Calculating the Speed of a River Current Using Relative Velocity

In summary, the task is to set the boat's velocity so that it travels straight across the river. The boat's velocity needs to be changed to 119.38 units and the angle to 100.3. To find the speed of the current, the formula used is cos(100.3) = -current/119.38. After some adjustments, the correct formula is 136.84/tan(angle). The interactive learning exercise allows for multiple attempts until the correct answer is submitted within 5% of the correct value.
  • #1
AnkhUNC
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Homework Statement


I have a boat trying to cross a river with a current flowing ->

The boat is attempting to make it straight across (90d). In order to do this I have to change the volocity to 119.38 and the angle to 100.3.

So what is the speed of the current?


Homework Equations





The Attempt at a Solution


I tried solving using: cos(100.3) = -current/119.38

So -current = -21.345 but this is incorrect. Can anyone help?
 
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  • #2
You're off by a minus sign and perhaps a significant digit. In terms of the velocity of the current, -V_c = -21.345; thus, V_c = 21.345 to the right. In terms of speed, that's the magnitude of the velocity, always a positive number.
 
  • #3
Its still saying its wrong. It was the speed of the current but I thought that's what I was getting.
 
  • #4
How is the problem worded? Is the current flowing exactly west to east, parallel to the x axis? Is the boat's velocity given as exactly 119.38 units? What do you mean when you say you had to 'change' the velocity to 119.38?
 
  • #5
Objective:
Your task is to set the velocity of the boat so that it travels straight across the river. From the selected boat velocity, you can then compute the speed of the current.

Instructions: (The little white spots are floating in the current.)
(1) Drag the blue arrow tip to set the velocity of the boat relative to the water.
(2) Press START to begin crossing the river.
(3) If the boat misses the dock, press RESET to begin again with the same current.
(4) You can select a new random value for the current with the NEW VELOCITY button.
(5) When you have found the necessary boat velocity vector, use it to figure out the speed of the current. Enter your calculated value in the box labeled “current” below. Press the CHECK button to check your answer.
(6) Keep trying until you get it right*, then press the SUBMIT button to record your score. Only one submission is allowed, but that’s enough, since you shouldn’t submit until you have it right.
* For your answer to be excepted, it must be within 5% of the correct answer.Its an interactive learning thing.
 
  • #6
136.84/tan(angle) worked :P

Thanks for the attempt at helping, much appreciated!
 

FAQ: Calculating the Speed of a River Current Using Relative Velocity

1. What is relative velocity and how is it different from absolute velocity?

Relative velocity is the velocity of an object in relation to another object. It takes into account the motion of both objects and is measured from the perspective of one object in relation to the other. Absolute velocity, on the other hand, is the velocity of an object in relation to a fixed reference point. It does not consider the motion of any other objects and is measured from a stationary point.

2. How is relative velocity calculated?

Relative velocity is calculated by subtracting the velocity of one object from the velocity of the other object. This can be done using vector addition or using the relative velocity formula, which is Vrel = V2 - V1.

3. Why is it important to understand relative velocity?

Understanding relative velocity is important in many fields of science, such as physics, astronomy, and engineering. It allows us to accurately describe and predict the motion of objects in relation to other objects, which is crucial in understanding the dynamics of complex systems. It also helps us to understand the concept of frame of reference and how different perspectives can affect our perception of motion.

4. How does relative velocity affect collisions between objects?

Relative velocity plays a significant role in collisions between objects. The relative velocity between two objects before a collision determines the amount of kinetic energy that will be transferred during the collision. This can affect the outcome of the collision and can be used to analyze and predict the behavior of objects in motion.

5. Can relative velocity be negative?

Yes, relative velocity can be negative. This happens when the two objects are moving in opposite directions. In this case, the relative velocity would be negative, indicating that the objects are getting closer to each other. However, the magnitude of the relative velocity would still be positive, as it represents the speed at which the objects are approaching each other.

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