How Does Elastic String Behavior Affect Particle Motion?

[ashokmittal]

A particle of mass m is suspended under gravity from a point of the ceiling by a light elastic string of natural length h. When it is in equilibrium the extension if the string is a. It is pulled down a further distance b, which is > a, and released. Show that when the string becomes slack the particle speed V is given by V^2 = g(b^2 – a^2)/a. Show that if b^2 > a(2h+a) it will hit the ceiling with a speed U given by U^2 = V^2 – 2gh.

Describe what happens i)if b<a, ii)if b>a, but b^2 < a(2h+a), iii) if b > a but the string is replaced by a spring


Any help would be great,

Thanks

 

[Andrew Mason]

A particle of mass m is suspended under gravity from a point of the ceiling by a light elastic string of natural length h. When it is in equilibrium the extension if the string is a. It is pulled down a further distance b, which is > a, and released. Show that when the string becomes slack the particle speed V is given by V^2 = g(b^2 – a^2)/a. Show that if b^2 > a(2h+a) it will hit the ceiling with a speed U given by U^2 = V^2 – 2gh.

Describe what happens i)if b<a, ii)if b>a, but b^2 < a(2h+a), iii) if b > a but the string is replaced by a spring

The first step is to find the spring constant. You know that a force of mg increases the length by a-h. So k(a-h) = mg; k= mg/(a-h)

Then use a potential energy approach. Initial potential energy of the string is:

\frac{1}{2}k(b-h)^2

String potential energy is converted into kinetic energy of the mass and gravitational potential energy of the mass.

The string begins to go slack when the string length is back to h.

So: KE_h + mg(b-h) = \frac{1}{2}k(b-h)^2

AM

 

[wais]

Hello,

I would be happy to assist you with your question about elastic strings and particle motion. Let's first define the variables used in the problem:

m = mass of the particle
h = natural length of the elastic string
a = extension of the string when the particle is in equilibrium
b = additional distance the particle is pulled down before being released
V = speed of the particle when the string becomes slack
g = acceleration due to gravity
U = speed of the particle when it hits the ceiling

Now, let's look at the first part of the problem where we need to show that when the string becomes slack, the particle speed V is given by V^2 = g(b^2 – a^2)/a.

We know that the force acting on the particle is its weight, which is mg, and this is balanced by the restoring force of the elastic string. When the particle is in equilibrium, these two forces are equal and opposite, and the extension of the string is a.

When the particle is pulled down a further distance b, the weight of the particle remains the same, but the restoring force of the string increases. This is because the string is stretched further, and therefore has a higher spring constant. When the particle is released, the restoring force of the string will cause it to move upwards, and the particle will continue to oscillate until it comes to rest at its equilibrium position.

Now, when the string becomes slack, it means that the restoring force of the string is no longer present, and the only force acting on the particle is its weight. Using Newton's second law, we can write the equation of motion for the particle as:

mg = ma

Where a is the acceleration of the particle. We can also express the acceleration in terms of the particle speed V as:

a = V^2/h

This is because the particle is moving in a circular motion with a radius equal to the natural length of the string, h. So, we can rewrite the equation as:

mg = mV^2/h

Solving for V^2, we get:

V^2 = gh

But we know that the extension of the string is a, so we can substitute this value into the equation:

V^2 = g(b^2 – a^2)/a

This is the required expression for the particle speed when the string becomes slack.

Moving on to the second part of the problem, where we need to show