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Topic: Forces and Motion

Content Statement:  There are different types of potential energy.

Concept:  Gravitational potential energy is associated with the mass of an object and its height above a reference point (e.g., above ground level, above floor level).  A change in the height of the object is evidence that the gravitational potential energy has changed.

Concept:  Elastic potential energy is associated with how much an elastic object has been stretched or compressed and how difficult such a compression or stretch is.  A change in the amount of compression or stretch of an elastic object is evidence that the elastic potential energy has changed.

Concept:  Chemical potential energy is associated with the position and arrangement of the atoms within substances.  Rearranging atoms into new positions to form new substances (chemical reaction) is evidence that the chemical potential energy has most likely changed.  The energy transferred when a chemical system undergoes a reaction is often thermal energy.

Concept:  Electrical potential energy is associated with the position of electrically charged objects relative to each other and the amount of charge they have.  A change in the position of charged particles relative to each other is evidence of a change in electrical potential energy.

Concept:  Magnetic potential energy is associated with the position of magnetic objects relative to each other.

 

Content Statement:  Forces have magnitude and direction.

Concept:  Motion can be described in different ways by different observers (e.g., a pencil held in someone’s hand may appear to be at rest, but to an observer in a car speeding by, the pencil may appear to be moving backward).

Concept:  A force is described by its strength (magnitude) and in what direction it is acting. Many forces can act on a single object simultaneously. The forces acting on an object can be represented by arrows drawn on an isolated picture of the object (a force diagram). The direction of each arrow shows the direction of push or pull. When many forces act on an object, their combined effect is what influences the motion of that object. The sum of all the forces acting on an object depends not only on how strong the forces are, but also in what directions they act. Forces can cancel to a net force of zero if they are equal in strength and act in opposite directions. Such forces are said to be balanced. If all forces are balanced by equal forces in the opposite direction, the object will maintain its current motion (both speed and direction). This means if the object is stationary, it will remain stationary. If the object is moving, it will continue moving in the same direction and at the same speed. Such qualitative, intuitive understandings and descriptions of inertia must be developed through inquiry activities.

Concept:  Kinetic friction is a force that occurs when two objects in contact interact by sliding past one another. Drag is a force that opposes the motion of an object when an object moves through a fluid (e.g., gas, liquid). Kinetic friction and drag affect the motion of objects and may even cause moving objects to slow to a stop unless another force is exerted in the direction of motion. This phenomenon leads to the misconception that objects require a sustained force to continue moving. Experimentation with objects that have limited friction (e.g., a puck on an air hockey table, dry ice on a surface) can address the misconception that objects with a net force of zero naturally slow down.

Concept:  If the forces are not balanced, the object’s motion will change, either by speeding up, slowing down or changing direction. Qualitative, intuitive understandings of the influence of unbalanced forces on objects must be developed through inquiry investigations.

 

Content Statement:  Forces between objects act when the objects are in direct contact or when they are not touching.           

Concept:  A field model can be used to explain how two objects can exert forces on each other without touching. An object is thought to have a region of influence, called a field, surrounding it. When a second object with an appropriate property is placed in this region, the field exerts a force on and can cause changes in the motion of the object.

Concept:  Electric fields exist around objects with charge. If a second object with charge is placed in the field, the two objects experience electric forces that can attract or repel them, depending on the charges involved. Electric force weakens rapidly with increasing distance.

Concept:  Magnetic fields exist around magnetic objects. If a second magnetic object is placed in the field, the two objects experience magnetic forces that can attract or repel them, depending on the objects involved. Magnetic force weakens rapidly with increasing distance. Magnetic field lines can be seen when iron filings are sprinkled around a magnet.

Concept:  Gravitational fields exist around objects with mass. If a second object with mass is placed in the field, the two objects experience attractive gravitational forces toward each other. Gravitational force weakens rapidly with increasing distance.

Concept:  Every object exerts a gravitational force on every other object with mass. These forces are hard to detect unless at least one of the objects is very massive (e.g., sun, planets). The gravitational force increases with the mass of the objects, decreases rapidly with increasing distance and points toward the center of objects. Weight is gravitational force and is often confused with mass. Weight is proportional to mass, but depends upon the gravitational field at a particular location. An object will have the same mass when it is on the moon as it does on Earth. However, the weight (force of gravity) will be different at these two locations.

Concept:  Electricity is related to magnetism. In some circumstances, magnetic fields can produce electrical currents in conductors. Electric currents produce magnetic fields. Electromagnets are temporary magnets that lose their magnetism when the electric current is turned off. Building an electromagnet to investigate magnetic properties and fields can demonstrate this concept.

Concept:  Generators convert mechanical energy into electrical energy and are used to produce electrical energy in power plants. Electric motors convert electrical energy into mechanical energy. Motors are in blenders and washing machines. Both motors and generators have magnets (or electromagnets) and a coil of wire that creates its own magnetic field when an electric current flows through it.