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A2L Item 023

Meta-Information
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.
Level: 
High School
Level: 
Undergraduate
Keywords: 
Forces
Keywords: 
Friction
Keywords: 
Mechanics
Keywords: 
Problem Solving
Keywords: 
Vectors
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?

  1. 0N
  2. 2N
  3. 5N
  4. 10N
  5. 15N
  6. 20N
  7. 32N
  8. None of the above
  9. 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.