The Wall Push

---

The Premise

To understand where the normal reaction force comes from and how it can seemingly match the magnitude of any incidence force, it is helpful to think of surfaces as "squishy" and "flexible". This demo is based off of the amazing blog post by Marc Reif on fysicsfool. I really just wanted to include this on my website as a way to route as many people to Marc's awesome walkthrough because it has totally changed the approach that I use to introduce the normal force. Marc's description of the build up to the demo is much better than mine so you should check it out right now.

Click here to read "A Physics Teacher's Strongman Trick"

The general idea is that all surfaces flex when they impart a normal force but for rigid surfaces like a brick wall, it's impossible to see it by just staring at the wall when someone pushes. This demo uses a rod attached to the wall that rolls a small dowel to turn a mirror and deflect a laser point when the wall is pushed. It's a pretty cool set up to walk students through and makes for an awesome introduction to the topic that all students can see (and even try for themselves).

---

Materials

• Balloon/Sponge/Foam/Meter Stick – Model to show deformation or flex when an object is placed on it
• Laser – Any laser works for this. A red laser is recommended to avoid any excess danger if the beam gets unintentionally deflected. It will also need to remain on so it should have a steady on feature or a button that can be compressed by the test tube holder.
• Ringstand and Test Tube Holder – This will hold the laser and direct it toward the mirror. This can be set up on a table or the floor.
• Small mirror and dowel/skewer – the mirror works best if it’s around 1-inch across. The dowel/skewer should be as small as possible to maximize the amount of rotation experienced by the mirror. This means it should be smaller than a pencil or the movement won’t be noticeable enough.
• Rigid Metal Pole – This will be affixed to the wall. A pole from a ring stand or a copper pipe works well for this.
• Mounting Putty – This is used to stick the pole to the wall and the mirror to the dowel/skewer
• Solid Wall or Column – Should be easy to access to support the set up and allow space to push
• Separate table/chair – This separate surface is used to support the mirror/dowel apparatus. This cannot be a counter that is also fixed to the wall because it wouldn’t show the relative motion of the wall. 

---

Setup

There are several different ways that you could get this demo to work. In the video above, I used a wall and a table and in Marc's post, he uses a beam and a chair. Just make sure that the laser and mirror are on surfaces that are not attached to (and moving with) the wall that you are pushing on and you should be good to go. I also recommend playing around with your laser and mirror set up so that the laser point ends up in a place that the entire class can see. Even though this apparatus is in the corner of my classroom, the point that gets deflected is on the ceiling in the very middle of the room.

---

Discussion Questions

I do this in my classroom as a full class introduction but there are still some opportunities for discussion in groups as we work through this demonstration.

1. Place a heavy object and a light object on the table. How do you know that there is a upward force (normal force) from the table on the object?
2. When an object is placed on a clearly flexible or “squishy” surface like a balloon, sponge, or meter stick, the surface deforms.
   
   1. ​How does the weight of the object affect the amount of deformation of the surface? 
   2. How does the amount of deformation relate to the normal force from the surface? 
3. In the “Strongman” demonstration, a clever apparatus makes the deformation of a “solid” wall visible. Describe how this evidence relates to normal force.

---

Extension

If students really want to get into it, they can use the deflection of the laser, distance to the ceiling, and the radius of the dowel to calculate how much the wall actually moved ;) See this article for more details.

---

CLICK HERE FOR MORE DEMOS

---

​Click for more Waves resources ​⬇