• 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)

• A system can be described in terms of its components and their interactions. (3-5)
• Models can be used to represent systems and their interactions. (6-8)

• Modeling in 3–5 builds on K–2 experiences and progresses to building and revising simple models and using models to represent events and design solutions. (3-5)
  • Use models to describe phenomena. (5)

• 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.

• 3-5: 11A/E1. In something that consists of many parts, the parts usually influence one another.
• 6-8: 11A/M2. Thinking about things as systems means looking for how every part relates to others. The output from one part of a system (which can include material, energy, or information) can become the input to other parts. Such feedback can serve to control what goes on in the system as a whole.

• 3-5: 11B/E4. Models are very useful for communicating ideas about objects, events, and processes. When using a model to communicate about something, it is important to keep in mind how it is different from the thing being modeled.
• 6-8: 11B/M4. Simulations are often useful in modeling events and processes.

This is a collection of interactive animations that depict how forces and simple machines work together to create the compound machine.  The authors designed it for Grades 3-6 to help children understand how compound machines function and how they are different from simple machines.  It would be a good follow-up to the resource directly above -- Edheads: Simple Machines.

• Physical Sciences K-8 Applications of Newton's Laws Unit