Mechanics

Location: Cabinet 2, Shelf 4

Description: This demo has two items, a straight armed and crooked arm. First, spin the straight arm making sure it is going fast enough that it is still spinning after you finish the next step. Next spin the crooked arm. While spinning the crooked arm you will have to hold down the bases. It is also important that crooked arms be spun fast enough that the bases begin to wobble once allowed to move.

Pedagogically, ask the students why the crooked arm rotor starts to wobble: if it accelerates, what is the external force that causes it to start moving? This demonstration is very useful in showing that angular momentum is not always parallel to angular velocity, unlike linear momentum and linear velocity. The rotational axis is not along a principal axis of the crooked arm rotor. To rotate about the vertical axis an external torque must be supplied (the normal force on the edge of the stand) and if that is not sufficient the system will start to wobble.

Location: Cabinet 2, Shelf 3

Description: The ballistic pendulum can be used to demonstrate conservation of energy. To load the canon, first remove the ball from the trap by using the release on the side facing the barrel of the canon.The release is highlighted by the green arrow in figure 2. You may need to swing the pendulum while holding the release to obtain the ball. Having the ball in hand grab the plunger. Place the ball at the face of the barrel and using the plunger push the ball into the barrel until one hears a click. Note that the canon has three possible settings. You can push the ball past the first click till you hear a second and then a third. Once the canon is loaded make sure that the protractor is set to zero degrees and is flush with the arm of the pendulum. To fire the canon, grab the yellow string and pull perpendicularly to the top of the canon until the string is taught. Once taught, continue applying pressure until the canon fires. Once the canon fires you can use the protractor to measure the maximum angle made by the pendulum arm.  

Location: Cabinet 1, Shelf 2

Description: This demo illustrates how the center of gravity of a body is connected to stability of the body. When performing this demo be ready to catch the balls as they do roll out of the tubes when the tubes fall. Below is a picture of what the demo looks like stored, and a picture of what should be in the box, including the instruction. It includes two plastic balls, a steel ball, a straight tube, a slanted tube, two caps, and instructions. The left image is the contents and the right is the demo stored.

Location: Cabinet 1, Shelf 4

Description: This demo will require a bowling ball with the hook attachment, a ladder, and the hooks on the ceiling of the lecture hall. It is advised that another person be present while setting up this demonstration. The red arrows show the hook that is attached to the ball and where they attach on the ceiling of the lecture hall. These hooks are located above the floor of the lecture hall. One will have to use the ladders located in the hallway between the demo room and the classroom to reach the anchor points. 

Location: (pending permanent placement)

Description: This demo has four pieces: two marbles, a stick, the base. First lift the bases and use the stick to jam the base in an elevated position as shown in the photo to the right. Note there is a small piece of felt on the lower board that marks where the stick should sit. This is highlighted with a green arrow in the right picture. Once the stick is in place, balance a marble on the nail. The nail is located at the highest point of the raised arm. The nail's location is marked by a red arrow in the right picture. Finally, in a quick motion pull the stick out of the set up and watch the marble fall into the cup.

Location: Service hallway by NH 170 (Accessible by 1R button in elevator)

Description: This is a simpler version of the famous brachistochrone curve, or path of quickest descent. Four gears of equal size (stored at the end of the track) can be released simultaneously from the same height at the top of the track. There are four paths, at angles 30, 45, 60, and 75 degrees with the horizontal. It is simple to calculate the time it takes an object to slide without friction down a ramp height H and base X and traverse the final horizontal distance L-X. Minimizing the time with respect to X will prove that the optimal angle is 60.

Rather than work with sliding objects, this demonstration has gears and the tracks all have “teeth” so that the gears rotate without slipping. Instructors can make it an exercise for the students to show that this does not change the answer for the optimal angle. The difference in time for the paths is slight, so an observer is best placed at the very end of the track looking down.

Pedagogically, many students (especially more advanced ones) may expect that the time is independent of the path, since this is true for many properties of the trajectory (e.g. final velocity). It is worth having them first consider if the time of a trajectory is a conserved quantity and if not, why not.

Location: Cabinet 1, Shelf 2

Description: This demo requires no setup. Simply set it onto a table, lift one of the outer balls as shown below, and let go. Next, repeat with two balls and release and two will rebound on the other side.

Pedagogically, ask the students what will happen when you release three or more balls (or four or more from the cradle with seven suspended balls). Will fewer balls on the other side leave but go higher?

Finally, you can release one ball on the left and a second ball on the right at a higher level. The subsequent motion will be a rebounding ball on the left going higher that the one on the right. Point out that is can be viewed as the “excitations” traveling through the cradle, independently.

Location: Cabinet 1, Shelf 2

Description: This little guy requires no setup. Make sure you have enough room on both sides for the cart to oscillate. Simply pull up the mass and let it swing. The cart will oscillate back and forth in opposition to the mass.

Location: Cabinet 2, Shelf 1

Description: This demo requires no setup other than ensuring that the spool is placed as shown in the images below. The spool consists of two disks connected by a cylinder of smaller radius, with the cylinder acting to spool a fabric ribbon. The spool should be placed so that the end of the ribbon comes upward from the bottom of the spool. Once in this position the ribbon can be pulled at different angles and speeds to illustrate different behaviors. It is important to ensure that there is room for the spool to roll out.

Pedagogically, the instructor can ask the students which way the spool will roll when the ribbon is pulled. The instructor can then confound every prediction in a humorous fashion. When pulled more “upward” the device will roll in a fashion to unspool the ribbon; when pulled more horizontally, the device will roll in a fashion to wind up the ribbon. Rolling without slipping implies:

F_ocos(θ) - F_fric = ma
-F_or + F_fricR = Iα = Ia/R

where θ is the angle between the ribbon and the horizontal. This can be solved (assuming that there is no slipping)

a = F_oR[r - Rcos(θ)]/(I + mR²)

which will change sign depending upon θ. Note that the force of friction is bounded by μN, which we assume is sufficient to cause rolling without slipping.

Location: Cabinet 1, Shelf 3

Description: This demo requires no setup, only the ramp and the conical ball. Place the conical shape at the bottom of the ramp and watch it roll ‘up’ the hill. Be ready to catch the roller as there is nothing to stop it from falling off the edge of the ramp.

Location: Cabinet 2, Shelf 3

Description: There are several tops that are available in the demo room. Some have strings and others do not. If you need a string, there are spools of string in the drawer labeled ‘string’ located under the workbench on the west side of the first floor of the demo room. To work the tops, take a string that is roughly the length of your wingspan. It is better to have a shorter string than a longer one. A longer string runs the risk of getting tangled if it is not completely pulled out when launching the top. Taking care, wrap the sting around the shaft that connects to the spinning mass Once wrapped, place the top on the table. With one hand hold the top while the other quickly pulls the string. Ensure to pull through until the string comes off the top. When the string comes loose, let the top go and watch it spin. Above, is an example of stringing one of the tops available in the demo room. The left picture shows the threading of the top. The right  picture shows what the top looks like after winding the string. 

Location: Near door

Equipment Needed:

• A heavy weight
• A 2-meter stick
• A colorful sticky note or ribbon

Description: Students may be confused (if they consider it at all), that it seems that by the physics definition of work, it costs no energy to hold out a weight at constant height. Pedagogically, the instructor can ask a student to hold out the weight at arm’s length and ask them if they are doing work. The goal is to get them to consider the conflict between their experience and the physics definition of work.

To resolve the matter, the same student should be asked to hold the two-meter stick with a colorful sticky-note or ribbon attached to the far end. It should be easy to see that it is not possible for the student to hold the stick motionless. That is because the muscles in their arm cannot stay contracted. In fact, the striped muscle tissue is broken into groups that work in concert, alternating between contracting and then releasing. This is because the cell membrane is “leaky,” a design that allows for fast contraction but cannot keep the cell contracted. Therefore, the student is doing lots of microscopic work when the attempt to hold the weight motionless.

In contrast, smooth muscle tissue, used in the digestive system, can stay contracted for long periods of time (thankfully). However, it is also much slower to contract and relax. Such tissue would be poorly suited for running, but well suited to keeping clenched for long times.