A2L Item 023
- Description: Early problem of child pulling dog in wagon with constant velocity.
- Goal: Develop good problem solving practices. Determining the value of procedure forces, those requiring use of the 2nd law.
- Source: UMPERG
- Keywords: Forces, Friction, Mechanics, Problem Solving, Vectors
The question for students:
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 for teachers:
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.