Page 85 - Physics - XI

It is clear that horizontal range of the projectile is directly proportional to the square of its velocity.
So, the range of projectile increases with increase in its velocity and decreases with decrease in its
velocity.

Q12. Why has a big protractor been taken? Would a protractor of radius about 10 cm be preferable?
(NCERT)
Ans. A big protractor of a large radius gives an exact and accurate value of angle of projection, so
corresponding to this, the range of a jet of water is also accurate. While in the case of a protractor
with radius 10 cm, the value of angle of projection becomes very diffi cult to measure. So, the
protractor with radius 10 cm is usually not preferred.

Q13. Why do you get the same range for angles of projection 15° and 75°?
Ans. The horizontal range is given as

u sin
2
2
R
g
if we replace θ by 90° – θ,

290
2
then, R = u sin( g )
1
2
2
= u sin( 180 2 ) = u sin 2θ = R
g g

So, for complementary angles, the range will be the same.

ACTIVITY - 6
Aim
To study the conservation of energy of a ball rolling down on an inclined plane (using a double inclined
plane).

Apparatus and Materials Required Ball
F Ball released from A reaches A
Double inclined plane, a steel ball of diameter about the same height at F
about 2.5 cm, two wooden blocks, plumb line,
metre scale, spirit level, and two weights of one kg
each w 1 w 1

E w 2 w 2 B
Principle/Theory
The law of conservation of energy states that Wooden block O Wooden block
"Energy can neither be created nor destroyed, but Fig. 6.1: Set up for studying the conservation of energy
can be transferred from one form to another". of a ball using double inclined tracks
During the rolling of a steel ball on a perfectly smooth inclined plane, a continuous transformation between
kinetic energy and potential energy takes place.

Thus, the sum of kinetic energy and potential energy of the ball remains constant provided there is no
dissipation of energy due to air resistance, friction, etc.

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