Question

(a) Newton's first law of motion is also called as the law of inertia. Justify this statement.
(b) A plastic ball and a cricket ball are rolled on the floor with same velocity. Which one will cover larger distance before stopping ? Give reason.
(c) A truck is moving with a velocity of 72 km/h and it takes 3 seconds to stop after the
brakes are applied. Calculate the force exerted by the brakes. Mass of truck is 1200 kg.

CBSE Class IX Science LA (5 Marks)

Answer 1

(a) According to Newton's first law of motion, everybody tends to resist change of state whether it is in rest or motion. Qualitatively, this tendency of bodies to stay at rest or keep moving with same velocity is called inertia.

(b)    Cricket ball will cover larger distance before stopping. As its mass is larger, its inertia of motion is larger too.

(c)    u = 72 km/h = 20 m/s, v = 0

v = u + at

a = 0 - 20/ 3 

     = 20/3

   = -6.66 m/s²

Now, force                                                             

  F = ma

= 1200 x — 6.6 = — 8000 N (Retarding force)

 

Hence, the force exerted by the brakes is 8000 N.