1
Physics 100
Lecture 2
Work and
Mechanical Energy
January 25, 2017
2
What is your year in school?
A.
B.
C.
D.
E.
First-year student
Sophomore
Junior
Senior
I’ve been here, like,
forever
41%
37%
15%
7%
0%
A.
B.
C.
D.
E.
3
What is your major?
A.
B.
C.
D.
E.
F.
Arts or Communication
23%
23%
Natural Resources
Professional Studies
One of the Sciences
One of the Humanities (languages,
philosophy, political science, etc.)
Undeclared
A.
B.
19%
15%
12%
8%
C.
D.
E.
F.
4
Why are you taking this class?
A.
B.
C.
D.
E.
The study of energy seems
interesting to me.
Only because it satisfies
GEP requirements.
It was the only class that
would fit in my schedule.
A friend recommended
19%
it to me.
Some other reason.
70%
4%
A.
B.
C.
4%
4%
D.
E.
5
Energy of Motion

When work is done on an object that
results in the motion of that object, the
object has an energy called kinetic
energy
KE  1 mv 2
2
KE 
1
2
 kg  m/s 
2
 kg  m/s 2  m
KE  N  m  Joule (J)
6
Weight versus Mass






Weight is a force (lb, N)
Newton’s Second Law: F = mg
g = 10 m/s/s = 10 m/s2
m = mass (slug, kg)
Mass is a measure of the amount of
material in an object
Weight depends upon location, mass
does not
7
If you push a 10-kg wagon for 3.0 m
with a 5.0 N force, how much work
have you done?
A.
B.
C.
D.
93%
Zero
10 J
15 J
30 J
4%
A.
4%
B.
0%
C.
D.
8
If you push a 10-kg wagon for 3.0 m
with a 5.0 N force, how much work
have you done?
A.
B.
C.
D.
Zero
10 J
15 J
30 J
W  F  d (assume parallel)
  5.0 N  3.0 m   15 J
9
If the wagon had 5.0 J of kinetic energy
when you started pushing in the direction it
was already traveling, what is its kinetic
energy now?
A.
B.
C.
D.
56%
5.0 J
10 J
15 J
20 J
37%
7%
0%
A.
B.
C.
D.
10
If the wagon had 5.0 J of kinetic energy
when you started pushing in the direction it
was already traveling, what is its kinetic
energy now?
A.
B.
C.
D.
5.0 J
10 J
15 J
20 J
Work transfers energy to the
wagon, and its kinetic energy
changes accordingly:
KEfinal  KEinitial  W
KE  5.0 J  15 J  20 J
11
What is the speed of the 10-kg wagon now?
A.
B.
C.
D.
92%
1.41 m/s
2.00 m/s
4.00 m/s
10.0 m/s
8%
0%
A.
B.
C.
0%
D.
12
What is the speed of the 10-kg wagon now?
A.
B.
C.
D.
1.41 m/s
2.00 m/s
4.00 m/s
10.0 m/s
KE  mv
1
2
2
2 KE
2
v
m
2  20 J 
2 KE
v

m
10 kg
 2.00 m/s
13
Work to Lift an Object




Force = weight = mg
g = acceleration of gravity = 10 m/s2
Distance = altitude = h
Force × Distance = mgh
14
How much work is done by lifting a
50-kg object 1.00 meter up?
A.
B.
C.
D.
500 J
250 J
50 J
zero
65%
23%
8%
A.
B.
4%
C.
D.
15
How much work is done by lifting a
50-kg object 1.00 meter up?
A.
B.
C.
D.
500 J
250 J
50 J
zero
W  mgh
  50 kg  10 m/s 2  1.0 m 
W  500 J
16
Lifting an Object



Gravitational Potential
Energy (PE)
Removing the constraint
Energy conversion
17
Mechanical Energy

ME = KE + PE
• conservation principle
• bowling ball pendulum
•
•
(applet)
Bucket analogy
roller coaster & video