Transcript 101KnightChp8_63.pptx

Textbook - Knight
Chapter 8: 63-65
(page 246)
8:63-65
• When you stand on your tiptoes,
your feet pivot around your
ankles. The forces on your foot
are an upward force on your toes
from the floor, a downward force
on your lower leg bone and an
upward force on the heal of your
foot from your Achillies tendon.
• Suppose a 60 kg woman sands on tiptoes with
the sole of her foot making a 25° angle with
the floor. Assume each foot supports half the
weight.
8:63-65
• Each foot supports half
the woman’s weight:
30 kg ⋅ 10N/kg = 300 N
• Torque is force times
perpendicular distances,
which are given.
• The force up on the toes is 300 Newtons
• Forces on foot = Fnet = Ffloor + Fbone + Ftendon = ma
•
= 300 + B + T
= 0
• Torque on foot (about ankle) =
• τnet = τfloor + τbone + τtendon = −300 ⋅ 15 + B⋅0 + T⋅5
τnet = τfloor + τbone + τtendon = −300 ⋅ 15 + B⋅0 + T⋅5 = 0
• = −300 ⋅ 15 + B⋅0 + T⋅5 = 0
• = −4500 + 0 + T⋅5 = 0
•
5T = 4500
•
5cm T = 4500 N-cm
•
T = 900 N
• Fnet = 300 + B + T = 0
•
300 + B + 900 = 0
•
B = − 1200 newtons