Summary

• Rolling motion
• Rotational energy
• Moment of inertia
• Torque

• Reading Quiz



• Newton's second law
  • derivation for rotation
  • moment of inertia
    Example #1
    
  • torque & rotation applet
  • net torque & acceleration applet
  
  
• Static equilibrium
  • 
  • examples
    Example #2
    
  
  
  
  
  
  
• Stability & balance [optional]
  • stable equilibrium
  • unstable equilibrium
  • neutral equilibrium
    Example #3
    
    Example #4
    
  
  
  

Walker4e
Reading quiz What is the torque caused by the tension T about the elbow?
A. L T sin 40.0°
B. L T cos 40.0°
C. L T tan 40.0°
D. L T
Answer















sj6 10.38
A 0.50 m-radius, 100 kg potter's wheel freely rotates at 50 rev/min. The potter stops it in 6.0 s by pressing inwards on the rim with 70 N force. What is μ_k between potter and wheel?
A. 0.156
B. 0.227
C. 0.312
D. 0.533
Answer















Wlk4 11.37
The figure below shows the forces acting on a sprinter's foot just before she takes off at the start of the race. Find the magnitude of the force exerted on the heel by the Achilles tendon, F_H, and the magnitude of the force exerted on the foot at the ankle joint, F_J.

A. F_H = 169 N, F_J = 507 N
B. F_H = 446 N, F_J = 223 N
C. F_H = 330 N, F_J = 534 N
D. F_H = 884 N, F_J = 722 N
Answer















sj6 12.24
Two identical, uniform bricks of length L are stacked as shown. What is the maximum x without falling?
A. L
B. 3L/4
C. L/2
D. L/3

Answer















gc6 9.36
The Leaning Tower of Pisa is 55 m tall and about 7.0 m in diameter. The top is 4.5 m off center. How much farther can it lean before it becomes unstable?
A. 1.0 m
B. 2.5 m
C. 3.5 m
D. 4.5 m
Answer















 



A. L T sin 40.0°

Only the component of the tension T that is perpendicular to the moment arm L will produce a torque. That component is T_y, and it is the side opposite to the angle, so T_y = Tsin 40°.





















 



C. 0.312





















 



C. F_H = 330 N, F_J = 534 N























 



B. 3L/4
The center of mass of the first brick must be above the point of support, and the center of mass of the two-brick combination must also be above its point of support.




















 



B. 2.5 m
The top would have to move one diameter in order to put the center gravity, assumed to be halfway down, over the pivot point of one side of the base. It can therefore move 7.0 m − 4.5 m = 2.5 m further.