When the car is moving along the curve, it receives a force that tries to take it from the road. It's called centripetal force and the formula to compute it is:
The centripetal acceleration a_c is computed as
Where v is the tangent speed of the car and r is the radius of curvature. Replacing the formula into the first one
For the car to keep on the track, the friction must have the exact same value of the centripetal force and balance the forces. The friction force is computed as
The normal force N is equal to the weight of the car, thus
The mathematical relationship between force and extension for a spring is F = -kx , where F is the restoring force, k is the spring constant and x is the extension. If a box that weighs 40 N is hung from a spring of content 400 N/m, then the extension is equal to x = -F / k = -400 / 40 = -10 cm. The negative sign simply shows the extension and restoring force are in opposite directions. The answer is 10 cm.