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Poll
You push a bobsled on ice.
There is a kinetic frictional
force on the bobsled by the
ice. The kinetic frictional force
on the bobsled while you are
pushing it
1. Is always equal to kFN
2. Depends on the acceleration of the bobsled
3. Depends on the velocity of the bobsled.
4. Depends on how hard you push it.
Poll
Suppose that you push
horizontally with a force F on
the bobsled. There is also a
frictional force fk on the
bobsled. It moves in the +x
direction. If it is speeding up,
1. F > fk
2. F < fk
3. F = fk
Poll
Suppose that you push
horizontally with a force F on
the bobsled. There is also a
frictional force fk on the
bobsled. It moves in the +x
direction. If it is slowing down,
1. F > fk
2. F < fk
3. F = fk
Poll
Suppose that you push
horizontally with a force F on
the bobsled. There is also a
frictional force fk on the
bobsled. It moves in the +x
direction. If it has a constant
velocity
1. F > fk
2. F < fk
3. F = fk
Center of Mass
Center of Mass
Equilibrium
If the net force on an object is zero, then the center of
mass of the object does not accelerate. This is called
translational equilibrium.
See-saw
Torque
F1
d1
Torque  = Fd where d is called the moment arm.
If F causes counterclockwise rotation, then the
torque by F on the object is positive.
If F causes clockwise rotation, then the torque by B
on the object is negative.
Equilibrium
This condition keeps the
object’s center of mass at
rest (or uniform motion,
actually).
Translational
equilibrium
This condition keeps the
object from rotating (or
uniform rotation, actually).
Rotational
equilibrium
Example
F1
d1
A child of mass 30 kg sits at the end of the seesaw at
a distance of 3-m from the center of the seesaw. If a
90-kg adult wants to balance the child (so that the
seesaw does not rotate), where should he sit?
F1
Example Continued…
F2
d1
d2
What is the normal force on the seesaw by the
fulcrum if the mass of the seesaw is 100 kg?
F1
Hint on HW3 #4
d1
What if F2 acts at an angle? Find F.
d2
F2
Hint on HW3 #3