# Maximum force on rope and wavefunction

## Homework Statement

A sinusoidal wave on a rope with linear density ##\mu=0.012 kg/m## is described in SI units by $$\xi(x,t)=A sin (kx-\omega t)= 0.15sin(0.8x-50t)$$
a) find the maximum acceleration of a rope element
b) find the maximum transverse force on a piece of rope ##1 cm## long
c) Show how the force depends on the tension of the rope

Wave equation

## The Attempt at a Solution

a) This is simply ##a_{max}=\omega^2 A=375 m/s^2##
b) This should be ##F_{max, y}=m a_{max}=\mu \cdot 0.01 \cdot \omega^2 A = 0.045 N##
c) Tension should be related to such force by ##F_{y}=-T \frac{\partial \xi}{\partial x}## but here is the problem, since
$$\frac{\partial \xi}{\partial x}=-0.12 sin(0.8x-50t)$$

So the maximum value is ##0.12 ## and the force should be

$$F_{max,y}= 0.12 T= 0.12 \cdot \mu \cdot v^2$$

Now ##v=\frac{\omega}{k}=\frac{50}{0.8}=62.5 m/s## so it should be

$$F_{max,y}=0.09 N$$

Different from the value found in b), which is strange, did I get something wrong?

Last edited:

physics user1
I think I have a clue but I'm not sure... when you say that Fy= -T de/dx ...

Isn't it Fy= -2T de/dx at the maximum point ? Since the tension acts on both sides of the rope element

And the derivative of the sine is the cosine

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haruspex