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A particle is under the influence of a force \(F=-k x+k x^{3} / \alpha^{2},\) where \(k\) and \(\alpha\) are constants and \(k\) is positive. Determine \(U(x)\) and discuss the motion. What happens when \(E=(1 / 4) k \alpha^{2} ?\)

Short Answer

Expert verified
The potential energy function is \(U(x) = \frac{1}{2}kx^2 - \frac{1}{4\alpha^2}kx^4 + C\). Using the conservation of energy, we have \(\frac{1}{2}mv^2 + \frac{1}{2}kx^2 - \frac{1}{4\alpha^2}kx^4 = E\). When \(E = \frac{1}{4}k\alpha^2\), we find multiple turning points. If the particle's energy level is lower than this value, it is in a stable equilibrium. If the energy level is higher, the particle experiences unstable equilibrium and escapes towards infinity.

Step by step solution

01

1. Find the potential energy function U(x)

To determine the potential energy function, we can integrate the force function with respect to x. Since the force term involving \(\alpha\) is simply \(F(x) = -kx + \frac{kx^3}{\alpha^2}\), we can integrate it directly: \(U(x) = -\int F(x)\, dx = -\int (-kx + \frac{kx^3}{\alpha^2})\, dx\) Now, we integrate the individual terms: \(U(x) = k\int x\, dx - \frac{k}{\alpha^2}\int x^3\, dx\) \(U(x) = \frac{1}{2}kx^2 - \frac{1}{4\alpha^2}kx^4 + C\) where C is the integration constant.
02

2. Analyze the motion of the particle using conservation of energy

We know that the total energy \(E\) of the particle is conserved. In this case, the total energy can be written as the sum of the kinetic energy T(x) and the potential energy U(x): \(E = T(x) + U(x)\) Since \(T(x) = \frac{1}{2}mv^2\), where \(m\) is the mass of the particle and \(v\) is its velocity, we can rewrite the conservation of energy equation as: \(\frac{1}{2}mv^2 + \frac{1}{2}kx^2 - \frac{1}{4\alpha^2}kx^4 = E\)
03

3. Discuss the particular case when the total energy \(E = \frac{1}{4}k\alpha^2\)

Let's examine the motion when the total energy is given as \(E = \frac{1}{4}k\alpha^2\). We can substitute this value into the energy conservation equation: \(\frac{1}{2}mv^2 + \frac{1}{2}kx^2 - \frac{1}{4\alpha^2}kx^4 = \frac{1}{4}k\alpha^2\) Now, we can analyze the motion in terms of the turning points. Turning points occur when the kinetic energy term is zero, i.e., when the particle momentarily comes to a stop, and changes direction. In this case, we have: \(v = 0 \Rightarrow \frac{1}{2}kx^2 - \frac{1}{4\alpha^2}kx^4 = \frac{1}{4}k\alpha^2\) We can rearrange the equation as: \(\frac{1}{2}x^2(1 - \frac{x^2}{\alpha^2}) = \frac{1}{4}\alpha^2\) From this rearranged equation, we can see that there are multiple turning points when the particle's energy level is equal to \(\frac{1}{4}k\alpha^2\). When the particle's energy level is lower than this value, the particle is in a stable equilibrium. When the energy level is higher, the particle experiences an unstable equilibrium, where it escapes towards infinity.

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Key Concepts

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

Potential Energy
Potential energy is a crucial concept in understanding the behavior of systems in classical mechanics. In our exercise, we look at the potential energy function, denoted by \( U(x) \), for a particle influenced by a specific force. The equation for this force is \( F = -kx + \frac{kx^3}{\alpha^2} \), where \( k \) and \( \alpha \) are constants, and \( k \) is positive.
To find the potential energy \( U(x) \), we integrate the force with respect to position \( x \):
\[ U(x) = -\int F(x) \, dx = \frac{1}{2}kx^2 - \frac{1}{4\alpha^2}kx^4 + C \]
Here, \( C \) is the integration constant, usually determined by the conditions of the problem. This function gives us the potential energy landscape in which the particle moves, setting the groundwork for analyzing the particle's motion.
Conservation of Energy
The law of conservation of energy is a fundamental principle in physics. It states that the total energy of an isolated system remains constant. In our scenario, the particle's total energy \( E \) combines its kinetic energy and potential energy.
The equation expressing this law for our particle is:
\[ E = T(x) + U(x) = \frac{1}{2}mv^2 + \frac{1}{2}kx^2 - \frac{1}{4\alpha^2}kx^4 \]
Here, \( T(x) = \frac{1}{2}mv^2 \) represents the kinetic energy, where \( m \) is the mass and \( v \) is the velocity. This formula helps us understand how the particle exchanges kinetic and potential energy as it moves, while its total energy \( E \) remains unchanged if there are no external forces acting on the system.
Turning Points
Turning points are essential in analyzing particle motion. They occur when the particle's velocity is zero, meaning its kinetic energy is zero, and thus, it temporarily stops before changing direction. When analyzing turning points, we set the velocity \( v = 0 \) in our equation for energy conservation:
\[ \frac{1}{2}kx^2 - \frac{1}{4\alpha^2}kx^4 = \frac{1}{4}k\alpha^2 \]
Solving this equation gives us the positions \( x \) where turning points occur. Specifically, in the case where the total energy \( E = \frac{1}{4}k\alpha^2 \), the possible solutions describe the particle's motion boundary.
  • When the total energy is less than \( \frac{1}{4}k\alpha^2 \), the particle experiences stable equilibrium and remains in a confined region.
  • If the energy exceeds \( \frac{1}{4}k\alpha^2 \), it leads to unstable equilibrium, potentially causing the particle to move away indefinitely.
These turning points are crucial for predicting and understanding the dynamic behavior of the particle in its energy landscape.

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Most popular questions from this chapter

The motion of a charged particle in an electromagnetic field can be obtained from the Lorentz equation* for the force on a particle in such a field. If the electric field vector is \(\mathbf{E}\) and the magnetic field vector is \(\mathbf{B}\), the force on a particle of mass \(m\) that carries a charge \(q\) and has a velocity \(\mathbf{v}\) is given by $$\mathbf{F}=q \mathbf{E}+q \mathbf{v} \times \mathbf{B}$$ where we assume that \(v \ll c\) (speed of light). (a) If there is no electric field and if the particle enters the magnetic field in a direction perpendicular to the lines of magnetic flux, show that the trajectory is a circle with radius $$r=\frac{m v}{q B}=\frac{v}{\omega_{c}}$$ where \(\omega_{c} \equiv q B / m\) is the cyclotron frequency. (b) Choose the \(z\) -axis to lie in the direction of \(\mathbf{B}\) and let the plane containing \(\mathbf{E}\) and B be the \(y z\) -plane. Thus $$\mathbf{B}=B \mathbf{k}, \quad \mathbf{E}=E_{y} \mathbf{j}+E_{z} \mathbf{k}$$ Show that the \(z\) component of the motion is given by $$z(t)=z_{0}+\dot{z}_{0} t+\frac{q E_{z}}{2 m} t^{2}$$ where $$z(0) \equiv z_{0} \quad \text { and } \quad \dot{z}(0) \equiv \dot{z}_{0}$$ (c) Continue the calculation and obtain expressions for \(\dot{x}(t)\) and \(\dot{y}(t) .\) Show that the time averages of these velocity components are $$\langle\dot{x}\rangle=\frac{E_{y}}{B}, \quad\langle\dot{y}\rangle=0$$ (Show that the motion is periodic and then average over one complete period.) (d) Integrate the velocity equations found in (c) and show (with the initial conditions \(\left.x(0)=-A / \omega_{c}, \dot{x}(0)=E_{y} / B, y(0)=0, \dot{y}(0)=A\right)\) that $$x(t)=\frac{-A}{\omega_{c}} \cos \omega_{c} t+\frac{E_{y}}{B} t, \quad y(t)=\frac{A}{\omega_{c}} \sin \omega_{c} t$$ These are the parametric equations of a trochoid. Sketch the projection of the trajectory on the \(x y-\) plane for the cases (i) \(A>\left|E_{y} / B\right|,\) (ii) \(A<\left|E_{y} / B\right|,\) and (iii) \(A=\left|E_{y} / B\right| .\) (The last case yields a cycloid.)

A particle is projected with an initial velocity \(v_{0}\) up a slope that makes an angle \(\alpha\) with the horizontal. Assume frictionless motion and find the time required for the particle to return to its starting position. Find the time for \(v_{0}=2.4 \mathrm{m} / \mathrm{s}\) and \(\alpha=26^{\circ}\).

Suppose that the force acting on a particle is factorable into one of the following forms: (a) \(F\left(x_{i}, t\right)=f\left(x_{i}\right) g(t)\) (b) \(F\left(\dot{x}_{i}, t\right)=f\left(\dot{x}_{i}\right) g(t)\) (c) \(F\left(x_{i}, \quad \dot{x}_{i}\right)=f\left(x_{i}\right) g\left(\dot{x}_{i}\right)\) For which cases are the equations of motion integrable?

If a projectile is fired from the origin of the coordinate system with an initial velocity \(v_{0}\) and in a direction making an angle \(\alpha\) with the horizontal, calculate the time required for the projectile to cross a line passing through the origin and making an angle \(\beta<\alpha\) with the horizontal.

A particle moves in a medium under the influence of a retarding force equal to \(m k\left(v^{3}+a^{2} v\right),\) where \(k\) and \(a\) are constants. Show that for any value of the initial speed the particle will never move a distance greater than \(\pi / 2 k a\) and that the particle comes to rest only for \(t \rightarrow \infty\).

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