In Sample Prob. 8.03, there are no non- conservative forces acting on the child as it slides down. At the highest points, the kinetic energy of the child is zero joules. At the lowest point as it slides, the potential energy is zero joules. Which one of the following describes the kinetic and potential energies of the child at the point in the middle between the highest and lowest points? a. K=0, U= U, %3D max b. K= U С. K>U K

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Sample Problem 8.03 Conservation of mechanical energy, water slide
The huge advantage of using the conservation of energy in-
stead of Newton's laws of motion is that we can jump from
the initial state to the final state without considering all the
intermediate motion. Here is an example. In Fig. 8-8, a child
of mass m is released from rest at the top of a water slide,
at height h = 8.5 m above the bottom of the slide.
Assuming that the slide is frictionless because of the water
on it, find the child's speed at the bottom of the slide.
The total mechanical
energy at the top
is equal to the total
at the bottom.
KEY IDEAS
(1) We cannot find her speed at the bottom by using her ac-
celeration along the slide as we might have in earlier chap-
ters because we do not know the slope (angle) of the slide.
However, because that speed is related to her kinetic en-
ergy, perhaps we can use the principle of conservation of
mechanical energy to get the speed. Then we would not
need to know the slope. (2) Mechanical energy is conserved
in a system if the system is isolated and if only conservative
forces cause energy transfers within it. Let's check.
Forces: Two forces act on the child. The gravitational
force, a conservative force, does work on her. The normal
force on her from the slide does no work because its direc-
Figure 8-8 A child slides down a water slide as she descends a
height h.
System: Because the only force doing work on the child
is the gravitational force, we choose the child-Earth system
as our system, which we can take to be isolated.
Thus, we have only a conservative force doing work in
an isolated system, so we can use the principle of conserva-
tion of mechanical energy.
Calculations: Let the mechanical energy be Emees when the
child is at the top of the slide and Emech when she is at the
bottom. Then the conservation principle tells us
tion at any point during the descent is always perpendicular
Ech= Emecr
to the direction in which the child moves.
(8-19)
Transcribed Image Text:Sample Problem 8.03 Conservation of mechanical energy, water slide The huge advantage of using the conservation of energy in- stead of Newton's laws of motion is that we can jump from the initial state to the final state without considering all the intermediate motion. Here is an example. In Fig. 8-8, a child of mass m is released from rest at the top of a water slide, at height h = 8.5 m above the bottom of the slide. Assuming that the slide is frictionless because of the water on it, find the child's speed at the bottom of the slide. The total mechanical energy at the top is equal to the total at the bottom. KEY IDEAS (1) We cannot find her speed at the bottom by using her ac- celeration along the slide as we might have in earlier chap- ters because we do not know the slope (angle) of the slide. However, because that speed is related to her kinetic en- ergy, perhaps we can use the principle of conservation of mechanical energy to get the speed. Then we would not need to know the slope. (2) Mechanical energy is conserved in a system if the system is isolated and if only conservative forces cause energy transfers within it. Let's check. Forces: Two forces act on the child. The gravitational force, a conservative force, does work on her. The normal force on her from the slide does no work because its direc- Figure 8-8 A child slides down a water slide as she descends a height h. System: Because the only force doing work on the child is the gravitational force, we choose the child-Earth system as our system, which we can take to be isolated. Thus, we have only a conservative force doing work in an isolated system, so we can use the principle of conserva- tion of mechanical energy. Calculations: Let the mechanical energy be Emees when the child is at the top of the slide and Emech when she is at the bottom. Then the conservation principle tells us tion at any point during the descent is always perpendicular Ech= Emecr to the direction in which the child moves. (8-19)
In Sample Prob. 8.03, there are no non-
conservative forces acting on the child as it
slides down. At the highest points, the
kinetic energy of the child is zero joules. At
the lowest point as it slides, the potential
energy is zero joules. Which one of the
following describes the kinetic and potential
energies of the child at the point in the
middle between the highest and lowest
points?
a. K=0, U=U
max
b.
K= U
С.
K>U
d.
K<U
U= 0, K= K
е.
ax
Transcribed Image Text:In Sample Prob. 8.03, there are no non- conservative forces acting on the child as it slides down. At the highest points, the kinetic energy of the child is zero joules. At the lowest point as it slides, the potential energy is zero joules. Which one of the following describes the kinetic and potential energies of the child at the point in the middle between the highest and lowest points? a. K=0, U=U max b. K= U С. K>U d. K<U U= 0, K= K е. ax
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