A2L Item 071
- Description: How many forces do work on a block.
- Goal: Recognize forces that do work, that is those with associated displacement.
- Source: UMPERG-ctqpe52
- Keywords: Displacement, Forces, Friction, Mechanics, Work
The question for students:
A block having mass m moves along an incline having friction as shown in the diagram above. The spring is extended from its relaxed length. As the block moves a small distance up the incline, how many forces do work on the block?
- One force
- Two forces
- Three forces
- Four forces
- Five forces
- Six forces
- Seven forces
- None of the above
- Cannot be determined
Commentary for teachers:
Answer
(4) Four forces do work on the block: gravitation, rope, spring, kinetic friction (because you are told the block moves). The normal force does no work.
Background
Recognizing those forces that do work is an important skill for students to master. They also need to recognize whether the work is positive or negative.
Questions to Reveal Student Reasoning
As the block moves up the plane, which forces do positive work? negative work? How are you determining which it is? How would your answer to the above question change if the spring were compressed rather than extended.
Suggestions
Set up some situations with blocks, springs and ropes and let students practice identifying all the forces doing work. This is a good activity to do in conjunction with drawing free body diagrams.