Miscellaneous Forces

Weight

• Weight is the force of gravity upon an object. This force is represented by the symbol Fg (units of Newtons)

• Acceleration of gravity on the earth is always -9.81 m/s/s, so weight can be calculated by multiplying the mass and -9.81 m/s/s

Normal Force

• Normal Force is the force of a surface on an object, and acts against the force of gravity (weight). It is always perpendicular to the contact surface. It is represented by Fn

Force of Friction

• Friction is a force that opposes applied force

• Static friction is the force that is exerted on a motionless object, and is always equal but opposite to an applied force. It is represented by Fs.

• There is a limit where the static friction can no longer keep the object motionless. This limit is called the Fs,max

Force of Friction

• Kinetic friction is the force that opposes motion of a moving object. It is a fixed amount that depends on the surfaces involved. It is represented by the symbol Fk

Force of Friction

• The coefficient of friction describes the relative amount of friction between two surfaces. It is represented by μs or μk

• It can be calculated by:

μs= Fs,max / Fn

• μk = Fk / Fn

• See page 102 for a table of values