Suppose a pendulum bob, suspended from a 1.2 m rope with angle to vertical=35 de

Question

Suppose a pendulum bob, suspended from a 1.2 m rope with angle to vertical=35 degrees, has a mass of 0.23 kg and is moving to the right at point B with a speed of 2.4 m/s}. Air resistance is negligible.

A.What is the change in the system's gravitational potential energy when the bob reaches point A?
B.What is the speed of the bob at point A?
c.If the mass of the bob is increased, does your answer to part (a) increase, decrease, or stay the same?
e.If the mass of the bob is increased, does your answer to part (b) increase, decrease, or stay the same?

Explanation / Answer

"Please, I need help understanding what they're even asking for."?? They told you exactly. Sigh, I never say this, but part (a) is obvious if you just draw a picture, right? Can you explain the particular problem you are running into? If I understand what you are thinking, then I can advise you. The second part of the equation follows from trig identities, but the left side (with the cosine in it) follows from direct geometry. part (b) find the velocity when the bob reaches the bottom of the path, hint: use conservation of energy. Edit: Draw a picture with the bob hanging vertically downward, then on the same picture draw the bob angled towards the left by a little (call the angle theta), in this position it is higher. After you draw the picture, you want to the vertical distance between the two heights. Trigonometry may be used. On the condescension, I personally do not think the word 'obvious' makes my entire answer condescending, neither does the sigh, and even the double question mark does not in my opinion. I think you were being too sensitive. I was sighing because I did not get how to explain it to you when all you said was that you did not understand what they were looking for and the question text was very specific. People often say in physics something is "immediately obvious" when drawing a picture or writing out an equation, etc. It is part of the language. I was lol-ing at saying it because I think it is funny when authors say that in textbooks for things that are not obvious. I think this is a culture clash here, I meant no condescension, I was moreso making fun of people that use that word. Truth be told though, it is "evident" if you draw a picture, please try your best and come back after you have thought about it more. The picture you get shows the height you need to find, from that you can figure out possibly how to find that height. Many problems in physics are facilitated by drawing pictures, which unfortunately many students have a tendency not to draw. Anyway, we can argue back and forth about whether or not I was being condescending, but the bottom line is that I told you I am not meaning to be. So, we can do that, or we can talk about physics. Sorry about how I came off, but how about we talk about physics? I can help you figure it out, but I am just not going to do it for you.

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