On dry concrete, a car can decelerate at 6.0m/s2, whereas on wet concrete it can decelerate at only 5.0m/s2.

Question

(a) Find the distance necessary to stop a car moving at 24.0m/s24.0m/s on dry concrete.

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(b) Find the distance necessary to stop a car moving at 24.0m/s24.0m/s on wet concrete.

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(c) Repeat both calculations, finding the displacement from the point where the driver sees a traffic light turn red, taking into account his reaction time of 0.5s0.5s to apply the brakes.

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Answer 1

(a) 0² - 24² = -2·6s; s = 48 m

(b) 0² - 24² = - 2·5s; s = 57.6 m

(c) Δs = 24·0.5 = 12 m

s₁ = 12 + 48 = 60 m;

s₂ = 12 + ,57.6 = 69.6 m

Answer 2

Use distance for constant deceleration = v_o²/2a

^{a) d=24^2/12= 49 feet}

^{b) d=24^2/10 = 58 feet}

^{c) +24*.5+49=61 and 70 feet}

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