# Pendulum Pair

https://isaacphysics.org/questions/pendulum_pair?board=d43a5a20-2f1c-4df2-a833-a457bf2cb206&stage=a_level

I've tried to just difference the times but I feel like im missing another equation to use or im doing smth wrong. Any help would be greatly appreciated.
Working:
Original post by mosaurlodon
Working:
You seem to be a fair bit off. As in the hints the time period for a pendulum is
2pisqrt(l/g)
and you want to think about how the period(s) relate to breaking the laser beam for the two pendulums. The actual perturbation is a bit of a red herring.

Also the hints suggest thinking about the shorter pendulum first, then the longer one and thats a good hint.
(edited 2 months ago)
ok I've considered the shorter pendulum first:

Though now im not sure what the time difference represents - is this value equal to 0.3s - how is the difference between the times determined?
Original post by mosaurlodon
ok I've considered the shorter pendulum first:

Though now im not sure what the time difference represents - is this value equal to 0.3s - how is the difference between the times determined?
The half period for the shorter pendulum is 0.3s (which I think youre saying) so the length is ...

Id use a more descriptive label(s) for time, so 1/4 period until it first cuts, then 3/4 period until it cuts again or by symmetry, 0.3 s must be a 1/2 period.

Then think about the second pendulum and how to minimise the (total) length.
(edited 2 months ago)
Ok I think I get what the question is asking?
So the time taken for the first pendulum to complete a half period is 0.3s so I can work out l1.
In this time the second pendulum completes a quarter period barely reaching the laser (which I think would give the lowest length) so I can work out l2.
Original post by mosaurlodon
Ok I think I get what the question is asking?
So the time taken for the first pendulum to complete a half period is 0.3s so I can work out l1.
In this time the second pendulum completes a quarter period barely reaching the laser (which I think would give the lowest length) so I can work out l2.
Sounds close but not quite right for the longer pendulum
Oh It just clicked! Thank you!
Original post by mosaurlodon
Oh It just clicked! Thank you!

I am still confused what is exact answer of total minimum length
Original post by mosaurlodon
Oh It just clicked! Thank you!