Rotational MotionEasy
Question
The torque
on a body about a given point is found to be equal to
, where
is a constant vector and
is the angular momentum of the body about the point. From this it follows that
on a body about a given point is found to be equal to
, where
is a constant vector and
is the angular momentum of the body about the point. From this it follows thatOptions
A.
is perpendicular to 
is perpendicular to 
B.the component of
in the direction of
does not change with time
in the direction of
does not change with timeC.the magnitude of
does not change with time
does not change with timeD.
does not change change with time
does not change change with timeSolution
(a)
i.e., 
This relation implies that
is perpendicular to both
and
.
Therefore, option (a) is correct.
(c) Here,
Differentiating w.r.t time, we get
= 2L
⇒
.
= 2L
But since,
⏊
∴
.
= 0
Therefore, from eq (i)
= 0
or magnitude of
i.e., L does not change with time.
(b) So far we are confirm about two points
(1)
or
⏊
and
(2)
= L is not changeing with time, therefore, it is a case when direction og
is changing but its magnitude is constant and
is perpendicular to
at all points. Theis can be written as
If
Here, a = positive constan
i.e., 
This relation implies that
is perpendicular to both
and
.Therefore, option (a) is correct.
(c) Here,

Differentiating w.r.t time, we get
= 2L
⇒
.
= 2L
But since,
⏊
∴
.
= 0 Therefore, from eq (i)
= 0or magnitude of
i.e., L does not change with time.(b) So far we are confirm about two points
(1)
or
⏊
and(2)
= L is not changeing with time, therefore, it is a case when direction og
is changing but its magnitude is constant and
is perpendicular to
at all points. Theis can be written asIf

Here, a = positive constan
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