18 January 2024

Mail Time: how do I have students describe normal and friction forces?

Vanessa asks:

How do you have students list the normal force and friction force on an object experiencing friction? Would both Fn and Ff be described as "the force of the surface on the object"?

Or do you have them specify "the normal force of the surface acting on the object" and "the friction force of the surface acting on the object"?

Just "force of track on cart" or "force of the ground on the cart" or similar, like you said.

I work so hard to get students to avoid excess language (like "the downward force of the earth pulling down on the upward moving cart") that I'd undo that work if I insisted on other language.  The simplicity helps substantially with Newton's 3rd law, for which we just switch the objects experiencing and applying the force.  

The 3rd law force pair to the friction force?  Well, friction is the force of the track on the cart, so the 3rd law pair is the force of the cart on the track.  That easy - but only if the friction force is originally written with this concise language.


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