enter image description here

I just don’t understand part (a). Why is the length vertically downwards, not 0.5 as that’s the length of the string in the pendulum. Why is it the other length instead that’s 0.5? I’m clearly being dumb but I can’t see why they would be different as the string doesn’t change length. Any replies would be very appreciated.

  • 1
    $\begingroup$ It's a triangle with .5m being the length of the longest side. $\endgroup$ Commented May 6, 2019 at 20:19
  • $\begingroup$ Hi @Mark, welcome to SE! Posting screenshots is discouraged -- if you write out the problem it will be easier for people to read! See math.meta.stackexchange.com/questions/5020/… for some hints about equation formatting. The screenshot is unfortunately probably why people downvoted, since you have a valid question. $\endgroup$
    – Will
    Commented May 7, 2019 at 0:35

1 Answer 1


They aren't looking at the length vertically downwards.

They are determining the change in potential energy compared to when the pendulum bob is being held straight down. When the bob is fully downwards, the pendulum arm has a length of $0.50 \ m$. The change in potential energy when that arm is lifted only depends on the change in height of the mass.

The change in height of the mass would only be equal to $0.50 \ m$ if $\theta$ was $90°$. You can see from the diagram where they labelled $\Delta h$, and the new height $0.50 \cos 30°$; which means the change in height is obviously $\Delta h = 0.50 - 0.50 \cos 30° $, just like they've done here.

  • $\begingroup$ I don’t get why it says I shouldn’t thank you guys but cheers regardless. $\endgroup$
    – Mark
    Commented May 6, 2019 at 21:08

Not the answer you're looking for? Browse other questions tagged or ask your own question.