1.    Which type of force can be a motive force relative to the system - internal or external?  Which can be a resistive force.

Only external forces relative to the system can be motive or resistive forces acting on that system.

4.    Consider the following forces that act on a system (be sure to define the system) and cite examples.

a.    Muscular tension serves as a motive force when it shortens to cause a bone to move; muscle that exerts tension during lengthening serves as a resistive force on the bone cause to move by some motive force.  Note: muscle forces can be external forces only when the system is a bone or bones

b.    Friction serves as a motive force when it is generated by surface or fluid external to the system and acts in a propulsive manner.  For example, the ground reaction friction force acting forward on the sole of the shoe in running causes the body to be moved forward.  If the external friction force acts to slow the system, or it works in opposition to some motive force, it is a resistive force; for example, the ground reaction friction force acting backward on the shoe as a runner slows to a stop.

c.    Gravitational force, or the weight of a system, will always be a motive force if the system is moving downward; for example, it causes objects to increase speed in falling.  The weight of an object is always a resistive force if the system is moving upward; for example, boy weight is a resistive force opposing the upward ground reaction force acting on the body of a jumper pushing against the ground.

d.    If the flow of air, such as a wind, moves past an object and causes the object to increase its speed or change direction, the air flow is a motive force.  Anything moving through air exerts forces on the air particles and, therefor, receives reaction forces from the air, which resist the object's motion through it (aerodynamic drag force is discussed in detail in Concept Module K and Chapter 13).

e.    Water is a fluid, as is air, and can serve to propel an object in a current, or resist a body moving through it, for the same reasons explained in question d (hydrodynamic drag force will be discussed in detail in Concept Module K and Chapter 14.)

f.    Any push, pull, blow, rub, etc., serves as a motive force if it causes an increase in speed or change in direction, and serves as a resistive force if it opposes the object's motion or resists the effects of a motive force.

5.    Differentiate among these three "pairs" of forces by giving an example that clarifies each pair: internal and external, action and reaction, motive and resistive.

Internal and external force are relative to the system.  Muscle and joint forces are internal forces relative to the whole body as the system, whereas the triceps muscle would exert an external force on the forearm if the forearm is the system.

Only external forces can changed the motion of the system being examined.

Action and reaction forces occur simultaneously on and by the two (or more) systems interacting.  The wall exerts reaction force on a person pressing on it (action force).

Motive and resistive forces are external forces acting on a system.  Motive forces act to to accelerate; resistive forces act against the motive force.  The "winning" force is called the motive force if opposing forces are also acting on the system.  The net external force acting on a system is the motive force - the resistive force.

6.    Describe several examples of Newton's action-reaction law.

For every force applied by a body on another, the other applies an equal and opposite force back.  The earth pulls down on your body with a force equal to your body weight and your body pulls up on the earth with the same amount of force.  If you apply your body weight to the floor, the floor applies the same amount of force up on your body.  If you apply a force of 100N to a door, the door applies a force of 100N back on you.