Question

(a) Newton's first law motion is also called law of inertia. Justify this statement with the help of an example.
(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 s to stop after the brakes are applied. Calculate the force exerted by brakes. Mass of truck is 1200 kg.

CBSE Class IX Science LA (5 Marks)

Answer 1

(a)  Inertia is a tendency of the object to resist change in its state. Newton's first law of motion also states similar i.e., the object will remain in its present state unless an external force is applied. That's why Newton's first law is called Law of inertia.

(b) A cricket ball as it will have more inertia due to its more mass as compared to the plastic ball.

(c)                                 F = m x a

                                       =  1200 x 20 / 3
                                      
                                        = 400 x 20

                                        = 8000 N.
                                          

Answer 2

1. according to Newton's 1st law of motion a body at rest will remain at rest,motion in uniform motion in straight line will move in straight line unless a force is applied and inertia also means inability of body to come out of its rest position so it is also called inertia of motion.
2. a cricket ball will cover more distance as it has more inertia
3. u=72km/h = 72×5/18 = 20 m/s v = 0 ,t=3s ,masss=1200kg

a = 20/3 f = m×a

= 1200× 20/3 = 8000N