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 Is always equal to kFN Depends on the acceleration of the bobsled Depends on the velocity of the bobsled. 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, F > fk F < fk 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, F > fk F < fk 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 F > fk F < fk 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). This condition keeps the object from rotating (or uniform rotation, actually). Translational equilibrium 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?

Example Continued… F1 F2 d1 d2 What is the normal force on the seesaw by the fulcrum if the mass of the seesaw is 100 kg?

Hint on HW3 #4 F1 d1 d2 F2 What if F2 acts at an angle? Find F.

Hint on HW3 #3