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Force
Diagrams
Some commonly used conventions:
Smooth, as in “A smooth surface” or “A smooth
pulley” implies that friction is ignored.
Rough, as in “A rough surface” or “A rough pulley”
implies that friction is taken into account.
Unless stated otherwise, objects are modelled as
particles.
Unless stated otherwise, all units are SI units which
are:
Force: Newtons (N)
Time: Seconds (s)
Distance: Metres (m)
Mass: Kilogrammes (kg)
Arrow conventions:
Forces:
Velocity:
Acceleration:
Apple falling in space (air resistance ignored).
W = mg
9.81 ms-2
Apple hanging from a tree.
T
W = mg
Book resting on table.
N
W = mg
Book being pulled across smooth surface.
N
P
W = mg
Book being pulled across rough surface.
N
Fr
P
W = mg
Book sliding down smooth surface.
N
Resolved
W = mg
q
W = mg Sin q
W = mg Cos q
q
Book sliding down rough surface.
N
Fr
W = mg
Book sliding down rough surface.
N
Fr
W = mg
Book sliding down rough surface.
N
Fr
W = mg
Book sliding down rough surface.
N
Fr
W = mg
Book sliding down rough surface.
N
Fr
W = mg
Book sliding down rough surface.
N
Fr
W = mg
Book sliding down rough surface.
N
Fr
W = mg
Book sliding down rough surface.
N
Fr
W = mg
Book sliding down rough surface.
N
Fr
W = mg
Two masses,
joined by a light
string, passing
over a smooth
pulley.
T
T
W1
W2
Important!
If a force is resolved into two perpendicular components
which are equivalent to the original force, never show the
original force and the resolved components on the same
diagram.