**Goal:** Hone the vector nature of force and interrelate model and procedure forces.

**Source:** UMPERG

A marble rolls on to a piece of felt and slows down.

Indicate the direction that most nearly corresponds to the direction of

the force that the marble exerts on the felt. If none of the directions

are appropriate, or if the answer cannot be determined, respond (9).

## Commentary:

## Answer

(3) The force the felt exerts on the marble is up (normal force) and to

the left (friction force). Newton’s third law tells us that the force

the marble exerts on the felt must be down and to the right. Students

may focus on the normal force alone (4) or the friction force alone (2).

These are not two forces, but the components of a single force.

Students also find it difficult to extract some information from the

dynamical statement “slows down” and integrate this with the familiar

normal force.

## Background

This presents an interesting twist to students. The friction force is

usually formulated in terms of a moving object and a fixed surface.

Students may not know for sure whether there is a friction force on the

felt – the felt is not moving. The analysis on the marble is reasonably

straightforward. Newton’s third law can be used to determine the force

on the felt if the force cannot be determined from the situation

directly.

## Questions to Reveal Student Reasoning

Question students about how they got their answer. Did they use the

force laws that they learned previously? Did they use Newton’s second

or third laws?

## Suggestions

Instead of a marble consider a sliding block and see if students think

differently – some students will have difficulty thinking about friction

with a rolling object.