Question

A car moving with a speed of 40 km/h can be stopped by applying brakes after atleast 2 m. If the same car is moving with a speed of 80 km/h, what is the minimum stopping distance

• A. 8 m
• B. 2 m
• C. 4 m
• D. 6 m

Answer 1

Answer: (B)

2 m

Answer 2

Let initial velocity of the bullet = u

After penetrating 3 cm its velocity becomes $\frac{u}{2}$

From $v^{2} = u^{2} - 2as$ $\left( \frac{u}{2} \right)^{2} = u^{2} - 2a(3)$

⇒ $6a = \frac{3u^{2}}{4}$ ⇒ $a = \frac{u^{2}}{8}$

Let further it will penetrate through distance x and stops at point C.

For distance BC, $v = 0,u = u/2,s = x,a = u^{2}/8$

From $v^{2} = u^{2} - 2as$⇒$0 = \left( \frac{u}{2} \right)^{2} - 2\left( \frac{u^{2}}{8} \right).x$ ⇒$x = 1$ cm.