|Directions: Read the following text and answer the questions.
Newton's Laws of Motion
Newton's first law states that every object remains at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. This is normally taken as the definition of inertia. The key point here is that if there is no net force acting on an object (if all the external forces cancel each other out) then the object maintains a constant velocity. Prior to Newton's investigation, people believed that one needed to constantly apply a force to maintain a constant velocity. Newton's law states quite the opposite; no force is needed to maintain a constant velocity. Forces produce a change in velocity not the velocity itself. If all external forces are balanced and the velocity is zero, then the object remains at rest. If an external force is applied, the velocity changes because of the force.
The second law explains how the velocity changes. The law defines a force to be equal to change in momentum (mass times velocity) per change in time. For an object with a constant mass, the second law can be more easily expressed as the product of an object's mass and it's acceleration.
F = m * a
Acceleration is the change in velocity with change in time. For a constant external applied force, the acceleration is inversely proportional to the mass. For the same force, a lighter object has a higher acceleration than a heavier object. A force causes a change in velocity; and likewise, a change in velocity generates a force. The equation works both ways.
The third law states that for every action (force) in nature there is an equal and opposite reaction. In other words, if object A exerts a force on object B, then object B also exerts an equal force on object A. Note that the forces are exerted on different objects. The third law can be used to explain the generation of lift by a wing and the production of thrust by a propeller.