Question

Consider an automobile moving at v mph that skids d feet after its brakes lock. Calculate how far it would skid if it was moving at 2v and the brakes were locked.   

Answer 1

By conservation of energy:

$$\frac{mv^2}2=\mu mgd,\\\space\\ \frac{m(2v)^2}2=4\frac{mv^2}2=\mu mg(4d).$$

4 times further.