johnson (mjj622) – HW 12 – Coker – (56625) 1
This print-out should have 18 questions.
Multiple-choice questions may continue on
the next column or page – find all choices
before answering.
This assignment deals with
...
johnson (mjj622) – HW 12 – Coker – (56625) 1
This print-out should have 18 questions.
Multiple-choice questions may continue on
the next column or page – find all choices
before answering.
This assignment deals with concepts from
Chapter 13, the next to last chapter to be
covered on Quiz 3, coming up November 10.
001 10.0 points
A force of F = F1ˆi + F2 ˆj+ F3 kˆ acts at the
point r = r0ˆi.
What is the total torque about the origin of
the coordinate system?
1. r0 F2 kˆ − F3 ˆj correct
2. r0 F3 kˆ + F2 ˆj + F1ˆi
3. r0 F2 kˆ − F1 ˆj
4. r0 F2 kˆ + F3 ˆj
5. r0qF12 + F22 + F32
6. r0 F1 kˆ + F3ˆi
7. r0 F1ˆi + F2 ˆj
Explanation:
From the definition of the cross product,
ˆi ׈j = +kˆ and ˆi × kˆ = −ˆj, and ˆi ׈i = 0.
Applying the definition of torque,
~τ = r × F
= r0ˆi × F1ˆi + F2ˆj + F3 kˆ
= r0 hF1 ˆi ׈i + F2 ˆi ׈j + F3 ˆi × kˆi
= r0 F2 kˆ − F3 ˆj
002 10.0 points
A horizontal bar with a mass m suspended
from one end is held by a cord with tension
T, fastened at a distance D from the end
of the bar that is attached to a wall with a
frictionless pin.
m
D
θ
Tension T
Pin
What is the torque about the pin exerted
on the bar by the cord?
1. D T (1 − sin θ)
2. D T (1 − cos θ)
3. D T sin θ
4. D T cos θ correct
5. D T tan θ
6. D T (1 − tan θ)
Explanation:
The distance from the pin to the cord is
D cos θ, so the torque is
T × D cos θ = D T cos θ .
003 10.0 points
A uniform, rigid rod of mass M and length L
is pivoted frictionlessly at its upper end. If
the rod is dropped from an initially horizontal
position, it swings freely through a vertical
position, as shown.
A
B
C
b D
In which of the four positions illustrated
is the net torque about the pivot on the rod
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