W from a
1. A particle with an initial velocity
v
(
t
= 0) = 7.5 m/s i has a constant acceleration
a
= –3 m/s
2
i.
At
t
= 0, its kinetic energy is 30 J.
Find the kinetic energy of the particle at
t
= 2 s.
To calculate final
KE
we need to know the final velocity:
2
0
ˆˆ
ˆ
(7.5 m/s)
( 3 m/s )(2 s)
1.5 m/s
vv a
t
i
i
i
=+=
+
−
=
GG G
and the mass of the particle, which can be derived from its kinetic energy at
t
= 0:
2
0
00
22
0
2
12
(
3
0
J
)
1.1 kg
2(
7
.
5
m
/
s
)
K
Km
v
m
v
=⇔
=
=
=
11
Thus,
(
2 s)
(1.1 kg)(1.5 m/s)
1.2 J
f
Kt
m
v
==
=
=
2. Find the kinetic energy of the particle at
t
= 4 s.
Same procedure:
2
0
ˆ
(7.5 m/s )
( 3 m/s )(4 s)
4.5 m/s
Thus,
( =4 s)
(1.1 kg)( 4.5 m/s)
11 J
f
t
i
i
i
m
v
+
−
=
−
−
=
3. Find the net work done on the particle between
t
= 0 and
t
= 2 s.
net
1.2 J 30 J
28.8 J
WK
=∆ =
−
=−
4. Find the net work done on the particle between
t
= 2 s and
t
= 4 s.
net
11 J 1.2 J
9.8 J
−
=
5. Find the net work done on the particle between
t
= 0 and
t
= 4 s.
net
net
11 J 30 J
19 J
Or:
28.8 9.8 19 J
W
−
+
=
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View Full DocumentJames Bond
1. After stopping at the cliff, James Bond (90 kg) finds out that it was not a cliff but
just a steep slope that a secret spy like him can easily handle. He lets himself go from
rest and smoothly slides down the
h
= 15 m high hill. A big parking lot lies at the
bottom of the hill. Since the parking area has been cleared of snow, the friction
between the ground and the skis brings our hero to a halt at point D, located at a
distance
d
= 12 m from point C . The descent can be considered frictionless.
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 Spring '08
 HerreraSiklody
 Acceleration, Energy, Force, Kinetic Energy, Mass, Potential Energy, m/s, James Bond

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