Physics Demo Number: 061

Approximate

Run Time: 10 min

Third Law Demonstration Involving Steel Plate, Instructor Arm , and Table Flexure

Demo Description

With the setup done as described below, the instructor has only to rest his/her arm on a steel plate and push down to observe a laser dot move upwards on the wall (in the last photo )due to the flexure of the table top in response to increased normal force.

Scientific Principles

You Can't Touch Without Being Touched
Also known as Newton's 3^rd Law

Equipment

The laser assembly
A flat front surface mirror
The 55-lb steel plate from (010)
A lecture room cart
Plastic mirror on attachment fixture on a big tripod stand .

Equipment Location

The laser assembly [C-2-4]
A flat front surface mirror in Kit(061) on [C-1-2]
The 55-lb steel plate from Kit (010), lower shelf of Steel Cart End of E-F Aisle
In Lecture Room or in Prep Room
Plastic mirror on [C-1-3] in Kit (132)
Big tripod stand is at Start of [H] columns

Instructions

The first four components are arranged as shown on the left in the first picture. The plastic mirror on the stand seen on

the right side of the picture can be moved to catch the laser spot being reflected from the mirror on the table. When this is actually done the second picture shows the resulting spot on wall. (Spot is on concrete block above door.)

With the setup done as just described , the instructor has only to rest his/her arm on the steel plate and push down to observe the laser dot move upwards on the wall in the last photo due to the flexure of the table top in response to the increased normal force.