A2L Item 040
- Description: Compare forces in the case of a hammer hitting a nail into wood.
- Goal: Understanding action-reaction forces.
- Source: UMPERG
- Keywords: Dynamics
The question for students:
A hammer strikes a nail driving it into a piece of wood. Which statement below is true about the forces exerted during the impact?
- The nail exerts a larger force on the wood than the wood does on the nail.
- The wood exerts a larger force on the nail than the nail does on the wood.
- The force that they exert on each other is the same size.
- One of the two forces is larger, but which is larger can’t be determined unless more information is provided.
- None of the above.
- Cannot be determined.
Commentary for teachers:
Answer
(3) The forces are the same size (according to Newton’s third law).
Background
Students’ natural inclination in situations like this is that a moving object is a more active agent and therefore exerts a larger force, while a stationary object is the more passive agent and exerts a smaller force. Students also look at effects: the object that has the largest change in motion has experienced the largest force.
Questions to Reveal Student Reasoning
How can you determine which object experiences the larger force? What are some of the clues? Do we have any way to relate the effects we observe to the size of the forces each object experiences?
Suggestions
Newton’s third law, while easily memorized as a principle, is hard to develop as an intuition and to employ in reasoning about situations. There is no single experience that can help. One needs to revisit the third law often in many different contexts.
There are many situations one can use with students. Try a moving block with a spring colliding with a wall (or another block that is stationary). In this situation one can use the spring law to help relate the forces.