加了一题新的physic问题，在第7楼。请帮帮忙。。。

jennykuwn

A ladder that is 3.0m long and weighs 200N has its center of gravity 120cm from the foot. At its top end is a 50N weight. Compute the work required to raise the ladder from a horizontal position on the ground to a vertical position.

ANS：390J

请大家帮帮忙，问卷的最后一题了。

数学系

First, we have to understand that what is work done and potential energy. I assume that you have understood what is the notion.

Next, the weights of the ladder are divided to two parts: 200N (center of gravity) and 50N (top end). To raise the weight 200N to a position of 1.2m, the work done is W_1 = Fs = 200(1.2) = 240N.

Similarly, to raise the weight 50N to a position of 3m, the work done is W_2 = 50(3) = 150 N.

Thus, W_1 + W_2 = 390N

数学系

Sorry, typo: should be J.

jennykuwn

噢，我明白了。谢谢

Lov瑜瑜4ever

First, we have to understand that what is work done and potential energy. I assume that you have und ...
数学系 发表于 1-8-2010 01:13 AM

   
奇怪。。。
可以这样做的吗？？？
分开2个来做？

数学系

To #5

You can assume that the ladder is consisting of two "point mass", according to this question. Making insightful assumptions is the essential of physics.

jennykuwn

A cylindrical tank 10m high has an internal diameter of 4m.How much work would be required to fill the tank with water if the water were pumped in
a) at the bottom.    Ans: 6.15MJ
b) over the tip edge.   Ans: 12.30MJ

数学系

Before writing up my solution, I would like to know how are you analyzing this problem.

jennykuwn

With my opinion,
I should calculate the volume first...then volume of water = mass of water
so volume of water= pi x 2^2 x 10=126m^3
work done =mgh=126 x 9.81x 10 =12360.6

where is mistake?

数学系

Well, you are nearly correct, but you have committed the following errors:

1. First, your assumption that the volume of the water is equivalent to mass of the water is correct, but the correct equivalent is 1cm^3 = 1g.

2. Second, it is correct to assume that the water in the water tank is a point mass. But, if you assume that the water is a point mass, the center of gravity should be a the center of the water, that is 5m above the horizontal surface. So, you should use 5m, instead of 10m.

Lov瑜瑜4ever

To #5

You can assume that the ladder is consisting of two "point mass", according to this questio ...
数学系 发表于 1-8-2010 10:47 AM

你用200Nx1.2m+50Nx3.0m
那不是moment of force的做法？

jennykuwn

回复 10# 数学系


谢谢你的解释。but wat is the different between the work done when the water pumped in at the bottom and over the top edge?
Their height oso count as 5 m?

数学系

To #11

Moment of force? Okay, let me know how are you analyzing the problem. I guess you have misunderstood my solution without properly understanding the question or my solution.

Lov瑜瑜4ever

To #11

Moment of force? Okay, let me know how are you analyzing the problem. I guess you have mis ...
数学系 发表于 1-8-2010 11:23 AM

那题我完全做不到。。。><

数学系

回复 12# jennykuwn

Hehe, if you are pumping the water from the bottom, you are against the gravity force, so work must be done.

If you are pumping the water from the tip, you are making your potential energy less, and it is raising from the bottom, so you have to do work. :p

数学系

回复 14# Lov瑜瑜4ever


Okay, two tips for you:
1. understand what is potential energy.


2. how to calculate work done for the vertical motion.

Lov瑜瑜4ever

A cylindrical tank 10m high has an internal diameter of 4m.How much work would be required to fill the tank with water if the water were pumped in
a) at the bottom.    Ans: 6.15MJ
这个啊
我的做法是W=PV, P=pressure
P=densityxgh
  =1000kg m^-3 x 9.81ms^-2 x 10m
  =98100kg m^-1 s^-2

V=2m^2 x 10m x pi
  =40pi m^3

W=98100kg m^-1 s^-2 x 40pi m^3
   =12327609.57 J
   =12.33MJ
这样哪里错？

jennykuwn

我还是不明白，y water pumped from downside the height will become 5m...
Pumped from top edge the height will remain as 10m...if u set the centre of gravity at 5m, then the height of water when pumped from upside should be 5 oso mah....><

数学系

回复 18# jennykuwn

Because you are assuming that the water in the water tank is a point mass...

你必须明白，这个问题必须简化；你不可能去考虑每个水的质量点在这个水桶里的每个位置所作的功，你所拥有的数学工具不能让你做这样的分析。如果你已经假定所有的水的质量是个集中点，那在这个问题中也就那么设定（就是所有质量集中在中心的那一点）。

数学系

(Continued)

You can see that the work done in part b) is double of part a), so the author of this question has done the same analysis as me.