Problem 1)
a)
W
=

F


s

cos(
θ
) and
θ
=90
°
so W=0
b)
0
°
<
θ
<90
°
and so cos(
θ
)>0 so that W>0
c)
Here
θ
=180 so W<0
d)
θ
=0 so W>0
e)
W< 0
f)
θ
=90 so W=0
g)
cos(
θ
)>0 for 0
°
>
θ
> 90
°
h)
a.

s

=160m ,

F

=18N,
θ
=0 so W=2900J
b.

F

=30N,
θ
=30 so W=4200J
c.

F

=12N,
θ
=180 so W=1900J
d.

F

=15N,
θ
=140 so W=1800J
Problem 2) (7.7)
perpendicular to each other. C is incorrect because work also depends on the direction of
displacement
Problem 3)
a)
Work is equal to the change in kinetic energy. Since the force on both blocks
is the same and the distance is the same, an equal amount of work is done on
each block. Therefore they have the same kinetic energy.
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View Full Documentb)
Setting the kinetic energies of the two blocks equal to each other, we find that
the lighter block is traveling twice as fast as the heavier one.
c)
Because the heavier block has four times the mass of the lighter block, when
the two blocks travel with the same speed, the heavier block will have four
times as much kinetic energy. The workenergy theorem implies that four
times more work must be done on the heavier block than on the lighter block.
Since the same force is applied to both blocks, the heavier block must be
pushed through four times the distance as the lighter block.
Problem 4) (7.26)
a)
The length the spring extends is x=2.2 cm. F=25N. The spring formula is F=
kx so k=1140 N/m
b)
W=.5kx
2
=.276 J
c)
From the spring formula, d= 21.4 cm
Problem 5)
a)
The workenergy theorem states that a force acting on a particle as it moves
over a distance changes the kinetic energy of a particle
b)
To calculate the change in energy, you must know the force as a function of
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 Spring '09
 Geller
 Energy, Potential Energy, Ugrav, Wfric= fkd

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