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Key Idea: When objects collide, contact forces can transfer energy from one object to another resulting in a change in the objects’ motion.

Students should know that:

  1. When two objects collide, their speeds change compared to their speeds before the collision and energy is transferred from the object that slows down to the object that speeds up. [Note: An exception to this is when two identical objects collide at a 45-degree angle. Assessment items will not assess students’ understanding of the exceptions.]
  2. Energy is transferred by forces as long as one object exerts a force on another object. The transfer of energy stops when one object no longer exerts a force on the other object.
  3. When two forces act over the same distance and in the same direction, the stronger force transfers more energy than the weaker force.


  1. This idea is limited to transfers of energy as a result of direct contact forces and elastic collisions and is limited to systems containing two objects that are moving or that can be moved. Examples of direct contact include when billiard balls hit each other, when a ball is thrown or kicked, when a baseball or golf ball is hit with a bat or club, when an object is set in motion by a rubber band or spring, or when a bobsled or swing is pushed.
  2. Assessment items include contexts where there is a clear transfer of energy (one object will slow down and the other will speed up). Items do not include collisions between objects that are significantly different in mass where the change in speed of the more massive object is negligible.
  3. For direct contact interactions, assessment items may include situations where both objects are moving or one object is moving and the other object is at rest.

No misconceptions are associated with this idea in the selected project.

No NGSS statements are associated with this idea in the selected project.