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Plunging into Forces…and ready for a bruising

February 8, 2012
In my physics class, we go way too fast and jump in way too deep on the first day of forces. Think about it: Our first day with forces jumps right into two-dimensional forces on ramps. This means we have to introduce the concept of force, understand different kinds of forces, draw force diagrams, find the components of forces along angled axes, figure out how the angle of ramp translates to angles in diagram, discuss how and why to sum forces, and finally apply Newton’s 2nd law for x- and y-directions. It’s going to be a mess; I can guarantee it.
I’m very much inclined to spend today’s class discussing and working problems with forces in one dimension. It would be nice to just make sure we have some time to discuss how to identify forces, to consider the mechanisms for normal and friction forces, to grapple with how a single force influences an object, and to introduce the idea that we can think of multiple forces as acting as if it were a single force. I could even foreshadow the need for a clever way to think about how to do this in 2D.
I haven’t quite settled on what I’ll do, but I’ll have to figure it out soon. For context of where my students are at after an hour of lecture and a reading, below are some student responses from my pre-class reading quiz:

I’m having trouble with understanding how forces effect the movement of that object.

How can both force and friction be in the same direction up the ramp and the object still slide down the ramp?

Based on the readings, I don’t grasp of the concepts of the forces that act on an object that is on a ramp.

In lecture today, Professor X was talking about the forces that do or do not act on an object while it’s at an incline and I didn’t understand how to determine whether or not the force is present.

With simulation, sometimes when I kept the applied force the same it kept speeding up.. I don’t get why it did that.

It is not much of a question but more of a wanting to know a deeper understanding on how an object on a hill creates a force of friction without having to move since force tends to be based on some acceleration.

Real Life

How is it possible that a ramp can have a perpendicular upward force?

Is ice the only surface that cancels out friction?  What about a waxed bowling lane or an oiled skating rink?


If an object is on a ramp and we are trying to find the acceleration, will the equation always be g sin or cos of the angle incline?

I don’t understand when the inverse of sin, cos, or tan should be used.

When drawing an FBD to represent the force of something on a ramp how do you determine the angles of the vectors if they are not already given.

When can we ignore the Y axis forces and when can we ignore the X axis forces?

I just want to know how do you figure out or what determines where the object that is being pushed up a ramp will end at the bottom of the ramp.

I really don’t understand how to apply the free body diagram.

2 Comments leave one →
  1. Chris Goedde permalink
    February 8, 2012 3:23 pm

    Hi Brian,

    Starting with blocks on inclines sounds crazy to me. I feel your pain.

    I don’t know the answer, but this student question: “When can we ignore the Y axis forces and when can we ignore the X axis forces?” is one reason that I have moved away from relying on 1D dynamics as the gateway to forces. It’s too easy for me to set up a situation where it’s obvious to me as an expert why some forces are ignored but not others. If you restrict yourself to vertical motion it can probably work. You can also do a lot with horizontal constant acceleration (e.g. a box in the back of a truck or a block on a sheet of paper.) I do think there’s something to be said for starting with static situations and balanced forces to introduce FBDs etc. (Is this how some modelers do it?)

    Sorry, no blog yet!

    • February 9, 2012 12:30 pm

      Yeah, it went pretty bad. I should have gone with my gut to scaffold more 1D stuff, or perhaps 2D with all on axis vectors. Or I should have had us just practice drawing lots of FBDs for a variety of situations and relate those FBDs to discussion of balanced or unbalanced forces. Next time.

      I think many modelers do constant velocity, then balanced forces (so not necessarily static), and then constant acceleration, then unbalanced forces.

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