1. Define mechanical equilibrium.

2.  What is “equilibrium”?  Equilibrium is a condition in which all acting influences are cancelled by others, resulting in a stable, balanced or unchanging system.

3.  Mechanical equilibrium is when the sum of all the forces acting on an object is zero, and the object’s motion does not change.  This rock is in mechanical equilibrium; the sum of the forces on it is zero. Upward push from hand Downward pull from Earth

4.  Mechanical equilibrium is expressed mathematically as:  The Σ stands for “the sum of”  F is the variable that represents forces.  So, in mechanical equilibrium, the sum of the forces is equal to zero … in all directions.

5.  The sum of the upward forces equals the sum of the downward forces.  F = 0, and the scaffold is in equilibrium.

9.  An object under the influence of only one force cannot be in mechanical equilibrium.  Only when there is no force at all, or when two or more forces combine to zero, can an object be in mechanical equilibrium.

10.  The state of rest is only one form of mechanical equilibrium.  An object moving at constant speed in a straight-line path is also in a state of equilibrium.  Once in motion, if there is no unbalanced force to change the state of motion, it is in mechanical equilibrium.

11.  When the push on the car is the same as the force of friction between the car and the ground, the sum of the forces is zero and the car moves at an unchanging speed.

12.  If the car moves steadily at constant speed, without change in its motion, it is in equilibrium.  In this case, Σ F = 0 means that the force of friction is equal in magnitude and opposite in direction to the pushing force.

13. There are two types of mechanical equilibrium.  Static Equilibrium: the object is not moving.  Dynamic Equilibrium: the object is moving with a constant, non-zero velocity (same speed and same direction).

14.  An airplane in constant, level flight  A car driving on cruise control on a perfectly straight road.  A train moving on straight tracks at a constant speed.  A boat moving at a constant speed in a constant direction  A skydiver falling at terminal velocity.

16. If the gymnast hangs with her weight evenly divided between the two rings, how would scale readings in both supporting ropes compare with her weight? A. Each would read half her weight B. Each would read less than half her weight C. Each would read more than half her weight D. Not enough information given

17. Suppose she hangs with slightly more of her weight supported by the left ring. What must be true for the rope on the right? A. It will have the same tension as the left rope. B. It will have more tension than the left rope. C. It will have slightly less tension than the left rope. D. The tension in the right rope is unrelated to the tension in the left rope - it could be any value.

18. When is an object in mechanical equilibrium? A. When it has no forces acting on it B. When it has only one force acting on it C. When it has only two forces acting on it D. When the net force is less than the object’s weight E. When the net force is zero

19. An object is moving at a constant velocity of 12 m/s in a straight line. How would you describe its condition? A. Chemical equilibrium B. Dynamic equilibrium C. Punctuated equilibrium D. Static equilibrium E. Thermal equilibrium