Biomedical Engineering Reference
In-Depth Information
Fig. 3.13
(
a
) Translational
motion of a racket. (
b
)
Motion of a racket thrown
by one person to another
a
Translational motion
C.G.
b
Projectile motion
3.7 Answers to Exercises
Exercise 3.1
Situation (a) the acrobat does not rotate.
Situation (b) the chair rotates clockwise.
Situation (c) the yoga practitioner does not rotate and remains in the posture.
Situation (d) while the person keeps the vertical line passing through his C.G. on his
support feet, he will not rotate.
Situation (e) when the vertical line passing through his C.G. extends beyond his
feet, he will rotate counterclockwise.
When the C.G. is not along the same vertical line passing through the support point,
there is a lever arm of the weight force and, hence, the torque will produce rotation.
In cases in which the C.G. lies in the vertical line passing through the support point,
the torque of the weight force is zero, since its lever arm is zero.
Exercise 3.2
This is the situation (e) illustrated in Fig.
3.1
. At the instant in which
the vertical line passing through his C.G. goes outside the support area,
corresponding to the area of the feet in this case, the lever arm of weight force
will no longer be zero and its weight will exert torque causing rotation, what could
mean his fall. To bend over without falling, the person must maintain the line
passing through his C.G. on the feet, moving the legs and hips backward.
Exercise 3.4
Depending on the material and shape, the C.G. of a hanger can be
outside the hanger (in the air) or on it, but always below the point where it is
supported when hanging in the wardrobe. This is a situation of great stability, since
