
Elements Of Electromagnetics
7th Edition
ISBN: 9780190698614
Author: Sadiku, Matthew N. O.
Publisher: Oxford University Press
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From Hibbeler's Engineerin
Why is the highlighted portion not negative? The car is accelerating to the left so it should make a counterclockwise rotation on point A right?

Transcribed Image Text:The car shown in Fig. 17–10a has a mass of 2 Mg and a center of mass
at G. Determine the acceleration if the rear “driving" wheels are always
slipping, whereas the front wheels are free to rotate. Neglect the mass
0.3 m
of the wheels. The coefficient of kinetic friction between the wheels and
1.25 m '0.75 m
the road is µz = 0.25.
(a)
SOLUTION I
Free-Body Diagram. As shown in Fig. 17-10b, the rear-wheel
frictional force Fg pushes the car forward, and since slipping occurs,
FB = 0.25N.. The frictional forces acting on the front wheels are zero,
since these wheels have negligible mass.* There are three unknowns in
the problem, NA, Ng, and aç. Here we will sum moments about the mass
center. The car (point G) accelerates to the left, i.e., in the negative x
direction, Fig. 17–10b.
aG
2000 (9.81) N
0.3 m
Equations of Motion.
F = 0.25 Ng
A
NA
|-1.25 m-
4ΣF, m (aς).;
-0.25NB = -(2000 kg)ac
(1)
0.75 m
+1£F, = m(ac),;
NA + Ng – 2000(9.81) N = 0
(2)
6+EMG = 0; -NĄ(1.25 m) – 0.25Ng(0.3 m) + Ng(0.75 m) = 0 (3)
(b)
Solving,
ag
1.59 m/s? +
Ans.
2000 (9.81) N
NA = 6.88 kN
0.3 m
Ng = 12.7 kN
- FB = 0.25 Ng
SOLUTION II
Free-Body and Kinetic Diagrams. If the "moment" equation is
applied about point A, then the unknown NA will be eliminated from
the equation. To "visualize" the moment of maç about A, we will include
the kinetic diagram as part of the analysis, Fig. 17–10c.
Equation of Motion.
|-1.25 m--
0,75 m
||
2000 aG
6+EMa = £(M)a;
Ng(2 m) – [2000(9.81) NJ(1.25 m) =
0.3 m
(2000 kg)ac(0.3 m)
(c)
Solving this and Eq. 1 for aç leads to a simpler solution than that
obtained from Eqs. 1 to 3.
Fig. 17-10
*With negligible wheel mass, la = 0 and the frictional force at A required to turn
the wheel is zero. If the wheels' mass were included, then the solution would be more
involved, since a general-plane-motion analysis of the wheels would have to be
considered (see Sec. 17.5).
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