Question

A boy is swinging in the standing position. how will the time period of the swing be affected if the boy sits down on the swing ?
a. increase
b. decrease
c. remain same
d. 1st decrease then increase

Answer 1

@Mashy

Answer 2

@Rina.r @RANE @ganeshie8 @GTXMUQSIT

Answer 3

do u know the distance and time graph ?

Answer 4

yeah but I think its related to law of conservation of momentum
because when the boy is sitting on the swing his moment of inertia ( l= mr^2) is increasing due to increase in distance r. According to the law of conservation of angular momentum IW=constant... w = 2pi/T .. w will decrease and therefore, T will increase. .....
IS my explanation correct?

Answer 5

Clearly application of conservation of angular momentum.

Answer 6

So its A ?

Answer 7

yes

Answer 8

ok thanks and I just wanna confirm :)

Answer 9

Yes you are right. Good work :)

Answer 10

thanks for medal ^_^ @AravindG

Answer 11

You deserve it! The question is a good one and tests your thinking ability+knowledge of concept. A similar qn is the one where a simple pendulum has water filled in its bob with a small hole on bottom. We are asked what will happen to the time period. Movement of center of mass is the key concept.

Answer 12

@sarah786

"yeah but I think its related to law of conservation of momentum
because when the boy is sitting on the swing his moment of inertia ( l= mr^2) is increasing due to increase in distance r. According to the law of conservation of angular momentum IW=constant... w = 2pi/T .. w will decrease and therefore, T will increase. .....
IS my explanation correct?"


i am so sorry to tell you.. but you are horribly wrong xD.. you cannot use conservation of angular momentum here..

cause angular momentum is only conserved when next external TORQUE is zero.. clearly gravity puts a torque on the system.. and therefore the angular momentum is continuously changing :P

But you are quite right when u are talking about moment of inertia.. do you know the formula for time period of a physical pendulum?

Answer 13

oh :O :P
Yeah T = 2pi under root l/g

Answer 14

no that is only for a pendulum with massless string and all mass in the bob

but if you have a physical pendulum.. or something.. have you done that? yet?

(even if you have not, u can easily answer this without the formula)

Answer 15

no :(

Answer 16

just think about the centre of mass.. if the centre of mass is near the point of pivot .. then u can think the length of pendulum is very small
if the centre of mass is lower.. then the length of pendulum is very high

so if the boy .. (imagine he is a giant :D and MASSSIVEEEEEE).. sits on the swing instead of standing, what happens to the centre of mass? does it become lower or higher?

Answer 17

high :/

Answer 18

when he sits.. u think the centre of mass of the system becomes HIgher? :O (near the pivot?)

Answer 19

I dunno what I'm doing :/

Answer 20

Thanks for pointing out @Mashy

But in a happy note the answer still remains the same. The correct method is center of mass concept+pendulum. L varies with center of mass.

Answer 21

yes.. but always the concept is important :) :)..

@sarah786
its not so hard.. m sure u understand this..
imagine the centre of mass of that boy is at his belly
so when he is standing, his belly is near the hinge the pivot
and when he sits, his belly lowers right?.. so the CM lowers right?