Question

The driver of a car travelling along a straight road with a speed of 90km/hr observes a sign board which gives the speed limit to be 54km/hr. The signboard is 70m ahead when the driver applies brakes. Calculate the acceleration of the car which will cause the car to pass the signboard at the stated speed limit.

Answer 1

Answer:

20/7 m's^-2

Explanation:

The initial speed is u = 90 km h^-1= 25 m s^-1

To have the stated speed limit, car's final speed must be,

v = 54 km h^-1= 15 m s^-1

To have the above final speed while crossing the sign board, he must do it within the given distance. Hence,

distance required to slow down, s = 70 m

Using the formula,

v^2 - u^2 = 2as

where a = acceleration

Putting the above values,

(25)^2 - (15)^2 = 2×70× a

Solving the equation

>>>>> a = -20/7

Here, the minus sign represents retardation, meaning the driver will have to slow down.