Biomedical Engineering Reference
In-Depth Information
Example 1.5
I decided to move some furniture. I have begun pushing a file cabinet
full of papers, with a mass of 100 kg. For this, I applied a force of 200 N, but the file
cabinet remained in its place. I had to ask a friend for help. Together we could
double the force to 400 N. Consider that the coefficient of static friction between the
table and the ground is 0.5 and the coefficient of kinetic friction is 0.3.
(a) Find the force of static friction that has acted on the file when I applied force
equal to 200 N.
(b) Evaluate if we had success in pushing the file cabinet with the help of a friend.
Justify your answer.
(c) Determine the intensity of force that must be applied to put the file cabinet in
motion.
(d) Verify if it was possible to dispense the help of friends, after the file cabinet was
in motion.
(a)
f
200 N, since the file cabinet remained at rest.
(b)
f
s
¼ μ
s
N
¼
0.5(100 kg)(10 m/s
2
)
500 N. This is the minimum force that
must be applied on the file cabinet to begin moving it. Therefore, the effort of
two persons was not enough and needed the help of a third person.
¼
¼
(c)
500 N.
(d)
f
k
¼ μ
k
N
>
0.3(100 kg)(10 m/s
2
)
300 N. Therefore, once in motion, it was
possible to dispense the help of the third friend, but not of the second.
¼
¼
Exercise 1.10
The static friction between a tennis shoe and the floor of a basketball
court is 0.56, and the normal force which acts on the shoe is 350 N. Determine the
horizontal force needed to cause slippage of the shoe.
Example 1.6
Consider a child with 20 kg mass playing on a slide that makes an
angle with the horizontal of 45
, as illustrated in the figure of Example 1.6. The
coefficients of static and kinetic friction between the body of the child and the slide
are 0.8 and 0.6, respectively.
N
f
W
x
W
y
