
Concept explainers
(a)
The units and dimensions of the constant b in the retarding force bυn if (a)n=1 .
(a)

Answer to Problem 27P
Dimension of b=MT−1
The unit of b is kg s-1
Explanation of Solution
Given Information:
n=1
Formula Used:
The drag force equation is Fd=bvn
Calculation:
The drag force equation is Fd=bvn
Dimension of force =[MLT−2]
The dimension of speed =[LT−1]
Solving the drag force equation for b with n=1
b=Fdv
Substitute the dimensions of Fd and v and simplify to obtain the dimension of b as
[b]=[MLT−2][LT−1][b]=MT−1
So, the units ofb is kg s-1 .
(b)
Using dimensional analysis, determine the units and dimensions of the constant b in the retarding force bυn if n=2 .
(b)

Answer to Problem 27P
Dimension of b=ML−1
The units of b is kg.m-1
Explanation of Solution
Given Information:
n=2
Calculation:
The drag force equation is Fd=bvn
Dimension of force =[MLT−2]
The dimension of speed =[LT−1]
Solving the drag force equation for b with n=2 ,
b=Fdv2
Substituting the dimensions of Fd and v and simplify to obtain the dimension of b as
[b]=[MLT−2][LT−1]2=ML−1
So, the units of b are kg m−1 .
(c)
To Show: Air resistance of a falling object with a circular cross section should be approximately 12ρπr2υ2, where ρ=1.20 kg/m3 , the density of air, is dimensionally consistent.
(c)

Explanation of Solution
Given Information:
Air resistance of a falling object with a circular cross section should be approximately 12ρπr2υ2, where ρ=1.20 kg/m3 , the density of air.
Calculation:
According to the expression of drag force,
Fd=12ρπr2υ2
So the dimensions of drag force is
[Fd]=[12ρπr2υ2]or =[ML−3][L]2[LT−1]2or =MLT−2
The drag force is given as
Fd=bv2
The dimension of drag force is
[Fd]=[bv2]=[ML−1][LT−1]2=MLT−2
Conclusion:
Hence, the expression for drag force is dimensionallyconsistent .
(d)
To Find: The terminal speed of the skydiver.
(d)

Answer to Problem 27P
56.9 m/s
Explanation of Solution
Given Information:
Mass of skydiver = 56 kg
Disk of radius = 0.30 m
Density of air near the surface of Earth is 120 kg/m3
Formula Used:
The terminal speed is given by relation
vt=√2mgρπr2
Calculation:
Mass m=56 kg
Acceleration due to gravity g=9.81 m/s2
Density ρ=1.2 kg/m3
Radius r=0.3 m
The terminal speed is given by relation
vt=√2mgρπr2 =√2(56 kg)(9.81m/s)2π(1.2 kg/m3)(0.3m)2=56.9 m/s
(e)
To Calculate: The terminal velocity at the given height.
(e)

Answer to Problem 27P
The terminal velocity is 86.9 m/s
Explanation of Solution
Given Information:
Height = 8 km
Density of air near the surface of Earth is 0.514kg/m3
Formula Used:
The terminal speed is given by relation
vt=√2mgρπr2
Calculation:
Mass, m=56 kg
Acceleration due to gravity, g=9.81 m/s2
Density, ρ=0.514 kg/m3
Radius r=0.3 m
Substitute the values:
vt=√2mgρπr2 =√2(56 kg)(9.81m/s)2π(0.514 kg/m3)(0.3 m)2 =272.12 m/s
Conclusion:
Thus, the terminal speed at the given height is 272.12 m/s
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Chapter 5 Solutions
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