Forces and Free-Body Diagrams

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Forces and Free-Body Diagrams
Common Forces
Gravity (Fg)

• Acts between every two masses in the known
• universe

• Always attractive

• On Earth, always acts down on an object (to-
• wards Earths centre)

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• Near the surface of a planetoid for which
  gravitational field strength is known, weight is
  calculated with

Normal Force (FN)

• Exerted on an object by the surface on which
• it rests

• NOT always equal in magnitude to gravity

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Tension (FT)

• A pulling force

• Is often exerted by a rope, a string or a cable

• Can also be exerted by a rigid object

Compression (Fc)

• A pushing force

• Exerted on/by rigid bodies like girder, ties rods
  and rafters

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• Can't be exerted on/by ropes, strings, etc.

Static Friction (Fs)

• Is exerted between surfaces that are in contact
  with each other and NOT sliding (unless applied
  force is zero)

• Will only be great enough to prevent the
  object(s) from sliding, until applied force is
  greater than maximum static friction force.

Kinetic Friction (Fk)

• Is exerted between surfaces that are in contact
  with each other and ARE sliding relative to each
  other.

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• Is always less than the maximum force of static
  friction

• Friction ALWAYS opposes the sliding of one object
  relative to another.

Free-Body Diagrams

• Used to show ALL the forces acting on an object

• Drawn before adding force vectors (to find net
  force) so that none are missed

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• NEVER include net force!!

Examples Draw FBDs for the following

• A car skidding to a stop (brakes are locked)

FN
Fk
Fg
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• A snowboarder going downhill

Fk
FN
Fgy
Fg
Fgx
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• Batman climbing up a rope (in contact with a
  wall) with constant speed

FT
FN
Fg
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• When adding force vectors in two dimensions, we
  add the x and y components of each force

Example Determine the net force acting on
Selmas barge while in the Sydney harbour, with
one tug pulling it with a force of 12500 N E
and another with a force of 9400 N S 40º E. Be
sure to draw an FBD.
N
E
Barge
12500 N
40º
9400 N
10
18542 N
?
7201 N
Fnet
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