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Motorway Speed Limit, Braking Forces and Wet Road Analysis

The legal speed limit on motorways is approximately 30m/s. In an incident on a motorway, a car of mass 900kg leaves a skid mark 75m long when stopping. The maximum deceleration of the car when skidding is approximately 10m/s2.
(i) Show that before the incidence, the car must have been travelling above the legal speed limit.
(ii) Calculate for this skid, the maximum average braking force between each of the four tyres and the road.
(iii) When the motorway is wet, the braking force provided by each wheel is reduced to 50% of the calculated in (ii) above. What is the effect of this reduced breaking force on stopping distance, explain your answer. Assume that the speed of the car before breaking is the same in both cases.

0
(0 Ratings)

(i) V2 = u2 + 2as
V2 = 0 + 2 x 10 x 75
V = 3.87m/s
(ii) F = ma
= 900 x 10
= 9000J (total for four tires)
Braking force for each tires = 9000J/4 =2250J
(iii) The breaking distance will increase because wet road offers less frictional force which is required for breaking.

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October 05, 2023

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