7/17/2019 Catalog of Forces

http://slidepdf.com/reader/full/catalog-of-forces 1/2

 

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