Lect08

# Lect08 - Physics 215 Physics for Elementary Education...

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Physics 215 Physics for Elementary Education Instructor: Dr. Mark Haugan Office: PHYS 282 TA: Mayra Cervantes Office: PHYS 222 TA: Jordan Kendall Office: PHYS 222 TA: Daniel Whitenack Office: PHYS 136 Office Hours: If you have questions, just email us to make an appointment. We enjoy talking about teaching and learning physics! Notices: The TAs are grading the midterm. We will return the exams to you after spring break.

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Interactions and Forces Two boys are pushing a heavy crate straight across a level floor. Q1. Your friend asks whether or not the forces acting on the crate are balanced. Which of the following statements could help you to tell? A) the boys push in the same direction, so, there must be unbalanced forces B) if the crate moves with constant speed, there must be unbalanced forces C) if the crate moves with constant speed, there must be balanced forces d) none of the above will help at all * As you’ve read and we’ve discussed, the idea that an unbalanced force must be acting on something if it is moving in a straight line at constant speed is a common one and difficult to change. Why do you think that is?
In this situation we would certainly expect the crate to stop quickly if the boys stopped exerting their obvious forces (pushes) on it. How do we reconcile what happens when the boys stop pushing with our idea that objects move in straight lines at steady speeds only when no force acts on them or when balanced forces do? Q2. What objects in the crate’s surroundings exert significant horizontal forces on the crate? A) the boys B) the floor C) the boys and the floor D) the boys, the floor and the Earth * Until we carefully examined many situations, the idea that objects often exert frictional forces on each other when they are in contact was not so natural. They aren’t as obvious as forces we exert by pushing and pulling on things. They do become “obvious” on reflection , however. For example, as we try to understand why a crate pushed across an ice surface slows down so much more slowly than one pushed across the floor when we stop pushing.

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Such reflection is the key to learning and to working in science. We are
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Lect08 - Physics 215 Physics for Elementary Education...

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