Many people have traveled on the river at least once in their lives by boat, kayak or boat. For such trips, it is important to know how fast the water flows in the river in order to be able to determine the time needed to travel a certain distance. In this article, we consider the question of how to find the speed of a river, and also solve two physical problems on this topic.
Features of the flow of water in rivers
Many have noticed that some rivers flow slowly and the surface of the water is smooth. Usually these are large rivers, for example, Don or Volga. From the point of view of physics, such a flow is called laminar, that is, the fluid layers move in straight lines and do not mix with each other. Smaller rivulets in some places literally "rage". This type of current is characteristic of mountain rivers. It is called turbulent. Unlike laminar, here small volumes of water move along chaotic trajectories, whirlpools and foam are observed on the surface.
The riverbed also has a significant effect on the flow rate. So, it is known that near the coast and bottom, water flows more slowly than in the central part of the channel inside its volume. During their movement, the water layers are delayed by obstacles, in the form of inhomogeneities of the bottom and banks, due to friction about them. Moreover, the rocky bottom reduces the rate of movement of water more than the clay or sandy bottom.
Channel width and water content
For a deeper understanding of the question of how to find the speed of the river, it is important to know one more thing. The fact is that the same river in different places can flow at different speeds. The reason is a change in the cross-sectional area of its channel, which is externally associated with a change in width. In fairness, we note that not only a change in width, but also fluctuations in depth affect the speed of water flow (the deeper, the slower).
In view of the above, it makes sense to talk about the speed of movement of water in a river if, on a sufficiently long section (kilometers or more), the parameters of its channel fluctuate slightly, and the river does not have tributaries in this section.
A more reliable characteristic for any river is its water content. Water bearing capacity is understood as the volume of water passing through the vertical section of the channel per unit time. The water content does not depend on the parameters of the channel, however, it, like the speed, will change if there is an influx in the considered section of the river.
In this article, we restrict ourselves to the information on water availability provided and move on to the question of how to find the speed of the river.
Practical method for determining the speed of water in a river
Consider a simple practical technique that answers the question of how to find the speed of a river.
First of all, it is necessary to choose a section of the river where the movement of water will be laminar, and the channel will not change its width. Then, a peg should be hammered on the shore. It will serve as the starting point. From the first peg, using a measuring tape, you should count the distance along the coast 10 meters, then, hammer the second peg. He will be the final mark. All the preparatory work is done. Now you can go directly to the measurements.
How to find the speed of the river? To do this, you need some light object that can swim. For example, a small stick, a cone, a sheet of paper, a feather of a bird, and so on. The item should be thrown into the water opposite the first peg. In this case, you must turn on the stopwatch. As soon as the object, moving along the river, reaches the second peg, the stopwatch must be stopped and the measured time t recorded.
The experiment described is recommended to be repeated several times (4-5). Then, you need to calculate the average value of the measured time. Denote it by t¯. It is equal to:
t¯ = ∑ i = 1 n (t i ) / n.
Here n is the number of experiments. The formula for finding the flow velocity is:
v = L / t¯.
Here L is the distance between the pegs on the shore (in this case, it is 10 meters).
Some recommendations for measuring speed and processing results
To get a more accurate value of the speed of water flow in the river, it is necessary to throw a floating object into the water at different distances from the coast. In addition, measurements should be carried out in calm weather.
As for the processing of results, it’s convenient to imagine speed for practical purposes not in meters per second, but in kilometers per hour. For this, the value in m / s should be multiplied by a conversion factor of 3.6. For example, 10 m / s is 36 km / h.
It was said above that the channel material determines the decrease in the measured average velocity of water in the river. Therefore, it is recommended that in the case of a sandy-clay channel, multiply the calculated value of v by 0.9, and in the case of a stony channel - by 0.8.
Challenge with fisherman and boat
Having figured out how to find the speed of the river, we will solve the following problem. It is known that a fisherman in a boat should swim along the river 10 km. After making the necessary measurements, he found that the flow in the river is 1 m / s. What time does it take for a fisherman to swim the indicated distance along the stream without using additional traction means (motor, oars).
We translate the speed from m / s to km / h, we get 3.6 km / h. Then the required time will be equal to:
t = S / v = 10 / 3.6 ≈ 2.8 hours
Task with a boat
The boat moves against the current from point A to point B, the distance between which is 5 km. The boat passed this distance in 30 minutes. What is the speed of the river, if it is known that the speed of the boat is three times that.
Denote the water velocity in the river x. Then the speed of the boat is 3 * x. Since he was moving against the current, we can write the following equation of motion:
(3 * x - x) * t = S.
Where do we get:
x = S / (2 * t).
The data from the conditions of the problem, substituted into the obtained equality, lead to the answer: the flow velocity is 5 km / h.