Transcript Work (print version) - Hartland High School

What about a math model for the energy transferred
into or out of a system by means of an external force?
In other words, what about…
WORK?
Consider an object that is pulled a distance d at

constant velocity by an applied force FA as in the diagram.

Constant v

FA
d
Consider an object that is pulled a distance d at

constant velocity by an applied force FA as in the diagram.

Constant v

The system is not isolated since FA

FA
d
1
2
is an external force. The object

exerting FA is outside the system.
3
System Box, Earth
1
2
3
Ek
Ek
Ek
Ediss
Ediss
The must be a force of kinetic
friction acting between the box
and the ground to balance the
applied force since the box is
moving at constant velocity.
Energy is flowing into the system
by means of the external force FA
and is being stored as Ediss .
Consider an object that is pulled a distance d at

constant velocity by an applied force FA as in the diagram.

Constant v

FA
d

Fext

FA
System Box, Earth
Work W  - the transfer of energy
into or out of a system
by means of an external force


- The area under the Fext vs. x graph,

where Fext is the external force that
transfers energy

FA  constant, positive

d

x
Area  lw
W  FAd
W  Fext d
For constant, 1D
external forces
Consider an object that is pulled a distance d at

constant velocity by an applied force FA as in the diagram.


Constantv

FN
FA
FA

d
System Box, Earth
What force actually pulls
the box across the floor?

F fk

FAx

Fg

FAy
Fg  FN  FAy
FAx  F fk

FAx  constant, positive

Fext

FAx

d

x
Consider an object that is pulled a distance d at

constant velocity by an applied force FA as in the diagram.


Constantv
FAx  constant,
FA


d
Fext

FAx
System Box, Earth

d
Area  lw
W  FAx d
W  FA cos d
W  FAd cos
W  Fext d cos
positive

For constant Fext


where   angle between Fext and d

x
W  Fext d cos

For constant Fext


where   angle between Fext and d

d

FA
  0
cos 0  1
W  Fext d
Maximum energy
transferred into the system
W  Fext d cos

FA

For constant Fext


where   angle between Fext and d

d
  180
cos180  1
W   Fext d
Maximum energy transferred
out of the system
W  Fext d cos

For constant Fext


where   angle between Fext and d

FA

d
  90
cos 90  0
W 0
No energy transferred into
or out of the system
because none of the force
is moving the box