Transcript Work

Work
Hard Work?

True or false, did you do work on the object listed?
• You hold a book in one hand while standing for 10 minutes.
• You start a resting ball rolling across a level table.
• You move a heavy box from one desk top to another of equal
height.
• You carry a pen up a flight of stairs.
Force over a Distance

In physics, work is the force
acting on object over a
distance.

Forces acting on objects at
rest do no work.
Creating Motion

v=0
F
Work is force acting over a
distance.
• Changing motion requires a
force: F = ma.
• Force is applied over a finite
distance.
v>0

Forces that change motion
may do work.
Work in a Direction

The force must act in the direction of the
displacement to be work.
Force acts in
this direction
The object moves
in this direction
Not all the force
is in the direction
of motion.
Beginning and End

Only the force acting in the
direction of displacement counts
towards work.
• Displacement considers the
beginning and end points.
FN
FN

FG
FG
Forces acting perpendicular to an
object’s motion do no work.
• Linear motion with perpendicular
forces
• Uniform circular motion
Scalar Product

Finding the component of a vector in the direction of
another vector is called the scalar product.
A
B

A||
 
A  Bdir  A||
 
A B  S
 
A  B  A|| B
 
A  B  AB cos
Scalar Product Properties

Perpendicular vectors
always have a scalar product
of zero.
A
B
A||=0


Scalar products can be
found with unit vectors and
components.
Scalar products can be
mixed with vector addition.

A  Ax iˆ  Ay ˆj  Az kˆ

B  Bx iˆ  B y ˆj  Bz kˆx
 
A  B  Ax Bx  Ay B y  Az Bz
  
   
A  (B  C)  A  B  A  C
Individual Force

Dx
• Constant velocity has net
force of zero.
• A net force of zero may
include forces that are not
zero.
FN
F
Ffr
Fg
Work can be done by
individual forces, or by a net
force.

Force acting through a
distance applies to separate
forces.
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