# What is the Total Force Exerted on a Section of a Wave-Carrying Rope?

• Ridonkulus
In summary, the wave equation for a 1/2 wavelength section of the rope between two points is almost in the form y = Acos(kx-wt). I changed it to y = Acos(-2.5x+1.7t) and found the k and w values. I found the tension using v^2 = F/(mue) and v = w/k. I used a force in the y direction equation to find Fy = 2Tsin(theta). I plugged in my tension and found angle Fy = 2*.1378N*sin(.13558) = .03737, which is not the correct answer. I would appreciate any help I can get.
Ridonkulus

## Homework Statement

At time t = 0, consider a 1/2 wavelength long section of the rope which is carrying the wave y = 0.08 cos(1.7 t - 2.5 x) between two points which have zero displacement (y = 0). Find the total force exerted by the rest of the rope on this section. Neglect any effects due to the weight of the rope. Use the small-angle approximation where q, sin(q), and tan(q) are all approximately equal to each other. Mue = .3 kg/m.

## Homework Equations

y = Acos(kx-wt)
k = 2pi/lamda
w = pi*f
v^2 = F/(mue) = (tension) / (mass per unit length)
v = w/k
-kAsin(kx-wt) ~= theta

## The Attempt at a Solution

I started out by recognizing the wave equation as being almost in the form y = Acos (kx - wt). So I changed it to y = Acos(-2.5x+1.7t). This equation gives the k and w values. Then, I found the tension using v^2 = F/(mue) and v = w/k. Since I knew that the force in the x direction had to cancel out because the wave isn't moving in the x direction, I used a force in the y direction equation. For this I got Fy = 2Tsin(theta). Plugging in my tension and found angle Fy = 2*.1378N*sin(.13558) = .03737, which is not the correct answer.

I would appreciate any help I can get, thank you.

I don't see where you are getting your sine factor. What is the slope of the rope at a point of zero displacement?

What I would suggest is to determine the acceleration of the rope $$\ddot{y}$$ at two x distances a half wavelength apart (both will be functions of time of cause) . Multiply this with the mass of the half wavelength piece of rope and add the two tensions that it experiences at its ends. What I am therefore suggesting amounts to consider the piece of rope responding as a unit to the two forces at its ends (in reality it deforms under these forces, but N2 is still applicable).

Last edited:
I'm sorry, I missed that part of my work. I used the equation -kAsin(kx-wt) (this came out to be -(-1.7)*.08*sin(-2.5*.25*2pi/-2.5)) to get the sine factor. The number I got was .136, using half of the wave length. This is what I plugged into the sin(theta).

The equation

$$c = \sqrt{\frac{T}{\mu}}$$

gives the tension in the rope in the x-direction (it controls the propagation of the wave along the rope).

For the SHM to exist the tension in the rope needs to oscillate with time as the disturbance propagates throught it (the y-component of the tension in the rope). In this case $$c$$ is the (constant) speed of propagation of the disturbance not the SHM speed (which varies with time).

Come to think about it one need to add the two tension components.

Last edited:
Ridonkulus said:
I'm sorry, I missed that part of my work. I used the equation -kAsin(kx-wt) (this came out to be -(-1.7)*.08*sin(-2.5*.25*2pi/-2.5)) to get the sine factor. The number I got was .136, using half of the wave length. This is what I plugged into the sin(theta).

That does not look right.

y(x,t) = Acos(kx-ωt) = Acos(ωt-kx) = 0.08 cos(1.7 t - 2.5 x)

I don't see that you gain anything by negating the argument of the cosine, but it's OK. You did not use the negated form to get -kAsin(kx-ωt)

You are interested in the shape of the rope at t = 0 (this time specification is not really needed in the problem; all you need is to look at adjacent points of zero displacement at some unspecified time). In particular, you need the angle between the rope and the line y = 0 at the points of intersection of y(x,0) and y = 0. Take a snapshot of the rope at t = 0 and you get

y(x,0) = 0.08 cos(2.5 x)

What you need is the slope of this function at points where y(x,0) = 0. Your expression

-kAsin(kx-ωt)

is the derivative of y(x,t) wrt x, which is the slope of the function at position x at any time t. The way the problem is stated, you want it at t = 0. So you are looking for the value of

dy(x,0)/dx = -kAsin(kx)

at points where y(x,0) = 0. Think about what has to be true if y(x,0) = 0, and use that in your slope expression.

Sorry I haven't been able to try this until just now. Thanks for all of the help everyone. I got the problem (for anyone that is curious the answer turned out to be .055128 N) using the help OlderDan gave me, thanks again.

## 1. What is a wave problem?

A wave problem is a type of scientific problem that involves studying the behavior and properties of waves, such as sound waves, light waves, or water waves. It often requires using mathematical equations and principles to understand and solve the problem.

## 2. How do I solve a difficult wave problem?

Solving a difficult wave problem requires a strong understanding of wave properties and mathematical principles. It is important to carefully read and analyze the problem, identify the relevant equations and variables, and use appropriate mathematical techniques to solve it. It may also be helpful to consult with peers or a mentor for guidance.

## 3. What are some common challenges in solving wave problems?

Some common challenges in solving wave problems include understanding and applying the correct equations, identifying the relevant variables, and accurately interpreting the problem. It may also be difficult to visualize and conceptualize the problem, especially for more complex wave phenomena.

## 4. Are there any tips or strategies for solving wave problems?

Some helpful tips and strategies for solving wave problems include practicing with simpler problems first, breaking down the problem into smaller, manageable parts, and using diagrams or graphs to visualize the wave behavior. It can also be helpful to review and understand the underlying principles and equations involved in the problem.

## 5. How can I check if my answer to a wave problem is correct?

One way to check if your answer to a wave problem is correct is to plug it back into the original problem and see if it satisfies all the given conditions. It can also be helpful to compare your answer to a known solution or to ask for feedback from peers or a mentor. Additionally, double-checking your calculations and units can help ensure the accuracy of your answer.

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