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Pendulum Problem: Potential energy equals? kinetic energy 
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#1
Oct1611, 09:36 PM

P: 36

1. The problem statement, all variables and given/known data
A pendulum consists of an object of mass m = 1.65 kg swinging on a massless string of length l. The object has a speed of 1.97 m/s when it passes through its lowest point. If the speed of the object is 0.87 m/s when the string is at 70° below the horizontal, what is the length of the string? Correct answer: 2.64 m (to 3 sig figs) 2. Relevant equations Ep = mgh Ek = 1/2mv^2 3. The attempt at a solution I tried the equation: mg(length(1cos20°)) = 1/2mv^2 and this did not work.. it worked when I had the length and needed the velocity? 1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution 


#2
Oct1711, 07:00 AM

HW Helper
P: 2,316

The reduction in kinetic energy will be accompanied by an equivalent increase in Potential energy  so you know the gain in height. A bit of trig on the triangle formed should yield the pendulum length you are after. 


#3
Oct1711, 12:41 PM

P: 36

Thank you so much!



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