30 + 6 = 36
36/12 = 3
So, it would take it 3 hours to go 12 kms downstream.
Note that we added 30 and 6 because it was going downstream. If it was going upstream, then we would have had to subtract.
Since you are looking for the speed, you need to rearrange the formula which is f = speed / wavelength. That should give you speed = f (wavelength.) All you need to do next is to substitute the value to the following equation. speed = 250 Hz (6.0m) that should leave you with 1500 m/s which is very fast.
To solve this problem we will use the work theorem, for which we have that the Force applied on the object multiplied by the distance traveled by it, is equivalent to the total work. From the measurements obtained we have that the width and the top are 14ft and 7ft respectively. In turn, the bottom of the tank is 15ft. Although the weight of the liquid is not given we will assume this value of 
(Whose variable will remain modifiable until the end of the equations subsequently presented to facilitate the change of this, in case be different). Now the general expression for the integral of work would be given as
Basically under this expression we are making it difficult for the weight of the liquid multiplied by the area (Top and widht) under the integral of the liquid path to be equivalent to the total work done, then replacing
![W = (14*7*62)\big [15y-\frac{y^2}{2}\big ]^{15}_0](https://tex.z-dn.net/?f=W%20%3D%20%2814%2A7%2A62%29%5Cbig%20%5B15y-%5Cfrac%7By%5E2%7D%7B2%7D%5Cbig%20%5D%5E%7B15%7D_0)
![W = (14*7*62)[15(15)-\frac{(15)^2}{2}]](https://tex.z-dn.net/?f=W%20%3D%20%2814%2A7%2A62%29%5B15%2815%29-%5Cfrac%7B%2815%29%5E2%7D%7B2%7D%5D)
Therefore the total work in the system is 
Answer: 
Explanation:
The density 
of a material is given by:
(1)
Where:
is the mass of the cube
is the volume of the cube
Now, the volume of a cube is equal to the length 
of its edge to the power of 3:
(2)
If we know 
, the volume of this cube is:
(3)
Substituting (3) in (1):
(4)
This is the density of the cube
Where Gravity rely's on only mass and distance and nothing else, so the weight on the planets will vary like you have stated. However Mars is smaller than Mercury, so the weight on Mars will be less, and the weight on Mercury will be more. Think this way.
More Mass = More Gravity = More Weight
Less Mass = Less Gravity = Less Weight
