# A2L Item 028

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. 1,000N
2. 2,000N
3. 4,000N
4. 5,000N
5. 6,000N
6. 12,000N
7. 14,000N
8. None of the above
9. Cannot be determined

### Commentary:

(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.