First slide

Instantaneous acceleration

Question

An automobile travelling with a speed of 60 km/h, can brake to stop within a distance of 2o m. If the car is going twice as fast, i.e., .120 km/h, the stopping distance will be

Moderate

A

20 m
B

40 m
C

60 m
D

80 m

Solution

Here, u -60 km/h = (60 x 5)/18 = 50/3 m/s
From equation, v² = u² -2as
we have $0 = {(\frac{50}{3})}^{2} â 2 a Ã 20$
or $a = (\frac{125}{18}) m / s^{2}$
when $u^{â²} = 2 u = 2 Ã (50 / 3) = (100 / 3) m / s$
then, $0 = {(\frac{100}{3})}^{2} â 2 Ã (\frac{125}{18}) Ã s$
or s=80m

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