Gravity
...interaction. They can then use Newton's laws to determine the motion of objects under a constant force along a straight line. They must be able to identify all forces acting on an object in one-dimensional motion, find the acceleration given the net force, and describe the motion or change in motion. At this level, vectors are not used, but the use of plus and minus to indicate direction serves as a precursor to later use of vectors. The distinction between weight and mass should be addressed. Students should learn that weight on Earth is the force due to gravity that accelerates objects downward. Knowing the acceleration due to gravity on Earth, they are able to calculate the weight of an object as a force in newtons Force must be exerted to extend a spring. Just prior to Newton's great discoveries Robert Hooke determined that the force of a spring is directly proportional to the distance that the spring is extended. This is the famous Hooke's law, that for a perfect spring F = -kx. For many springs this is true over a portion of the extension. If the spring is over-extended then this simple relation breaks down, and the spring becomes deformed. For an atomic bond in a molecule this is the separation into component parts. If the force is known, and the expansion measured then the k value may be determined. The units of k should be Newtons/meter. In order to maintain a constant force, one of several different weights is suspended from the end of the spring. The total elongation of...