A2L Item 047
- Description: Compare the force on a water balloon in three different situations.
- Goal: Hone the concept of force, recognize the presence of force.
- Source: UMPERG
- Keywords: Forces, Mechanics
The question for students:
A water balloon is shown at rest in three different situations. In each case the water balloon is in contact with a system that supports it.
Which system exerts the largest force on the balloon?
- Spring support in A
- Cardboard and blocks in B
- Table in C
- None of the systems exert a force
- None of the above
- Cannot be determined
Commentary for teachers:
Answer
(5). Each system exerts the same magnitude force on the balloon. The size of the force equals the weight of the balloon.
Background
Context for Use: Give to students before they receive a formal introduction to interactions.
Assessment Issues: (1) What are students’ naive views about interactions? (2) Can students perceive when an interaction is occurring? (3) What factors do students attend to when determining whether a force is present? (4) What factors do students use to compare the relative magnitude of two forces?
Questions to Reveal Student Reasoning
Which systems exert a force on the balloon? How did you decide whether the system exerts a force on the balloon? In each case how did the force affect the balloon? How did the interaction between the balloon and system affect the system? When comparing two forces, how can you tell which force is larger?
Suggestions
Recognizing the presence of an interaction is a difficult problem for students. One must return to the topic repeatedly as additional understanding and principles are learned by the students. Initially focus students on the effects of interaction: (1) Do the motions of the objects change? (2) Do the shapes of the objects change?