Unit IV: Force

Forces in Static (Non-Moving) SituationsPer. 3, 6

Force: What is it?

• Force is an interaction between two objects• Force is an interaction that can have an effect

on the motion of an object• Has units of Newtons (N)

Consider the Rock below:

Notes on: Friction Force

• Static Friction, Ffs , is a friction force exerted by a surface that prevents an object from moving.

• Friction always OPPOSES motion– Friction never causes an object to move, it only

resists motion• Friction is always directed parallel to the

surface.

Notes on: Normal Force

• Normal Force, FN , is the force exerted perpendicularly by a surface.

• Normal Force is a reaction force; – It arises from the atoms or molecules in the

surface pushing back, whenever they are pushed upon.

• “Normal” is Math-speak for “Perpendicular”• Normal Force is always perpendicular to the

surface.

Notes on: Tension Force

• Force exerted along the length of an object, most commonly a rope, chain or string

• Tension can be exerted by a rigid object (example, the handle of a wagon)

• Tension is exerted along the length of the rope, string or chain.

• Tension can be redirected by a pulley, it always goes the way the rope is going

• For purposes of this class, Tension is constant within a rope, string or chain.

Drawing Force Diagrams

• Identify a system (draw dotted line around the object that will be your system)

• Identify every force acting on the system• Draw a Force vector arrow for every force– Vector goes in direction that force acts on the

object– Magnitude represents relative strength– Label Force, FN , Fg, Ffs, etc.

– Identify using “on-by”: FN on apple by floor

Half of Newton’s First Law

• If an object is at rest, the Forces acting on that object are balanced– How can we judge whether forces are balanced?

Remember:

• Friction Force parallel to surface, opposes motion

• Normal Force perpendicular to the surface that is exerting the force

• Tension Force along the length of the rope or string.