1
18
1. Vector Dot Product or Scalar Vector Product
Multiply two vectors and get an answer that is a scalar!
How can we multiply two vectors by each other?
A
B
Project A onto B = A cos
θ
A
.
B = A B cos
θ
θ
A cos
θ
19
Clicker Question
A person is pulling a rock with a
rope.
The tension in the rope is
10 Newtons.
After dragging the
rock 5 meters, how much work
has the person done?
A) Work = 10 Joules
B) Work = 25 Joules
C) Work = 50 Joules
D) Work = 100 Joules
θ
=60
0
W = F
.
Δ
r = F 
Δ
r cos
θ
= 10 N 5 m (1/2) = 25 Nm
20
field
ext
i
f
W
W
U
U
U
−
=
+
=
−
=
Δ
Potential Energy and the Relationship with Work….
r
Q
kQ
U
f
2
1
=
Potential Energy between two
electrical charges.
21
r
U(r)
r
U(r)
r
Q
kQ
r
U
2
1
)
(
=
Both cases below are for two point
charges separated by a distance r.
Which graph is correct for two negative
charges?
A)
Left Plot,
B) Right Plot
Clicker Question
Just like a compressed
spring stores potential
energy.
22
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 Fall '06
 DUBSON
 Electric Potential, Potential Energy, Electric charge

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