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Exam
2
Physics 101: Lecture 9
Work and Kinetic Energy
Hour Exam 1, Tomorrow!
–
Physics 101: Lecture 9, Pg 1
Energy (and mass) is Conserved
 Energy
is “Conserved” meaning it
can not be created nor destroyed
Can change form
Can be transferred
 Total
Energy does not change with
time.
This
is a BIG deal!
Physics 101: Lecture 9, Pg 2
10
Energy
 Forms
Kinetic Energy
Potential Energy
Heat
Mass (E=mc2)
 Units
Motion (Today)
Stored (Thursday)
later
p102
Joules = kg m2 / s2
Physics 101: Lecture 9, Pg 3
12
Work: Energy Transfer due to Force
 Force
to lift trunk at constant speed
Case a Ta – mg = 0 T = mg
Case b 2Tb - mg =0 or T = ½ mg
 But
in case b, trunk only moves ½
distance you pull rope.
F
* distance is same in both!
Ta
Tb Tb
W = F dcos(q)
mg
mg
Physics 101: Lecture 9, Pg 4
15
Work by Constant Force
A) W>0
B) W=0
C) W<0
 Only
component of force parallel to
direction of motion does work!
W = F Dr cos q
F
1)
Dr
F
2)
Dr
F
3)
4)
Dr
Dr
F
Physics 101: Lecture 9, Pg 5
18
Work by Constant Force
 Example: You pull a 30 N chest 5 meters
across the floor at a constant speed by applying
a force of 50 N at an angle of 30 degrees. How
much work is done by the 50 N force?
N
T
f
mg
50 N
30
Physics 101: Lecture 9, Pg 7
21
Where did the energy go?
 Example: You pull a 30 N chest 5 meters
across the floor at a constant speed, by applying
a force of 50 N at an angle of 30 degrees.
 How much work did gravity do?
Dr
90
mg

N
How much work did friction do?
T
f
mg
f
Dr
180
Physics 101: Lecture 9, Pg 8
25
Kinetic Energy: Motion
 Apply
constant force along x-direction to
a point particle m.
1 2 2
recall : ax Dx  (vx  vx 0 )
2

Work changes ½ m v2
 Define
W=DK
Kinetic Energy K = ½ m v2
For Point Particles
Physics 101: Lecture 9, Pg 9
35
Example: Block w/ friction

A block is sliding on a surface with an initial speed of 5
m/s. If the coefficent of kinetic friction between the block
and table is 0.4, how far does the block travel before
y
stopping?
N
Y direction: F=ma
f
x
mg
W=DK
Work
5 m/s
Physics 101: Lecture 9, Pg 10 44
Falling Ball Example
 Ball
falls a distance 5 meters, What
is final speed?
Only force/work done is gravity
mg
Physics 101: Lecture 9, Pg 11
Example: block on incline
A 5kg block is at rest on a frictionless incline. It is pulled 4.5m by a
25N force. What is the final velocity of the block?
25N
5kg
30o
Physics 101: Lecture 9, Pg 12
Work by Variable Force
W
= Fx Dx
Force
Work is area under F vs x plot
Work
Distance
Spring F = k x
» Area = ½ k x2 =Wspring
Force
Work
Distance
Physics 101: Lecture 9, Pg 13 48
Summary
 Energy
is Conserved
 Work = transfer of energy using force
Can be positive, negative or zero
W = F d cos(q)
 Kinetic
Energy (Motion)
K = ½ m v2
 Work
= Change in Kinetic Energy
S W = DK
Physics 101: Lecture 9, Pg 14 50