Transcript Physics 130 - UND: University of North Dakota

10/9 About “Work”
Text: Chapter 6 Energy
HW 10/9 “Skidding Objects”
due Friday 10/11
Exam 2 Thursday, 10/17
5-7 Wit 116
6-8 Wit 114 (only if needed)
Please send email if other time needed
Work, Fother x
Other means
not Weight
How should we multiply vectors?
Moving in a circle at constant speed as in lab.
0
+ Work = 0
PEg,i KEi
PEg,f KEf
Since Ei = Ef, no Work is done on the ball. But...
There is a net Force that points toward the center of the
circle!
We are OK if we note that the Force is perpendicular to the
velocity and x.
Multiplying Vectors
x = 4m
F = 6N

+24J
Force in the direction of motion

-24J

0J
Force opposite the direction of
motion
Force perpendicular to the
direction of motion

-24J
Force opposite the direction of
motion

0J
Force perpendicular to the
direction of motion

?J
Hmm…...
How much Work is done?
F = 1000 N
37o
Frictionless Table
How much Work is done?
Vertical component does no Work, horizontal component does.
Work = F x cos
 is the angle between F and x
Work = 1000(5)cos37°  4000 J
F = 1000 N
37o
x = 5m
Frictionless Table
F = 1000 N
37o
How much Work is done?
What is the block’s velocity if it starts from rest? m = 10 kg
0J
+ 4000 J
=
4000 J
T = 1000 N
T = 1000 N
37o
37o
x = 5m
Kef = 1/2 mvf2 = 4000
vf = 28 m/s
How much Work is done?
A block slides down a frictionless ramp.
Since, at any instant, the Normal
Force is perpendicular to the
velocity and x, no Work is done at
all!
You can find velocities anywhere if you
know the altitude difference.