7/17/2019 Catalog of Forces
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1
Type of
Force
When to include
this force
How to draw
this force
Equation for this
force
Third Law action-reaction
Normalcontact
force
Wheneverobject A and
object B
touches
Draw it normal(perpendicular)
to the point of
contact
No equation.Typical
notation: ! or!!
A touches B, force is onB
B touches A, force is on
A
Weight or
gravitation
force
Whenever there
are masses
Draw it pointing
from one
object’s centre
of gravity (CG)
to the other
object’s CG
! ! !! A attracts B,
force is on B
B attracts A,
force is on A
Friction
(2 types:
Static and
Kinetic)
Whenever the
contact
surfaces are not
smooth. We
quantify the
roughness by
static coefficient
!! or kinetic
coefficient !!.
Static: Draw it
opposite to the
anticipated
direction of
motion
Kinetic: Draw it
opposite to the
direction ofmotion
Static: !! !
!!!!. Note that
this !! is the
max value that
the rough
surface can
“provide”.
Kinetic: !! ! !!!!
Note the above
are not laws!
A rubs against B, force is
on B
B rubs against A,
force is on A
Upthrust
or
Buoyancy
Whenever an
object is
partially or fully
submerged in a
fluid (liquid orgas)
Draw it opposite
to the weight of
the object and it
acts at the CG
of the displacedfluid
Archimedes’
Principle:! ! !"#
where ! is the
density of thefluid, ! is the
volume of the
displaced fluid
Fluid B is displaced by
object A, force is on A
Object A displaces fluid
B, force is on B
PH1011 Physics: Catalog of 9 Forces
f s = µsF N
f k = µkF N
7/17/2019 Catalog of Forces
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2
Drag Whenever an
object is moving
in a fluid (liquid
or gas)
Draw it opposite
to the direction
to the direction
of motion
Low speed:! ! !"
High speed:
! ! !!!
where!
and!
are some
constants and
the above are
not laws
Fluid B rubs against
object A, force is on A
Object A rubs against
Fluid B, force is on B
Spring
Force
Whenever
something is
compressed or
stretched
Draw the force
pointing
towards the
equilibrium
position
Hooke’s law:
! ! !!! (within limits)
Spring B is stretched/
compressed by object A,
force is on A
Object A stretches/
compresses spring B,
force is on B
Tension Whenever an
inextensible
string/rope is
taut
Draw the force
such that a
string/rope
always pull
No equation.
Typical
notation: ! or !!
String B pulls object A,
force is on A
Object A pulls string B,
force is on B
ElectricForce
Whenever thereare charges
Depending onwhether we
have attraction
(draw towards)
or repulsion
(draw away)
Lorentz force
law: !! ! !!
where ! is the
charge of the
object and ! is
the electric field
the object is in
Charge A attracts/repelscharge B, force is on B
Charge B attracts/repels
charge A, force is on A
Magnetic
force
Whenever a
current is in a
magnetic field
Use Fleming’s
left hand rule
Lorentz force
law: !! ! !!!!
or !! ! ! !!!
Magnetic field A acts on
current B, force is on B
Current B acts onmagnetic field A, force is
on A
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