Download Chapter 9: Rotational Dynamics

Static Equilibrium
 In Chap. 6 we studied the equilibrium of pointobjects (mass m) with the application of Newton’s
Laws
F
x
 0,
F
y
0
 Therefore, no linear (translational) acceleration,
a=0
 For rigid bodies (non-point-like objects), we can
apply another condition which describes the lack of
rotational motion
  0
 If the net of all the applied torques is zero, we
have no rotational (angular) acceleration, =0
(don’t need to know moment of inertia)
 We can now use these three relations to solve
problems for rigid bodies in equilibrium (a=0, =0)
Example Problem
The wheels, axle, and handles of a wheelbarrow
weigh 60.0 N. The load chamber and its contents
weigh 525 N. It is well known that the wheelbarrow is much easier to use if the center of
gravity of the load is placed directly over the axle.
Verify this fact by calculating the vertical lifting
load required to support the wheelbarrow for the
two situations shown.
FL
FD
L1
FL
FD
Fw
L2
Fw
L2
L3
L3
L1 = 0.400 m, L2 = 0.700 m, L3 = 1.300 m
 First, draw a FBD labeling forces and lengths
from the axis of rotation
FL
FD
Choose a direction for the
rotation, CCW being
axis
Fw
positive is the convention
L
a)
  0
 D W  L  0
1
L2
L3
 FD L1  FW L2  FL L3  0
FD L1  FW L2
FL 
L3
(525 N)(0.400 m)  (60.0 N)(0.700 m)
FL 
1.300 m
FL  194 N
Apply to case with load
over wheel
 Torque due to dirt is zero,
since lever arm is zero
axis
b)
  0
 D W  L  0
FL
FD
Fw
L2
 FD L1  FW L2  FL L3  0
FD L1  FW L2
FL 
L3
(525 N)(0 m)  (60.0 N)(0.700 m)
FL 
1.300 m
FL  32.3 N
 Who? What is carrying the balance of the
load?
 Consider sum of forces in y-direction
F
0
FL  FD  FW  FN  0
FN  FD  FW  FL
FN  525  60  194  391 N
FN  525  60  32.3  553 N
FD
y
a)
b)
FL
Fw
FN
•
We did not consider the Normal Force when
calculating the torques since its lever arm is
zero
Center of Gravity
FD
 The point at which the weight of a rigid body
can be considered to act when determining the
torque due to its weight
 Consider a uniform rod of length L. Its center
of gravity (cg) corresponds to its geometric
L
center, L/2.
L/2
cg
 Each particle which makes up the rod creates a
torque about cg, but the sum of all torques due
to each particle is zero
 So, we treat the weight of an extended object
as if it acts at one point
 Consider a collection of point-particles on a
massless rod
 The sum of the torques
Mg
xcg
m1 gx1  m2 gx2 m g
1
m2g m3g
 m3 gx3  Mgxcg
x2
M  m1  m2  m3
x1
m1 x1  m2 x2  m3 x3
 xcg 
 xcm
M

x3