• 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)
• If a system interacts with objects outside itself, the total momentum of the system can change; however, any such change is balanced by changes in the momentum of objects outside the system. (9-12)

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

• Within a natural or designed system, the transfer of energy drives the motion and/or cycling of matter. (6-8)

• 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.

• 6-8: 4G/M2. The sun's gravitational pull holds the earth and other planets in their orbits, just as the planets' gravitational pull keeps their moons in orbit around them.

Exactly what IS centripetal force and what does it do? An astronaut on board the International Space Station demonstrates this force in ways students cannot observe in daily life. The environment is "almost" weightless, making it easy to observe the center-seeking motion without the complicating effects of gravity.

• 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

Exactly what IS centripetal force and what does it do? This short video shot from NASA's International Space Station will help students understand the center-seeking force that results in circular motion. The environment is weightless, making it easy to watch the motion without the complicating effects of gravity.

• Physical Sciences K-8 Rotational Dynamics Unit
• Physics First Rotational Dynamics Unit
• Conceptual Physics Rotational Dynamics Unit