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2
2
vf = v0 + 2 g ( h0 − hf ) = ( 0 m/s ) + 2 ( 9.80 m/s 2 ) (104 m ) = 45.1 m/s b. The work Wnc done by the nonconservative forces follows directly from Equation 6.6: ( ) 2
Wnc = 1 m vf2 − v0 + mg ( hf − h0 )
2 = 1
2 ( 86.0 kg ) ( 35.8 m/s )2 − ( 0 m/s )2 + (86.0 kg ) ( 9.80 m/s 2 ) ( −104 m ) = −3.25 ×104 J Note that the difference in heights, hf − h0 = −104 m , is a negative number because the final
height hf is less than the initial height h0.
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57. REASONING
a. Because the kinetic frictional force, a nonconservative force, is present, it does negative
work on the skier. The workenergy theorem, in the form of Equation 6.6, may be used to
find the work done by this force.
b. Once the work done by the kinetic frictional force is known, the magnitude of the kinetic
frictional force can be determined by using the definition of work, Equation 6.1, since the
magnitude of the skier’s displacement is known. 320 WORK AND ENERGY SOLUTION
a. The work Wnc done by the kinetic frictional force is related to the object’s kinetic and
potential energies by Equation 6.6:
Wnc = ( 1
2 ) + ( mgh 2
mvf2 − 1 mv0
2 − mgh0 ) f s The initial height of the skier at the bottom of the hill is h0 = 0 m, and the final height is hf = s sin 25° (see the drawing). Thus, the work is
Wnc = ( = 1
2 1 mv 2
f
2 2 − 1 mv0
2 25° ) + mg ( s sin 25° − h )
0 ( 63 kg )( 4.4 m / s )2 − 1 ( 63 kg )( 6.6 m / s )2
2 ( + ( 63 kg ) 9.80 m / s 2 ) (1.9 m ) sin 25° − 0 m = −270 J b. The work done by the kinetic frictional force is, according to Equation 6.1,
Wnc = ( f k cos 180° ) s , where fk is the magnitude of the kinetic frictional force, and s is the
magnitude of the skier’s displacement. The displacement of the skier is up the hill and the
kinetic frictional force is directed down the hill, so the angle between the two vectors is
θ = 180° and cos θ = –1. Solving the equation above for fk, we have
Wnc −270 J
=
= 140 N
− s −1.9 m
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fk = 58. REASONING AND SOLUTION
Equation 6.8, we have Wnc = ( According to the workenergy theorem as given in 1
mvf2
2 + mghf )−( 1
2
mv0
2 + mgh0 ) The metal piece starts at rest and is at rest just as it barely strikes the bell, so that ( ) vf = v0 = 0 m/s. In addition, hf = h and h0 = 0 m, while Wnc = 0.25 1 M v , where M and
2
v are the mass and speed of the hammer. Thus, the workenergy theorem becomes 0.25 ( 1 M v2
2 Solving for the speed of the hammer, we find ) = mgh 2 hf Chapter 6 Problems 321 2mgh
2(0.400 kg)(9.80 m/s2 )(5.00 m)
=
= 4.17 m/s
0.25 M
0.25 (9.00 kg)
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v= 59. REASONING We can determine the force exerted on the diver by the water if we first find
the work done by the force. This is because the work can be expressed using Equation 6.1
as Wnc = (F cos θ)s, where F is the magnitude of the force from the water and s is the
magnitude of the displacement in the water. Since the force that the water exerts is
nonconservative, we have included the subscript “nc” in labeling the work. To calculate the
work we will use the workenergy theorem in the form of Equation 6.8:
2
Wnc = ( 1 mvf2 + mghf ) − ( 1 mv0 + mgh0 ) . In this theorem Wnc is the work done by the net
2
2
nonconservative force, which, in this case, is just the force exerted by the water. SOLUTION We write Equation 6.8 as follows: Wnc = mv + mgh
(144244 )
3
1
2 2
f f Final mechanical energy − mv + mgh
(144244 )
3
1
2 2
0 0 = mghf − mgh0 (1) Initial mechanical energy where we have used the fact that the diver is at rest initially and finally, so v0 = vf = 0 m/s.
According to Equation 6.1 the work done on the diver by the force of the water is Wnc = (F cos θ)s = (F cos 180º)s = −Fs
The angle θ between the force of the water and the displacement is 180º, because the force
opposes the motion. Substituting this result into Equation (1) gives
− Fs = mghf − mgh0 (2) Identifying the final position under the water as hf = 0 m und using upward as the positive
direction, we know that the initial position on the tower must be
h0 = 3.00 m + 1.10 m = 4.10 m. Thus, solving Equation (2) for F, we find that F= mg ( h0 − hf ) = ( 67.0 kg ) ( 9.80 m/s 2 ) ( 4.10 m ) − ( 0 m ) = 2450 N
s
1.10 m
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60. REASONING AND SOLUTION The force exerted by the bat on the ball is the only nonconservative force acting. The work due to this force is
2
Wnc = mvf2 – mv0 + mg ( hf – h0 )
1 1 2 2 322 WORK AND ENERGY Taking h0 = 0 m at the level of the bat, v0 = 40.0 m/s just before the bat strikes the ball and
vf to be the speed of the ball at hf = 25.0 m, we have vf = 2Wnc
m 2 + v0 – 2 ghf 2(70.0 J)
2
2
+ ( 40.0 m/s ) –2(9.80 m/s )(25.0 m) = 45.9 m/s
0.140 kg
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This note was uploaded on 04/30/2013 for the course PHYS 1100 and 2 taught by Professor Chastain during the Spring '13 term at LSU.
 Spring '13
 CHASTAIN
 Physics, The Lottery

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