**Goal:** Reasoning using the 2nd law.

**Source:** UMPERG

A tow truck (2,000kg) pushing a car (1000kg) experiences an average

friction force of 13,000N while accelerating from rest to a final

velocity of 36 mi/hr (16 m/s). The air and the road exert an average

resistive force of 1,000N on the car. What force does the car exert on

the tow truck?

- 1,000N
- 2,000N
- 4,000N
- 5,000N
- 6,000N
- 12,000N
- 14,000N
- None of the above
- Cannot be determined

## Commentary:

## Answer

(4) The net force on the car and tow truck is 12,000N (13,000N –

1,000N). The acceleration is 4m/s2. The magnitude of the force between

the two vehicles is 5,000N.

## Background

Answers are not as important as approach. What did students do to

understand the physical situation? Did they draw pictures. Did they

draw a free-body diagram.

## Questions to Reveal Student Reasoning

Ask a couple of students to describe how they approached the problem.

Ask them to describe the steps they took without getting into

mathematical details. For example, did they draw a free-body diagram?

What forces did they consider? What system did they analyze?

## Suggestions

After a couple of descriptions of how to approach solving the problem,

work through the problem with help from the class.