Physics问题..(Lower 6)

lonely_world

请问下..如果题目叫showthat the...........是要怎样回答的?
比如说这题..
Bernoulli equation which relates the pressure p with the velocity of flow v of the liquid is
                           p + hρg + ½ρv² = k
where h is height, ρ is density, gis acceleration due to gravity and k is a constant. Show that theequation is dimensionally consistent and give the SI unit for k.


The drag coefficient CD of a car travelling with a speed v through air of density ρ is given by
CD = F/(½ρv²A)
where F is the drag force and A is the maximum cross sectional area of the car perpendicular to the direction of motion.
(a)Show the CD is dimensionless.
(b)When a certain car is travelling at 15 ms^-1 it experiences the drag force of 300 N. What is the drag force when the car travels at 45 ms^-1?

求求帮帮忙..开学就要交功课了..

[ 本帖最后由 lonely_world 于 13-6-2009 08:52 PM 编辑 ]

walterchoo

bernoulli那题， 你找出pressure的dimension先。然后找HPG 和1/2pv^2。 然后compare， 看一样没有。
k 是pressure的unit



a）Cd 是force的dimension。
b）f direct proportional to v^2, then F1/F2=V1^2/V2^2

lonely_world

原帖由 walterchoo 于 13-6-2009 06:44 PM 发表
bernoulli那题， 你找出pressure的dimension先。然后找HPG 和1/2pv^2。 然后compare， 看一样没有。
k 是pressure的unit



a）Cd 是force的dimension。
b）f direct proportional to v^2, then F1/F2=V1^2/V ...

那个dimension我知道怎样做..
我不懂要怎样show (a)..
(b)的我会了..
谢谢!

walterchoo

另外一个dimension做出来, 答案是1/1所以是dimensionless

lonely_world

原帖由 walterchoo 于 13-6-2009 07:07 PM 发表
另外一个dimension做出来, 答案是1/1所以是dimensionless

不是很明白你说什么..
可以做来看看吗?

爱妮丝

原帖由 lonely_world 于 13-6-2009 03:03 PM 发表
he drag coefficient CD of a car travelling with a speed v through air of density ρ is given by
CD = F/(½ρv²A)
where F is the drag force and A is the maximum cross sectional area of the car perpendicular to the direction of motion.
(a)Show the CD is dimensionless.

CD = F/(½ρv²A)
[CD] = [F/(½ρv²A)]
       =         MLT^-2________
          (ML^-3)(L^2T^-2)(L^2)
       = MLT^-2
          MLT^-2
       =1
=>CD IS DIMENSIONLESS

很久没做了，有错别见怪。。





[ 本帖最后由 爱妮丝 于 13-6-2009 10:52 PM 编辑 ]

lonely_world

原帖由 爱妮丝 于 13-6-2009 10:48 PM 发表


CD = F/(½ρv²A)
[CD] = [F/(½ρv²A)]
       =         MLT^-2________
          (ML^-3)(L^2T^-2)(L^2)
       = MLT^-2
          MLT^-2
       =1
=>CD IS DIMENSIONLESS

很 ...

谢谢你哦!
我做到了..
你做对了..

lonely_world

请问这题怎样回答?

How do you check a formula for dimensional consistency?State two ways in which a dimensionally consistent equation may nevertheless be physically incorrect.