car from skidding on a curve of radius 570 feet at 50mph? round to the nearest pound of force. 
The equation comes out to be F=k[(ws^2)/r]
F=force, W=weight, R=radius, what does k stand for?