**Goal:** Understanding action-reaction forces.

**Source:** UMPERG

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:

## 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.