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P127. Max/Min Net Force Problems

Goal: In one hour or less, to solve a net force problem with the participation of your assigned group and to prepare a formal written presentation

Strategies to increase efficiency in use of time

  • Assign a leader to keep the discussion on track and coordinate work on the whiteboard.
  • Assign a scribe to prepare a formal solution on paper as the parts of the solution come together on the whiteboard.
  • Start by setting up the problem in the usual way with given, goal, picture, and force diagram. Reduce writing by ignoring all numerical information and solving symbolically.
  • Don't write out units and sign checks, but do make sure that your solution meets those checks.

Requirements for the solution

  • Must clearly state the goal as a maximum or minimum condition on the unknown. Use a phrase.
  • Must be a complete, formal net force solution, done symbolically
  • Must make appropriate use of the static friction inequality. Introduce the inequality at the appropriate time and carry through its use in the solution to obtain the final result in the form of an inequality.
  • Must provide two special case checks as described in the table below. A check should tell i) the result of the substitution and ii) whether the result makes sense.

What to submit

  • At the end of the group session, submit screen captures of the whiteboards by email. The Presenter is the person responsible for submitting these.
  • The scribe scans the formal solution and emails it to the instructor by 5 PM. Make sure the problem number and the names of the group members appear at the top of the solution. Name the file with your group name.

Presentation of final solutions: The instructor will post the solutions on the physics website.

Problem Assignments

Problems are from the end of Ch. 6.

Individuals will be assigned to the groups during the WebEx session. There will be 2 or 3 people per group.

Group Problem Checks required Notes
Curie 66 θ = 0o and 90o Be sure to identify the correct direction of the static friction force for the maximum stretch stretch of the spring.
Bohr 68 θ = 0o and 90o You'll need to do 2 net force problems as part of the solution. One of them will be for the knot joining the strings. The other will be for block A.
Franklin 73 Part b serves as one of the checks. For the other, consider the situation for no friction between the blocks. This is a connected object problem. You'll need to do a net force problem for each block. Let the accelerations of the blocks be a1 and a2 in general. The no slipping condition is a1 = a2.
Planck 79 Consider the situations where i) there is no friction between the box and cart and ii) where the friction approaches an infinite amount. You'll need to decide what to identify as the normal force in fs <= μsN.
Meitner 84b θ = 0o and 90o Find the minimum force needed for the block to remain at rest (ignore the maximum part). Note that it's the direction of fs that determines whether the force pushing up the plane is a maximum or a minimum.
Mach 91 Part b serves for both checks. Note that there are 2 conditions embodied in < or =. Consider each separately. Remember that T contributes to both horizontal and vertical net force equations.

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