Reflection: Checks for Understanding Gravity (Part 3) - Section 5: Explain


Getting students to be able to understand and explain the inventory of forces acting on an object as it is moving up or down a slope can be challenging for middle school students. There are two unknown forces that they encounter. the first opposes the normal force acting away from the ramp and that is a perpendicular force underneath the objects.

While not an actual force it is consistent with one of Newton's laws and that is equal and opposite and helps to explain the presence of the normal force. The other force which isn't necessarily an actual force, is opposing friction.  

Because the object is making contact with the surface friction exists. As the ball rolls down the hill experiences friction in the opposite direction. So again, consistent with Newton's laws of people in opposite forces, what then is the force opposite friction? We call this a parallel force as the forces parallel to the service the ball is rolling down.

My students struggle making sense of both of these, and we do our best to work through them. As my focus is generating a conceptual understanding of the physics a force in motion I don't get too concerned that I have established deep understanding particular with the mathematics that are involved.

  The challenge of explaining motion on a slope
  Checks for Understanding: The challenge of explaining motion on a slope
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Gravity (Part 3)

Unit 7: Movement of Objects in Space & Time
Lesson 10 of 12

Objective: SWBAT explain the forces that act on an object on a slope and the acceleration that results and create a model representing motion on a slope in a velocity over time graphs.

Big Idea: Motion on a slope can be difficult to explain. This lesson makes it easy to understand.

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