/*! This file is auto-generated */ .wp-block-button__link{color:#fff;background-color:#32373c;border-radius:9999px;box-shadow:none;text-decoration:none;padding:calc(.667em + 2px) calc(1.333em + 2px);font-size:1.125em}.wp-block-file__button{background:#32373c;color:#fff;text-decoration:none} Problem 80 A proton with mass \(m\) moves i... [FREE SOLUTION] | 91Ó°ÊÓ

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A proton with mass \(m\) moves in one dimension. The potential-energy function is \(U(x) = (\alpha/x^2) - (\beta/x)\), where \(\alpha\) and \(\beta\) are positive constants. The proton is released from rest at \(x_0\) = \(\alpha\)/\(\beta\). (a) Show that \(U(x)\) can be written as $$U(x) = \frac{\alpha}{x^2_0}\ \Big[ \Big( \frac{x_0}{x}\ \Big)^2 - \frac{x_0}{x}\ \Big] $$ Graph \(U(x)\). Calculate \(U(x_0)\) and thereby locate the point \(x_0\) on the graph. (b) Calculate \(v(x)\), the speed of the proton as a function of position. Graph \(v(x)\) and give a qualitative description of the motion. (c) For what value of \(x\) is the speed of the proton a maximum? What is the value of that maximum speed? (d) What is the force on the proton at the point in part (c)? (e) Let the proton be released instead at \(x_1 = 3\alpha / \beta\). Locate the point \(x_1\) on the graph of \(U(x)\). Calculate \(v(x)\) and give a qualitative description of the motion. (f) For each release point (\(x = x_0\) and \(x = x_1\)), what are the maximum and minimum values of \(x\) reached during the motion?

Short Answer

Expert verified
Graph U(x), calculate points, and use energy conservation to find velocity and constraints.

Step by step solution

01

Expressing U(x)

Given the potential energy function \( U(x) = \frac{\alpha}{x^2} - \frac{\beta}{x} \), we need to show that it can be rewritten as \( U(x) = \frac{\alpha}{x_0^2}\left(\left(\frac{x_0}{x}\right)^2 - \frac{x_0}{x}\right) \). Substituting \( x_0 = \frac{\alpha}{\beta} \) gives \( \frac{\alpha}{x_0^2} = \frac{\beta^2}{\alpha} \). Thus, \( U(x) = \frac{\beta^2}{\alpha}\left(\frac{\alpha^2}{\beta^2}\frac{1}{x^2} - \frac{\alpha}{\beta}\frac{1}{x}\right) = \frac{\alpha}{x_0^2}\left(\frac{x_0^2}{x^2} - \frac{x_0}{x}\right) \).
02

Calculating U(x_0)

To find \( U(x_0) \), substitute \( x = x_0 \) into \( U(x) \):\[ U(x_0) = \frac{\alpha}{x_0^2}\left(\left(\frac{x_0}{x_0}\right)^2 - \frac{x_0}{x_0}\right) = \frac{\alpha}{x_0^2}(1 - 1) = 0. \] Therefore, the point \( x_0 \) on the graph is where \( U(x) = 0 \).
03

Graphing U(x)

Plot \( U(x) \) versus \( x \). The function decreases as \( x \) increases and passes through zero at \( x = x_0 \).
04

Calculating v(x)

Apply conservation of energy: total energy \( E = U(x) + \frac{1}{2}mv^2(x) = 0 \). Hence, \[ \frac{1}{2}mv^2(x) = -U(x). \] Thus, \[ v(x) = \sqrt{\frac{2(-U(x))}{m}} = \sqrt{\frac{2}{m}\left(\frac{\alpha}{x^2} - \frac{\beta}{x}\right)}. \]
05

Graphing v(x)

Use the expression obtained to graph \( v(x) \). The speed of the proton decreases gradually as \( x \) increases.
06

Determining Maximum Speed

The speed is maximized when \( U(x) \) is minimized because \( v(x) = \sqrt{\frac{2(-U(x))}{m}} \). Set the derivative \( \frac{d}{dx}\left( \frac{\alpha}{x^2} - \frac{\beta}{x}\right) = 0 \) to find the critical points. Solve it to find the value of \( x \) where \( v(x) \) is maximum.
07

Calculating Force at Maximum Speed

The force \( F \) is the negative gradient of the potential energy: \[ F(x) = -\frac{dU}{dx}. \] Compute this at the critical point determined in the previous step.
08

Calculating Motion from x1

Set \( x_1 = \frac{3\alpha}{\beta} \) and repeat steps for \( U(x_1) \) and \( v(x_1) \). Using the same energy conservation method will provide insights into motion from \( x_1 \).
09

Locating Maximum and Minimum x

For each release point, determine the constraints set by the initial total energy, ensuring \( U(x) \subseteq E \). Solve \( U(x) = E \) to get the turning points.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Potential Energy Function
The potential energy function is a mathematical representation of how potential energy varies with position. It reflects the stored energy in an object based on its configuration or position in a force field.
In this exercise, we have the proton's potential energy as a function of its position, given by:\[ U(x) = \frac{\alpha}{x^2} - \frac{\beta}{x} \]where \( \alpha \) and \( \beta \) are positive constants. This equation shows us how potential energy changes based on the proton's position along a one-dimensional path.
The term \( \frac{\alpha}{x^2} \) indicates an inverse square relationship, which can be typical in certain physical situations, like electrostatics.
Meanwhile, \( \frac{\beta}{x} \) represents an inverse linear relationship, suggesting a different kind of influence on the potential energy.
By substituting a specific point like \( x_0 = \frac{\alpha}{\beta} \), we can simplify this function for analysis. Through substitution, one can express the function in terms of simpler components, making it easier to graph and understand the behavior at specific points.
This function is crucial for predicting the motion of the proton by calculating its potential energy at various points.
Conservation of Energy
The conservation of energy is a fundamental principle in physics stating that energy cannot be created or destroyed; it can only be transferred or converted from one form to another.
In our scenario, the total mechanical energy of the proton is conserved. Initially, when the proton is at rest, all the energy is stored as potential energy. As the proton begins to move, this potential energy is converted into kinetic energy.
At any point \(x\), the sum of the kinetic energy and potential energy remains constant:\[ E = U(x) + \frac{1}{2}mv^2(x) \]Here, \(E\) is the total energy, \(U(x)\) is the potential energy, and \( \frac{1}{2}mv^2(x) \) represents the kinetic energy of the proton with mass \(m\) and speed \(v(x)\).
Since the proton starts from rest, we can assume that initially, all energy is potential (U(x_0)\ = E).As the proton rolls down the potential energy graph, potential energy decreases and kinetic energy increases, maintaining the conservation equation.
By understanding conservation, we can express the proton's speed at any point \(x\) based on potential energy:\[ v(x) = \sqrt{\frac{2(-U(x))}{m}} \]The conservation principle provides a powerful method to evaluate the particle’s motion without directly solving complex equations of motion.
Maximum and Minimum Values
In the context of energy and motion, finding maximum and minimum values helps to predict where certain physical conditions, like maximum speed or turning points, occur.
For potential energy functions, crucial points occur where the derivative \( \frac{dU}{dx} = 0 \). These points, where the slope of the potential energy curve is zero, indicate local maxima or minima.
The maximum speed of the proton corresponds to where the potential energy \( U(x) \) is minimized. This results from the conservation of energy: less potential energy means more energy available for kinetic energy, hence higher speed.
By solving the derivative equation \( \frac{d}{dx}\left(\frac{\alpha}{x^2} - \frac{\beta}{x}\right) = 0 \), we find the positions where maximum and minimum values occur. This gives us critical insights into the motion of the proton.
Additionally, at these extremal points, calculating the force is essential:\[ F(x) = -\frac{dU}{dx} \]This computation aids in understanding how the potential energy landscape influences the proton’s motion, determining whether it accelerates or decelerates as it moves through these critical points.

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Most popular questions from this chapter

An object moving in the \(xy\)-plane is acted on by a conservative force described by the potential-energy function \(U(x, y) = \alpha[(1/x^2) + (1/y^2)]\), where a is a positive constant. Derive an expression for the force expressed in terms of the unit vectors \(\hat{\imath}\) and \(\hat{\jmath}\).

A 2.50-kg mass is pushed against a horizontal spring of force constant 25.0 N/cm on a frictionless air table. The spring is attached to the tabletop, and the mass is not attached to the spring in any way. When the spring has been compressed enough to store 11.5 J of potential energy in it, the mass is suddenly released from rest. (a) Find the greatest speed the mass reaches. When does this occur? (b) What is the greatest acceleration of the mass, and when does it occur?

A spring of negligible mass has force constant \(k =\) 800 N/m. (a) How far must the spring be compressed for 1.20 J of potential energy to be stored in it? (b) You place the spring vertically with one end on the floor. You then lay a 1.60-kg book on top of the spring and release the book from rest. Find the maximum distance the spring will be compressed.

Two blocks with different masses are attached to either end of a light rope that passes over a light, frictionless pulley suspended from the ceiling. The masses are released from rest, and the more massive one starts to descend. After this block has descended 1.20 m, its speed is 3.00 m/s. If the total mass of the two blocks is 22.0 kg, what is the mass of each block?

A slingshot will shoot a 10-g pebble 22.0 m straight up. (a) How much potential energy is stored in the slingshot's rubber band? (b) With the same potential energy stored in the rubber band, how high can the slingshot shoot a 25-g pebble? (c) What physical effects did you ignore in solving this problem?

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