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Pendulum swing basic trig problems 
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#1
Aug907, 02:55 AM

P: 104

1. The problem statement, all variables and given/known data
lets say you have a pendulum swinging. the length of string is 1m it swings to the right so that the angle is 45deg. i want to find difference in height from when it is at rest to its new position at 45deg. How do i do this, trigonomically speaking i mean? i mean, where is the right triangle??? 2. Relevant equations not exactly a homework prob, but related to everything in homework 3. The attempt at a solution 1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution i think i forgot sum basic trig, cause i cant figure out where the right angle triangle is to get my lengths from/???? 


#2
Aug907, 03:04 AM

HW Helper
P: 4,124

The string of the pendulum is the hypoteneuse. Draw a vertical line through the top of the pendulum (like when the pendulum is at rest)... now draw a horizontal line from the swinging end of the pendulum to this vertical line. do you see the right triangle now?



#3
Aug907, 03:19 AM

P: 104

but if i do that, then vertical line is no longer 1m??



#4
Aug907, 03:19 AM

P: 104

Pendulum swing basic trig problems
because when it swings it is higher than it was at rest



#5
Aug907, 03:21 AM

P: 104

book says to go something like LLcos45 = L(1cos45)
i dont undetsand why they say that, because when i apply basic trig (cos45 = adj/hyp) i get a totally differnt number than book ?? 


#6
Aug907, 03:21 AM

HW Helper
P: 4,124




#7
Aug907, 03:22 AM

P: 104

so even though the right triangle cuts off the hypotenuse, i should still use hyp as 1m



#8
Aug907, 03:23 AM

HW Helper
P: 4,124




#9
Aug907, 03:26 AM

P: 104

pleae allow the dumb question
what is Lcos45 derive from??? i cant match it up to SOHCAHTOA? 


#10
Aug907, 03:28 AM

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P: 4,124




#11
Aug907, 03:29 AM

Sci Advisor
HW Helper
P: 2,884

Ithink that you are still drawing the position of the string when it is at zero degree and this creates confusion. It might be better to draw on a separate figure, next to the first one, the position of the string when the object is at the lowets position. It should be clear that the difference of height is L  L cos 45. 


#12
Aug907, 03:30 AM

HW Helper
P: 4,124

adj/hypoteneuse = cos 45 adj/L = cos 45 adj = Lcos 45 


#13
Aug907, 04:40 AM

P: 104

thanks you
understand now 


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