The following table shows the names and brief descriptions of the activities^{1} and the homework extensions in the ‘Force based Model for Interactions’ unit, which is the first unit in the Interactions and Forces module. There are also links to the small and large class versions of three of the activities and the associated homework extension for two of them. The focus of this unit is on developing and using Newton’s Second Law for situations where a single force acts on an object.
The unit that follows this one is the ‘Combination of Forces’ unit, where the idea of a net force is applied and students consider situations where multiple forces act on an object at the same time.
Activity or 
Title 
Brief Description 
UFM Act1 
Interactions and Force 
This activity focuses on two issues: when does a force stop pushing on an object, and when an object is moving, does that mean that there must be a force pushing it in the direction of its motion? Students collect experimental evidence with regard to the first issue and draw conclusions about both issues. At the end of the activity they address the question of what is being transferred during the interaction: force, energy, both or neither? 
UFM Ext A 
Force Diagrams 
This extension activity introduces students to the use of force diagrams, how to draw and interpret them. 
UFM A2 
Motion with a Continuous Force 
Students explore what happens when a constant and continuous force acts on an object. 
Pushing a Skateboarder 
Students interpret and make connections between speedtime and forcetime graphs for an object that is acted on by a force for a certain interval of time. 

UFM A3 
Pushes and Slowing Down 
Students explore what happens to the motion of an object when first an impulsive force and then a continuous force acts on the object in a direction opposite to its motion. 
Connecting Force and Energy Models 
Students compare the energy and force descriptions of an interaction. They compare both energy diagram and force diagram representations, and also energybased and forcebased explanations. 

UFM A4 
Forces and Slowing (Friction) 
In this activity students first discuss whether they think friction is a force, and if so, in what direction does it act. Then they compare the motion of two moving carts after they were given an initial shove to move them in a particular direction: one cart has a fan unit mounted on it that continuously pushes the cart in the direction opposite to its motion; the other cart has an adjustable friction pad attached to increase the amount of rubbing (friction). From these experiments students conclude that friction is a force that acts in a direction opposite to the motion of the object. 
UFM Ext D 
How Does Friction Work? 
Students are introduced to a mechanism (via a sticky notes analogy) to account for friction. They also consider the phenomenon of air drag and its affect on the motion of a car. 
UFM A5 
Changing Force Strength and Mass 
Students explore how the motion of an object is affected by: (1) changing the strength of the force acting on it; (2) changing its mass; and (3) changing both force strength and mass. At the end they have a version of Newton’s Second Law that applies to single forces acting on an object. 
UFM Ext E 
Changing Direction 
Students first explore the affects of sideways taps (impulsive forces applied at right angle to the motion) on a moving object, and then consider the conditions necessary to cause an object to move in a circular path. 
UFM A6 
Falling Objects 
Students explore whether the mass of an object affects how it falls (ignoring air resistance) 
UFM A7 
Engineering Design: Brainstorm ways to achieve new fuel efficiency standards 
Students read about a brainstorming session involving workers for an automobile company who are discussing possible ways of reducing fuel efficiency in the cars they manufacture. The students need to analyze each suggestion in terms of Newton’s Second Law and decide which approach they believe is most promising. 
^{1} The term ‘activity’ is used in the small class version of NextGenPET, and ‘lesson’ is used in the large class version. For simplicity we will just use the term ‘activity’ to mean both.