**Goal:** Develop good problem solving practices. Determining the value of procedure forces, those requiring use of the 2nd law.

**Source:** UMPERG

A child is walking along the sidewalk at a constant speed of 1 m/s while

pulling his dog sitting in a wagon. The dog has a mass of 30kg and the

wagon weighs 50N. If the child pulls the wagon with a force of 60N at an

angle of 30°, what is the frictional force exerted by the wagon on

the dog?

- 0N
- 2N
- 5N
- 10N
- 15N
- 20N
- 32N
- None of the above
- Cannot be determined

## Commentary:

## Answer

(1) The dog is moving at a constant speed, presumably in a straight

line. The net force on the dog must be zero. Since there are no other

possible horizontal forces (if we ignore air resistance) other than the

friction force, the friction force must be zero.

## Background

This problem provides students with a lot of information. The challenge

of the problem (and most real problems) is to come to an understanding

of the situation independent of the specific details. Then based on an

understanding of the situation one can attempt to address specific

questions and make use of detailed information.

## Questions to Reveal Student Reasoning

Ask students to describe the situation. Ask them to describe how they

approached the problem. Did you describe all the forces? Did you draw

any free-body diagrams?

## Suggestions

Have students draw a free-body diagram (drawing all possible forces) for

the dog. Have them describe the motion for the dog. Ask them to

re-answer the question.