Dynamics

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Dynamics: Motion and Forces



Dynamics is the study of the cause of motion and why things change in the linear motion and/or maintain circular motion. Also, it is the study of torque and its effect on motion. The three main fundamentals of dynamics come from the one and only, Isaac Newton.
  1. Newton's First Law of Dynamics - An object in motion will stay in motion, unless acted upon by another force. Also known as inertia.
  2. Newton's Second Law of Dynamics - Force is equal to mass times acceleration. F=ma
  3. Newton's Third Law of Dynamics - For every action, there is an equal but opposite reaction.


Weight - The Force of Gravity; and the Normal Force

  • Definition of gravitational force; Fg = mg (g = 9.8 m/s^2). This gravitational force is called weight. This force is in the direction that points to the center of the Earth.
  • When the object is on the surface of the Earth there is an equal and opposite force contact force. There is a net force of 0 on the object at this point because of a force called a normal force. The normal force is perpendicular to the contact force, so now there is a net force of zero.


normal_force.gif
Normal Force


Kinetic Friction & Static Friction

  • Kinetic force is present when a an object slides across a surface and another force (kinetic friction) is acting on it in the opposite direction. This force will slightly - greatly effect the object moving across the surface.
  • Static friction is present when there is an object on a surface and there is a force (static friction) opposing the movement of the object.
  • Static friction is the opposing force one feels first when trying to slide something across a surface. One cannot move an object until one breaks past this static friction.
  • After one breaks the static friction, they will still be opposing the kinetic friction. Kinetic is all present when the object is in motion. Kinetic friction is never greater than static friction.
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Problem Solving - Vector Forces & Free Body Diagrams

  • Free body diagrams - show the forces acting on an object and the angles at which the forces are being implied. Should draw a free body diagram at the beginning of each problem.
  • Vector Forces - The sum of all force vector components makes up the components of the net force. Use a vector force diagram to distinguish between forces on the x-axis and the y-axis.
  • Apply necessary equations to finish the problem.

Vector Force Diagram
Vector Force Diagram
Free Body Diagram
Free Body Diagram

Equations:

Kinetic Friction: F friction = (Constant k)(F normal)
Static Friction: F friction less than or equal to (constant s)(F normal)
See Links - 1D Kinematics & 2D Kinematics for more equations


Links:

1D Kinematics
2D Kinematics
Cramster.com