• Each force acts on one particular object and has both strength and a direction. An object at rest typically has multiple forces acting on it, but they add to give zero net force on the object. Forces that do not sum to zero can cause changes in the object's speed or direction of motion. (Boundary: Qualitative and conceptual, but not quantitative addition of forces are used at this level.) (3)
• The motion of an object is determined by the sum of the forces acting on it; if the total force on the object is not zero, its motion will change. The greater the mass of the object, the greater the force needed to achieve the same change in motion. For any given object, a larger force causes a larger change in motion. (6-8)
• Newton's second law accurately predicts changes in the motion of macroscopic objects. (9-12)

• Cause and effect relationships may be used to predict phenomena in natural or designed systems. (6-8)

• A system can be described in terms of its components and their interactions. (3-5)

• Structures can be designed to serve particular functions. (6-8)

• Knowledge of relevant scientific concepts and research findings is important in engineering. (3-4)

• Science affects everyday life. (3-4)

• 6-8: 4B/M3. Everything on or anywhere near the earth is pulled toward the earth's center by gravitational force.

• 3-5: 4F/E1bc. The greater the force is, the greater the change in motion will be. The more massive an object is, the less effect a given force will have.
• 6-8: 4F/M3a. An unbalanced force acting on an object changes its speed or direction of motion, or both.
• 6-8: 4F/M3b. If a force acts towards a single center, the object's path may curve into an orbit around the center.
• 9-12: 4F/H1. The change in motion (direction or speed) of an object is proportional to the applied force and inversely proportional to the mass.
• 9-12: 4F/H2. All motion is relative to whatever frame of reference is chosen, for there is no motionless frame from which to judge all motion.

This short video does a great job of demonstrating centripetal force and how it acts to keep objects moving along a curved path. What makes a rider on a roller coaster feel a sensation of being thrown outward from the center during a loop, although there is no outward net force? The video serves to help beginners understand the dynamics of circular motion.

• Physical Sciences K-8 The Case of Roller Coasters Unit
• Physics First The Case of Roller Coasters Unit
• Conceptual Physics The Case of Roller Coasters Unit
• Algebra-Based Physics The Case of Roller Coasters Unit