# Day 27: Bridging Analogies & Normal Force

Normal force can be a complicated concept for some students. When a book is resting on a table, students may initially think the table doesn’t push up on the book, but rather “gets in the way” to keep the book from falling.

And we all know that simply telling students that the table pushes up isn’t going to change their thinking, even if they can draw the free-body diagram correctly.

I have found success using a sequence of bridging analogies1:

1. Anchor: A book is placed on a compression spring (like in a mattress). Kids see (or imagine) the spring deform and they intuitively get that the spring pushes up on the book. Remove the book and spring moves back up to its original position.
2. Bridge: A book placed on a wooden meter stick (see photo). Kids see the meter stick deform and when the book is removed, it returns to its unflexed state. Kids get that the meter stick must push up on the book, like in the Anchor.
3. Bridge: Add additional meter sticks to support the book. The deflection is less, but the meter sticks are still pushing up.
4. Target: A book resting on a table. The deflection is unnoticable (microscopic) but is still there. The ball and spring model of a solid (on the cover of the textbook in the photo) is very useful here.
5. Experiment: A small mirror is laying flat on a table and reflects a laser beam to a spot on the wall. As a student walks along the table, the laser spot moves, showing the shifting of the mirror and thus the deflection of the table.

1The must read article about bridging analogies: Using Bridging Analogies and Anchoring Intuitions to Deal with Students’ Preconceptions in Physics (John Clement)

HS Physics Teacher constantly questioning my teaching.

### 3 responses to “Day 27: Bridging Analogies & Normal Force”

1. Charles Tuttle says :

I just did this today…but just a little different. I had a student push down on the stick and then I used the stick to push up on her finger. Then I put a 500g mas on the stick. I also used a stack a four, foot-feet long 1x4s and had a student sit on them…removed one…removed another…made two stacks of 2..etc. Then I had a student walk up a 2×4 at various inclines…the 2×4 doesn’t smile as much when the angle is large…small smile, small push up. Takes a little silly convention about the "min of the chin for a smiling parabola"

2. Anonymous says :

I really like the standing on a 2×4 on an incline! A great visual for how the normal force changes with ange.