Equilibrium

Force Force is  A push or pull on an object.  A vector with magnitude and direction. Force is not  Energy.  Power.  Momentum.  Velocity.

Fundamental Forces  Gravity is a fundamental force.  It acts upon objects from a distance away from the source (such as the Earth).  There are two other fundamental forces.  Electroweak force is common in everyday life. ElectricityElectricity MagnetismMagnetism LightLight Radioactive decayRadioactive decay  Nuclear force is uncommon in everyday life. Nuclear fission (nuclear power plants)Nuclear fission (nuclear power plants) Nuclear fusion (stars)Nuclear fusion (stars)

Contact Forces  Many forces are due to contact between objects. Kick a ballKick a ball Push with a bulldozerPush with a bulldozer Tug from a ropeTug from a rope Friction due to the groundFriction due to the ground  The actual force is electricity, but the atoms are so small we can treat the forces as coming from contact by larger objects.

Newton’s Laws  Ancient scientists looked to the natural properties of objects. Motion was a result of the object’s properties.Motion was a result of the object’s properties.  Newton defined motion based on forces acting from outside an object. Motion was the result of external forces.Motion was the result of external forces.  Three laws were used to define the behavior of forces on objects.

First Law: Law of Inertia 1 An object continues at rest, or in uniform motion in a straight line, unless a force is imposed on it.  This describes constant velocity, including zero.  No change means no force, and vice versa. rocket no force constant velocity

Zero Net Force  An object at rest with no net force is in static equilibrium.  The net force is due to the sum of forces acting on the object. The forces are vectorsThe forces are vectors

Static Forces  An advertising sign weighs 210 N. It is supported from a post with a horizontal beam, and by a chain making an angle of 35  from the horizontal. What is the force in the chain? W = 210 N  = 35º Newton Legal

Vector Forces  With no motion, forces must sum to zero.  Identify forces on the sign. C is the force on the chain B is the force on the beam W is the weight  Vector sum is zero.

Force Components  To find the values, use components  Find the vertical components for the force on the chain. C y = C sin  W y = -210 N 0 = C y + W y = C sin  + W y C = -W y / sin  = 370 N  Use horizontal components for the force on the beam. 0 = B x + C x = B x + (-C cos  ) B x = C cos  = 300 N W y = -210 N  = 35º BxBx CxCx CyCy

Constant Velocity  Constant velocity means no change in motion.  Dynamic equilibrium applies in states of constant, non- zero velocity.  Zero net force used here: F N + F g + F y = 0F N + F g + F y = 0 F x + F fr = 0F x + F fr = 0 v0v0 FgFg FyFy FxFx F fr FNFN next