A2L Item 028
- Description: Analyze the forces acting between a tow truck pushing a car.
- Goal: Reasoning using the 2nd law.
- Source: UMPERG
- Keywords: Dynamics, Forces, Friction, Mechanics, Newton's Third Law, Problem Solving
The question for students:
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 for teachers:
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.