🏎️Engineering Mechanics – Dynamics Unit 8 – 3D Dynamics in Engineering Mechanics

Key Concepts and Terminology

• 3D dynamics involves the study of motion and forces acting on particles and rigid bodies in three-dimensional space
• Particles are considered as point masses with no size or shape, while rigid bodies have a fixed shape and size
• Position, velocity, and acceleration vectors describe the motion of particles and rigid bodies in 3D space
• Forces and moments act on particles and rigid bodies, causing them to move or rotate
• Newton's laws of motion (First, Second, and Third laws) form the foundation for analyzing the motion and forces in 3D dynamics
• Work, energy, and power concepts are used to analyze the motion and forces in 3D dynamics
  • Work is the product of force and displacement in the direction of the force
  • Energy can be in the form of kinetic energy (energy of motion) or potential energy (energy due to position or configuration)
  • Power is the rate of doing work or transferring energy
• Momentum and impulse are important concepts in 3D dynamics
  • Linear momentum is the product of mass and velocity
  • Angular momentum is the product of moment of inertia and angular velocity
  • Impulse is the product of force and time, causing a change in momentum

Coordinate Systems and Reference Frames

• Cartesian coordinate system (x, y, z) is commonly used to describe the position and orientation of particles and rigid bodies in 3D space
• Cylindrical coordinate system (r, θ, z) is useful for problems with axial symmetry or rotation about a fixed axis
• Spherical coordinate system (ρ, θ, φ) is used for problems with spherical symmetry or motion in three dimensions
• Reference frames can be fixed (inertial) or moving (non-inertial) relative to each other
• Inertial reference frames are those in which Newton's laws of motion are valid without any fictitious forces
• Non-inertial reference frames are accelerating or rotating relative to an inertial frame, and fictitious forces (Coriolis, centrifugal) must be considered
• Transformation matrices are used to convert vectors and coordinates between different reference frames
• Euler angles (roll, pitch, yaw) describe the orientation of a rigid body relative to a fixed reference frame

Kinematics of Particles in 3D

• Kinematics is the study of motion without considering the forces causing the motion
• Position vector $\vec{r}(t)$ describes the location of a particle in 3D space as a function of time
• Velocity vector $\vec{v}(t)$ is the first time derivative of the position vector, representing the rate of change of position
• Acceleration vector $\vec{a}(t)$ is the second time derivative of the position vector or the first time derivative of the velocity vector
• Trajectory is the path followed by a particle in 3D space, described by the position vector as a function of time
• Relative motion analysis is used when particles move relative to each other or to different reference frames
  • Relative position, velocity, and acceleration vectors are determined using vector addition or subtraction
• Projectile motion is a special case of 3D particle kinematics, where a particle is launched with an initial velocity and follows a parabolic trajectory under the influence of gravity

Kinetics of Particles in 3D

• Kinetics is the study of motion considering the forces causing the motion
• Newton's Second Law $\vec{F} = m\vec{a}$ relates the net force acting on a particle to its mass and acceleration
• Free body diagrams are used to identify and visualize all the forces acting on a particle
• Equations of motion are derived by applying Newton's Second Law in each coordinate direction (x, y, z)
• Friction forces (static, kinetic) oppose the relative motion between surfaces in contact
• Drag forces (air resistance, fluid drag) oppose the motion of a particle through a fluid medium
• Constraint forces (normal, tension, reaction) maintain the prescribed motion or trajectory of a particle
• Work-energy principle states that the net work done by all the forces acting on a particle equals the change in its kinetic energy
• Conservation of energy principle applies when the total energy (kinetic + potential) of a particle remains constant in the absence of non-conservative forces
• Impulse-momentum principle relates the net impulse applied to a particle to the change in its linear momentum

Kinematics of Rigid Bodies in 3D

• Rigid body motion in 3D involves both translation and rotation
• Translation is described by the motion of the center of mass (COM) of the rigid body
• Rotation is described by the angular velocity vector $\vec{\omega}$ and angular acceleration vector $\vec{\alpha}$
• Rotation matrices (direction cosine matrices) are used to describe the orientation of a rigid body relative to a fixed reference frame
• Euler angles (roll, pitch, yaw) or quaternions are used to parameterize the rotation matrices and avoid singularities
• Angular velocity vector $\vec{\omega}$ is related to the time derivatives of the Euler angles or quaternions
• Instantaneous axis of rotation is the direction of the angular velocity vector at a given instant
• Velocity and acceleration of any point on a rigid body can be determined using the velocity and acceleration of a reference point (e.g., COM) and the angular velocity and acceleration of the body
  • $\vec{v}_P = \vec{v}_{ref} + \vec{\omega} \times \vec{r}_{P/ref}$
  • $\vec{a}_P = \vec{a}_{ref} + \vec{\alpha} \times \vec{r}_{P/ref} + \vec{\omega} \times (\vec{\omega} \times \vec{r}_{P/ref})$

Kinetics of Rigid Bodies in 3D

• Newton-Euler equations describe the translational and rotational motion of a rigid body under the action of external forces and moments
  • $\vec{F} = m\vec{a}_{COM}$ (translational motion)
  • $\vec{M}_{COM} = I_{COM} \vec{\alpha} + \vec{\omega} \times (I_{COM} \vec{\omega})$ (rotational motion)
• Inertia tensor $I_{COM}$ is a 3x3 matrix that relates the angular velocity to the angular momentum of a rigid body about its COM
• Principal axes of inertia are the eigenvectors of the inertia tensor, along which the products of inertia are zero
• Moments of inertia $I_{xx}, I_{yy}, I_{zz}$ are the diagonal elements of the inertia tensor, representing the resistance to rotation about each principal axis
• Products of inertia $I_{xy}, I_{yz}, I_{xz}$ are the off-diagonal elements of the inertia tensor, representing the coupling between rotations about different axes
• Parallel axis theorem is used to transfer moments and products of inertia from one point to another
• Work-energy principle for rigid bodies includes both translational and rotational kinetic energy terms
• Conservation of angular momentum applies when the net external moment acting on a rigid body is zero

Energy and Momentum Methods in 3D

• Work-energy principle states that the net work done by all the forces acting on a system equals the change in its total kinetic energy (translational + rotational)
• Potential energy (gravitational, elastic) is the energy stored in a system due to its position or configuration
• Conservation of mechanical energy applies when the total energy (kinetic + potential) of a system remains constant in the absence of non-conservative forces
• Power is the rate of doing work or transferring energy, given by the dot product of force and velocity vectors or moment and angular velocity vectors
• Linear momentum $\vec{p} = m\vec{v}$ is a vector quantity representing the product of mass and velocity
• Angular momentum $\vec{L} = I \vec{\omega}$ is a vector quantity representing the product of moment of inertia and angular velocity
• Conservation of linear momentum applies when the net external force acting on a system is zero
• Conservation of angular momentum applies when the net external moment acting on a system is zero
• Impulse-momentum principle relates the net impulse (force × time) to the change in linear momentum, and the net angular impulse (moment × time) to the change in angular momentum
• Collisions (elastic, inelastic) involve the exchange of momentum and energy between colliding bodies, subject to conservation laws

Practical Applications and Problem-Solving Techniques

• Identify the system of interest (particle, rigid body, or system of particles/bodies) and the relevant coordinate system or reference frame
• Draw free body diagrams and kinetic diagrams to visualize the forces, moments, velocities, and accelerations acting on the system
• Determine the knowns and unknowns in the problem, and identify the appropriate equations or principles to solve for the unknowns
• Apply Newton's laws, work-energy principle, or impulse-momentum principle to set up the equations of motion or conservation laws
• Use vector algebra and calculus to solve the equations, taking into account initial conditions and constraints
• Verify the results by checking units, signs, and orders of magnitude, and by considering special cases or limiting conditions
• Interpret the results physically and discuss their implications for the system's behavior or performance
• Consider simplifying assumptions or approximations (e.g., small angles, negligible friction) when appropriate, and evaluate their validity
• Use numerical methods or software tools (e.g., MATLAB, Python) for complex problems involving coupled equations or non-linearities
• Develop intuition and physical insight by studying various examples and applications, such as:
  • Projectile motion (sports, ballistics)
  • Orbital motion (satellites, planets)
  • Gyroscopic motion (spinning tops, spacecraft attitude control)
  • Collisions (billiards, particle colliders)
  • Vibrations (machines, structures)