Science Assessment

Students should know that:

1. Objects that are zero distance from the reference point (i.e. the surface of the earth) have no gravitational potential energy.
2. The gravitational potential energy of an object is related to both the distance the object is above the surface of the earth and the mass of the object, and the amount of gravitational potential energy an object has can be determined from these two factors alone.
3. When an object is lifted above the surface of the earth, the change in its gravitational potential energy is related to the change in the height, not to the path taken to get there. Another way of saying this is that the amount of gravitational potential energy an object has is independent of its history (e.g. the path the object takes to get to the height above the reference point). 
4. For objects that have the same mass, the object farthest from the surface of the earth will have the most gravitational potential energy and the object closest to the surface of the earth will have the least gravitational potential energy.
5. For objects that are equal distances from the surface of the earth (greater than zero), the object with the greatest mass will have the most gravitational potential energy, and the object with the least mass will have the least gravitational potential energy.

Boundaries:

1. The distance used in the assessment items is the distance above the surface of the earth unless a substitute reference plane, such as the floor, is explicitly stated in the assessment item.  When this is the case, the reference plane is chosen so that there will be only positive values for the distance.
2. Assessment items are limited to systems involving the earth.
3. Students are not expected to know the difference between “weight” and “mass.”  All of the contexts used in the assessment items are ones where “mass” and “weight” are proportional to each other.  The earth is used as the context for all assessment items aligned to this idea.
4. Assessment items use the phrase “gravitational potential energy” and not “gravitational energy” because students often confuse “gravitational energy” with “gravitational force.”
5. Assessment items expect students to compare relative distances and masses to determine relative amounts of gravitational potential energy. Students are not expected to calculate the exact amount of gravitational potential energy.

NOTE: Any plane on which all points are essentially equidistant from the center of the earth, such as the floor of a room, can be used as a substitute for the center of the earth in determining the amount of gravitational potential energy an object has. Because all points on this reference plane are considered to be equidistant from the center of the earth, all objects on the plane can be considered to have zero gravitational potential energy. (There will be only one reference plane that applies to all objects in the situation.) In assessment items the “reference plane” will be referred to as a “reference point.”