Page 109 - Understanding NCERT Science 09

–2
stopped with a negative acceleration of 1.7 m s ?
9. What is the momentum of an object of mass m, moving with
a velocity v?
(a) (mv) 2 (b) mv 2 (c) ½ mv 2 (d) mv
10. Using a horizontal force of 200 N, we intend to move a wooden
cabinet across a floor at a constant velocity. What is the
friction force that will be exerted on the cabinet?
11. According to the third law of motion when we push on an
object, the object pushes back on us with an equal and
opposite force. If the object is a massive truck parked along
the roadside, it will probably not move. A student justifies
this by answering that the two opposite and equal forces
cancel each other. Comment on this logic and explain why
the truck does not move.
12. A hockey ball of mass 200 g travelling at 10 m s –1 is struck
by a hockey stick so as to return it along its original path
–1
with a velocity at 5 m s . Calculate the magnitude of change
of momentum occurred in the motion of the hockey ball by
the force applied by the hockey stick.
13. A bullet of mass 10 g travelling horizontally with a velocity
–1
of 150 m s strikes a stationary wooden block and comes to
rest in 0.03 s. Calculate the distance of penetration of the
bullet into the block. Also calculate the magnitude of the
force exerted by the wooden block on the bullet.
14. An object of mass 1 kg travelling in a straight line with a
–1
velocity of 10 m s collides with, and sticks to, a stationary
wooden block of mass 5 kg. Then they both move off together
in the same straight line. Calculate the total momentum
just before the impact and just after the impact. Also,
calculate the velocity of the combined object.
15. An object of mass 100 kg is accelerated uniformly from a
–1
–1
velocity of 5 m s to 8 m s in 6 s. Calculate the initial and
final momentum of the object. Also, find the magnitude of
the force exerted on the object.
16. Akhtar, Kiran and Rahul were riding in a motorcar that
was moving with a high velocity on an expressway when an
insect hit the windshield and got stuck on the windscreen.
Akhtar and Kiran started pondering over the situation. Kiran
suggested that the insect suffered a greater change in
momentum as compared to the change in momentum of the
motorcar (because the change in the velocity of the insect
was much more than that of the motorcar). Akhtar said
that since the motorcar was moving with a larger velocity,
it exerted a larger force on the insect. And as a result the
insect died. Rahul while putting an entirely new explanation

98 SCIENCE

Rationalised 2023-24