  ## Overview

The cardstock input on a MechAnimation or Mystery Mechanism often buckles because it cannot withstand the force needed to move the mechanism.  There are two ways to solve the buckling problem:

• Reduce the force the input must withstand by redesigning the mechanism.
• Redesign the input link so it can withstand the force that’s required.
In Lesson 8, students explore both approaches.  In Experiment A students examine how the force on the input link varies with the length of the input arm.  In Experiments B and C students redesign the input link and test it. In all three experiments, the role of input link is played by the Force Probe - a 1 ″ x 8 ″ strip of cardstock

## Materials

For the class

One mechanism with buckled input link.

One assembled Force Machine with several cardstock Force Probes

For each pair of students:

The parts for the Force Machine and ten cardstock Force Probes 8 ½″x 1″,

## Procedure

1. Force machine principles. Buckling of the input link occurs when the input link must exert too much force to move the mechanism’s lever. The Force Machine is used to find out about the force the input link must exert.

a. How does the Force Machine work?  Demonstrate the assembled Force Machine (see video). Students observe that

• The Force Machine uses a rubber band to resist the motion of a lever.
• The Force Probe is the input to the lever. It buckles when the force required to push the lever down (to stretch the rubber band) is too great.

b. How is the Force Machine assembled? Here are video and print directions to guide students as they assemble the Force Machine

2. Force Machine experiments.

a. Experiment A: How does the force exerted by the Force Probe (the input link) change with different input arm lengths?  This video introduces Experiment A. Here are some points to emphasize in introducing Experiment A.

Outcome: The force required to push the input arm down increases as the input arm (the distance from the fixed pivot) gets shorter.

b. Experiments B and C: How can the input link be made stronger? In Experiment B students double the Force Probe to see what difference that makes.  In Experiment C students fold the Force Probe lengthwise.  This video introduces Experiments B and C.

Outcomes: When the Force Probe is doubled, it withstands more force. When the Force Probe is folded lengthwise it withstands much more force.

3. Science Notebooks.  Students complete the data sheets for Experiments A, B and C and record their conclusions in their science notebooks.

## Troubleshooting

1. What if I don't have two #16 rubberbands to use with the Force Machine? Or what if the lever arm is slanted down instead of slanted up? 2. What if the Force Probes bend too much, even at 11"?