A police officer measured a skid mark to be 150 ft long. The driver said he was going under the speed limit, which was 45 miles per hour. Was the driver correct? How long would the skid mark be if he was indeed going 45 miles per hour when the brakes were applied?

Question

dumbcow · Answer

what is the deceleration force applied by the brakes?

anonymous · Answer

That is all it says

dumbcow · Answer

If d is length of skid mark going 45mph or 66 ft/sec
d = 66^2/2k
where k is acceleration of the brakes

anonymous · Answer

ty

dumbcow · Answer

found a formula that might be useful
$$v = \sqrt{30(0.7)d}$$
where 0.7 refers to drag coefficient for asphalt and assuming ideal braking efficiency 
similarly then
$$d = \frac{v^{2}}{(30)(0.7)}$$
using this for v=45
$$d = 96.4 ft$$
driver was obviously not going 45 mph
for d=150
$$v = 56.1 mph$$