Warning: foreach() argument must be of type array|object, bool given in /var/www/html/web/app/themes/studypress-core-theme/template-parts/header/mobile-offcanvas.php on line 20

Write down the Lagrangian for a cylinder (mass \(m\), radius \(R\), and moment of inertia \(I\) ) that rolls without slipping straight down an inclined plane which is at an angle \(\alpha\) from the horizontal. Use as your generalized coordinate the cylinder's distance \(x\) measured down the plane from its starting point. Write down the Lagrange equation and solve it for the cylinder's acceleration \(\ddot{x}\). Remember that \(T=\frac{1}{2} m v^{2}+\frac{1}{2} I \omega^{2},\) where \(v\) is the velocity of the center of mass and \(\omega\) is the angular velocity.

Short Answer

Expert verified
The cylinder's acceleration is \( \ddot{x} = -\frac{mg\sin(\alpha)}{m + \frac{I}{R^2}} \).

Step by step solution

01

Define the Kinetic Energy

The kinetic energy of the rolling cylinder is the sum of translational and rotational components. Using the formulas given, the translational kinetic energy is \( T_{trans} = \frac{1}{2} mv^2 \), and the rotational kinetic energy is \( T_{rot} = \frac{1}{2} I \omega^2 \). For rolling without slipping, the velocity \( v = \dot{x} \) and the angular velocity \( \omega = \frac{\dot{x}}{R} \). Substituting for \( \omega \) into the rotational energy, we have \( T_{rot} = \frac{1}{2} I \left(\frac{\dot{x}}{R}\right)^2 \). Therefore, the total kinetic energy \( T \) is \( T = \frac{1}{2}m\dot{x}^2 + \frac{1}{2}I\left(\frac{\dot{x}}{R}\right)^2 \).
02

Define the Potential Energy

The potential energy \( V \) of the cylinder when it has fallen a distance \( x \) down the incline is \( V = mgx\sin(\alpha) \), as the component of gravitational force acting along the incline is \( mg \sin(\alpha) \).
03

Calculate the Lagrangian

The Lagrangian \( L \) is obtained by taking the difference between the kinetic energy and potential energy: \( L = T - V = \frac{1}{2}m\dot{x}^2 + \frac{1}{2}I\left(\frac{\dot{x}}{R}\right)^2 - mgx\sin(\alpha) \).
04

Write the Lagrange's Equation

Lagrange's equation is given by \( \frac{d}{dt}\left(\frac{\partial L}{\partial \dot{x}}\right) - \frac{\partial L}{\partial x} = 0 \). First, calculate \( \frac{\partial L}{\partial \dot{x}} = m \dot{x} + \frac{I}{R^2}\dot{x} \). Differentiating this with respect to time, we get \( \frac{d}{dt}\left(\frac{\partial L}{\partial \dot{x}}\right) = (m + \frac{I}{R^2})\ddot{x} \). Next, compute \( \frac{\partial L}{\partial x} = -mg\sin(\alpha) \). Use these results in Lagrange's equation: \((m + \frac{I}{R^2})\ddot{x} + mg\sin(\alpha) = 0\).
05

Solve for the Acceleration

Rearrange the equation from Step 4 to solve for \( \ddot{x} \): \( \ddot{x} = -\frac{mg\sin(\alpha)}{m + \frac{I}{R^2}} \). This is the linear acceleration of the cylinder down the incline.

Unlock Step-by-Step Solutions & Ace Your Exams!

  • Full Textbook Solutions

    Get detailed explanations and key concepts

  • Unlimited Al creation

    Al flashcards, explanations, exams and more...

  • Ads-free access

    To over 500 millions flashcards

  • Money-back guarantee

    We refund you if you fail your exam.

Over 30 million students worldwide already upgrade their learning with Vaia!

Key Concepts

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

Kinetic Energy
Kinetic energy is a fundamental concept in physics that deals with the energy an object possesses due to its motion. For a rolling cylinder, kinetic energy is divided into two types: translational and rotational.
  • Translational kinetic energy is the energy due to the motion of the cylinder's center of mass. It is given by the formula: \( T_{trans} = \frac{1}{2} m v^2 \), where \( m \) is the mass and \( v \) is the velocity.
  • Rotational kinetic energy is the energy due to the cylinder spinning around its own axis. It is calculated using: \( T_{rot} = \frac{1}{2} I \omega^2 \), where \( I \) is the moment of inertia and \( \omega \) is the angular velocity.
In the case of a rolling cylinder without slipping, the linear velocity \( v \) relates to the angular velocity \( \omega \) through the radius \( R \), as \( \omega = \frac{v}{R} \). Hence, the total kinetic energy \( T \) of the cylinder becomes: \[T = \frac{1}{2} m \dot{x}^2 + \frac{1}{2} I \left(\frac{\dot{x}}{R}\right)^2\] This combined energy tells us how much energy the cylinder has because of its overall motion.
Potential Energy
Potential energy is associated with the position of an object in a force field, typically a gravitational field. In the scenario of a cylinder rolling down an incline, the potential energy is influenced by its position along the slope.
  • The potential energy \( V \) of the cylinder is given by \( V = mgx\sin(\alpha) \).
  • Here, \( x \) is the distance the cylinder has traveled down the incline.
  • \( \alpha \) is the angle of the incline with respect to the horizontal, while \( m \) and \( g \) represent the mass of the cylinder and gravitational acceleration, respectively.
Potential energy essentially quantifies the energy stored within the cylinder because of its elevated position on the slope. As it rolls downward, this energy is converted into kinetic energy, specifically the translational and rotational energies discussed before.
Lagrange's Equation
Lagrange's equation is a powerful tool in mechanics that allows us to derive equations of motion for systems. It is particularly useful when dealing with systems possessing complex constraints like the rolling cylinder. This equation links the kinetic and potential energies through the concept of the Lagrangian, \( L \), defined as the difference between kinetic energy \( T \) and potential energy \( V \):
\[ L = T - V \] For the rolling cylinder, the Lagrangian is computed as: \[ L = \frac{1}{2}m\dot{x}^2 + \frac{1}{2}I\left(\frac{\dot{x}}{R}\right)^2 - mgx\sin(\alpha) \]Using Lagrange's equation \( \frac{d}{dt}\left(\frac{\partial L}{\partial \dot{x}}\right) - \frac{\partial L}{\partial x} = 0 \), we can derive the motion equation for the cylinder. This involves calculating the derivative of the Lagrangian with respect to velocity \( \dot{x} \) and position \( x \), and setting up the equation:
  • The term \( \frac{\partial L}{\partial \dot{x}} = m \dot{x} + \frac{I}{R^2}\dot{x} \) is differentiated over time \( t \).
  • Simultaneously, \( \frac{\partial L}{\partial x} = -mg\sin(\alpha) \) is calculated.
  • Inserting these into Lagrange's equation gives the acceleration \( \ddot{x} \): \[ \ddot{x} = -\frac{mg\sin(\alpha)}{m + \frac{I}{R^2}} \]
This result provides the acceleration with which the cylinder rolls down the incline. Understanding Lagrange's formulation helps simplify complex systems and is a cornerstone in classical mechanics.

One App. One Place for Learning.

All the tools & learning materials you need for study success - in one app.

Get started for free

Most popular questions from this chapter

Write down the Lagrangian for a one-dimensional particle moving along the \(x\) axis and subject to a force \(F=-k x\) (with \(k\) positive). Find the Lagrange equation of motion and solve it.

Consider a bead that is threaded on a rigid circular hoop of radius \(R\) lying in the \(x y\) plane with its center at \(O,\) and use the angle \(\phi\) of two- dimensional polar coordinates as the one generalized coordinate to describe the bead's position. Write down the equations that give the Cartesian coordinates \((x, y)\) in terms of \(\phi\) and the equation that gives the generalized coordinate \(\phi\) in terms of \((x, y)\).

Lagrange's equations in the form discussed in this chapter hold only if the forces (at least the nonconstraint forces) are derivable from a potential energy. To get an idea how they can be modified to include forces like friction, consider the following: A single particle in one dimension is subject to various conservative forces (net conservative force \(=F=-\partial U / \partial x)\) and a nonconservative force (let's call it \(F_{\text {fric }}\) ). Define the Lagrangian as \(\mathcal{L}=T-U\) and show that the appropriate modification is $$\frac{\partial \mathcal{L}}{\partial x}+F_{\mathrm{fric}}=\frac{d}{d t} \frac{\partial \mathcal{L}}{\partial \dot{x}}.$$

A smooth wire is bent into the shape of a helix, with cylindrical polar coordinates \(\rho=R\) and \(z=\lambda \phi,\) where \(R\) and \(\lambda\) are constants and the \(z\) axis is vertically up (and gravity vertically down). Using \(z\) as your generalized coordinate, write down the Lagrangian for a bead of mass \(m\) threaded on the wire. Find the Lagrange equation and hence the bead's vertical acceleration \(\ddot{z}\). In the limit that \(R \rightarrow 0\), what is \(\ddot{z} ?\) Does this make sense?

Noether's theorem asserts a connection between invariance principles and conservation laws. In Section 7.8 we saw that translational invariance of the Lagrangian implies conservation of total linear momentum. Here you will prove that rotational invariance of \(\mathcal{L}\) implies conservation of total angular momentum. Suppose that the Lagrangian of an \(N\) -particle system is unchanged by rotations about a certain symmetry axis. (a) Without loss of generality, take this axis to be the \(z\) axis, and show that the Lagrangian is unchanged when all of the particles are simultaneously moved from \(\left(r_{\alpha}, \theta_{\alpha}, \phi_{\alpha}\right)\) to \(\left(r_{\alpha}, \theta_{\alpha}, \phi_{\alpha}+\epsilon\right)\) (same \(\epsilon\) for all particles). Hence show that $$\sum_{\alpha=1}^{N} \frac{\partial \mathcal{L}}{\partial \phi_{\alpha}}=0.$$ (b) Use Lagrange's equations to show that this implies that the total angular momentum \(L_{z}\) about the symmetry axis is constant. In particular, if the Lagrangian is invariant under rotations about all axes, then all components of \(\mathbf{L}\) are conserved.

See all solutions

Recommended explanations on Physics Textbooks

View all explanations

What do you think about this solution?

We value your feedback to improve our textbook solutions.

Study anywhere. Anytime. Across all devices.

Sign-up for free