🏎️Engineering Mechanics – Dynamics Unit 6 – Kinetics of Rigid Bodies in Dynamics

Key Concepts and Definitions

• Rigid body a solid object whose deformation is negligible compared to its overall motion
• Kinetics the study of forces and their effects on the motion of objects
• Translation motion in which all points in a body move in the same direction and with the same velocity
• Rotation motion around a fixed axis where all points in the body move in circular paths centered on the axis
• Center of mass the point in a body or system where the total mass can be considered to be concentrated
• Moment of inertia a measure of an object's resistance to rotational acceleration, depends on the object's mass distribution
• Angular momentum the rotational equivalent of linear momentum, product of moment of inertia and angular velocity
• Torque the rotational equivalent of force, causes angular acceleration

Rigid Body Motion Analysis

• Rigid body motion can be broken down into translation and rotation components
• Translation of the center of mass is analyzed using the same equations as for a particle
• Rotation about the center of mass is analyzed using the moment of inertia and angular acceleration
• The total kinetic energy of a rigid body is the sum of translational and rotational kinetic energies
• Rigid bodies can also undergo general plane motion, a combination of translation and rotation in a plane
• Instantaneous center of rotation the point in a body or its extension about which the body is rotating at a given instant
• Velocity and acceleration analysis can be performed using the relative motion equations

Angular Velocity and Acceleration

• Angular velocity $\omega$ measures the rate of rotation, typically in radians per second (rad/s)
• Angular acceleration $\alpha$ measures the rate of change of angular velocity, typically in rad/s²
• The right-hand rule determines the direction of angular velocity and acceleration vectors
  • Curl the fingers of your right hand in the direction of rotation
  • Your thumb points in the direction of the angular velocity or acceleration vector
• Angular velocity and acceleration are related to linear velocity and acceleration by $v = r\omega$ and $a_t = r\alpha$
  • $r$ is the perpendicular distance from the axis of rotation
  • $a_t$ is the tangential component of acceleration
• Angular velocity and acceleration can be constant or vary with time

Moment of Inertia and Mass Moments

• Moment of inertia $I$ is a measure of an object's resistance to rotational acceleration
• Moment of inertia depends on the object's mass distribution and the axis of rotation
• The parallel axis theorem allows the calculation of moment of inertia about any parallel axis, given the moment of inertia about an axis through the center of mass
  • $I = I_{cm} + md^2$, where $d$ is the perpendicular distance between the axes
• Mass moments of inertia $I_{xx}, I_{yy}, I_{zz}$ are moments of inertia about the $x$, $y$, and $z$ axes, respectively
• Products of inertia $I_{xy}, I_{yz}, I_{zx}$ measure the symmetry of mass distribution about the coordinate planes
• The inertia tensor is a 3x3 matrix containing the mass moments and products of inertia, used for 3D rotational dynamics

Equations of Motion for Rigid Bodies

• The rotational equation of motion relates the net torque $\sum \vec{\tau}$ to the angular acceleration $\vec{\alpha}$
  • $\sum \vec{\tau} = I \vec{\alpha}$, where $I$ is the moment of inertia about the axis of rotation
• The translational equation of motion relates the net force $\sum \vec{F}$ to the linear acceleration of the center of mass $\vec{a}_{cm}$
  • $\sum \vec{F} = m \vec{a}_{cm}$, where $m$ is the total mass of the rigid body
• These equations are analogous to Newton's second law for particles, $\sum \vec{F} = m \vec{a}$
• For general plane motion, both translational and rotational equations of motion must be solved simultaneously
• Initial conditions (initial position, velocity, and angular velocity) are required to solve the equations of motion

Energy and Momentum Methods

• The work-energy principle states that the net work done on a rigid body equals the change in its kinetic energy
  • $W_{net} = \Delta KE = \Delta KE_{translation} + \Delta KE_{rotation}$
• The principle of conservation of energy applies to rigid bodies, including potential energy (gravitational, elastic, etc.)
• Linear momentum $\vec{p}$ is conserved in the absence of external forces, $\vec{p} = m \vec{v}_{cm}$
• Angular momentum $\vec{L}$ is conserved in the absence of external torques, $\vec{L} = I \vec{\omega}$
• The impact between rigid bodies can be analyzed using the coefficient of restitution and conservation of momentum
  • The coefficient of restitution $e$ relates the relative velocities before and after impact, $e = -\frac{v_{2f} - v_{1f}}{v_{2i} - v_{1i}}$

Applications and Problem-Solving Strategies

• Identify the type of motion (translation, rotation, general plane motion) and the relevant variables
• Draw free-body diagrams and kinetic diagrams to visualize forces, torques, velocities, and accelerations
• Use the equations of motion, work-energy principle, and conservation laws as appropriate
• For complex shapes, break the problem into simpler components or use the parallel axis theorem
• Consider constraints and initial conditions when formulating the problem
• Verify the results using alternative methods or by checking the units and reasonableness of the solution
• Apply the concepts to real-world problems, such as:
  • Rotating machinery (gears, flywheels, engines)
  • Vehicles (cars, bicycles, airplanes)
  • Sports equipment (bats, clubs, rackets)

Common Pitfalls and Tips

• Ensure consistent units throughout the problem, converting if necessary
• Be mindful of the sign conventions for angles, torques, and moments of inertia
• Remember that the moment of inertia depends on the axis of rotation and is not always constant
• Do not confuse angular velocity $\omega$ with angular speed $|\omega|$
• When using the right-hand rule, be consistent with the direction of the angular velocity and acceleration vectors
• Consider the reference frame when analyzing the motion of multiple rigid bodies
• Double-check the equations and calculations for accuracy
• Practice solving a variety of problems to develop proficiency and intuition in rigid body dynamics