the jamaican bobsled team hit the brakes on their sled so that it decelerates at a uniform rate of 0.43 m/s, how long does it take to stop if it travels 85 m before coming to rest
here's a cheap trick it would take the same time to accelerate from rest to top speed as it would take to decelerate from top speed to zero so instead of d = Vi t + 1/2 a t^2 where Vi is positive and a is negative we'll use Vi = 0 and a is positive giving 85 = 0 + 1/2 (0.43) t^2 = 0.215 t^2 t^2 = 395.345 t = 19.88s or 20. s to 2 sig figs
or we ccould find Vi from Vf*2 = Vi^2 + 2 a d 0 = Vi^2 + 2 (0.43) 85 Vi^2 = 71.4 Vi = 8.45m/s then 85 = 8.45 t + 1/2 (-0.43) t^2 85 = 8.45 t - 0.215 t^2 0.215 t^2 - 8.45 + 85 = 0 t = 19.65s or 20. s to 2 s.f.(minor difference arises from rounding Vi) or another cheap trick when a is constant Vavg = (Vf + Vi) /2 = 8.45/2 = 4.225 and d = Vavg t 85 = 4.225 t t = 20.12 or 20. s to 2 s.f. (minor differences from intermidiate roundings)
