SPH4C: Energy Transformations

Teacher Demo: The Nose vs. the Wrecking Ball

TopicsConservation of Mechanical Energy

Timingpreparation: less than 5 mindemonstration: 10-15 min.

Specific Expectations SPH4C

IntroductionA pendulum is constructed from a 1.0-kg mass suspended from the ceiling, with a length of strong string long enough so that, when pulled about 40o from the vertical, the mass comes to nose level. The pendulum will be released from this position. The demonstrator is to remain absolutely still until the pendulum makes at least one full cycle. This very-simple-to-perform demo can be made very memorable by playing up the drama. The possibility of seeing the teacher’s blood will excite the class. It will even test one’s faith in the laws of Physics.

Materials

one large mass with a hook (1 kg is good)several metres of strong stringa way of securing the pendulum to the ceiling (see notes below)safety glasses

Safety ConsiderationsThe mass will be traveling at pretty high speed at the bottom of the swing; ensure nobody is in the path of the swinging mass.Ensure that the string is strong enough and attached well enough to the mass so that there is no chance of an accidental release.Do not move your nose forward at any time during the demonstration.Ensure that only the force of gravity works on the pendulum. Do not let the students do any extra work. Safety glasses protect eyes from unexpected action.

ProcedurePreparation

1. The pendulum should be set-up and tested before class, as described in the introduction.

2. Once the class is safely seated at a discrete distance from the apparatus, review the Law of Conservation of Mechanical Energy, noting that a pendulum experiences very little friction because there is no contact between moving surfaces.

Predict/Explain3. Pull the pendulum to the side; stand behind the pendulum and bring it up until it

just touches your nose. 4. Ask students to meet in small groups to predict the outcome of releasing the mass.

Each prediction requires some explanation to support this prediction.Observe

5. Increase the drama by saying some more words about inertia and pausing to indicate uncertainty and trepidation. Release the mass. Do not move.

Explain6. Ask student groups to review their predictions and revisit their explanations to

revise with their observations.

DisposalBroken string can be thrown into the garbage.

What happens?The mass will swing out and then return, moving at pretty good speed, until it reverses direction, just as it reaches your nose. This is the turning point, where the velocity of the mass is equal to zero.

How does it work? The constant total energy is shared between the kinetic and gravitational potential energy at any point during the swing. As the height decreases, the speed increases; but as the height increases, the speed decreases.

Etot = Ek + Eg

Etot = ½ mv2 + mgh

When the mass is released, its speed is zero but its height is at a maximum. The potential energy is transformed to kinetic energy as it loses height but then is transformed back to potential energy as it rises to its original height. With luck the speed will be back to zero as it reaches your nose.

Teaching Suggestions/Hints1. As with most demonstrations, timing and drama contribute to the success. Play up

to your audience. 2. Insert as much good physics vocabulary and relevant concept review as you can

while you appear to assess the danger and possible outcomes. This should hold their attention.

3. This is a photo-op. The students might use their cell phones to take a video which can be played back in slow motion or freeze-framed. (This could also be used as a great Physics promo for the school).

4. Support the demonstration with an opportunity to make measurements and calculations to explore the explanations further.

Next Steps1. This demo leads naturally into roller coaster and other conservation of energy

problems.2. Once conservation of mechanical energy is understood, trebuchet construction

cannot be too far away.

Additional Resources1. From The Physics Classroom - An animation of a swinging pendulum matched

with a data table: http://www.physicsclassroom.com/mmedia/energy/pe.cfm2. You Tube of the demo using a bowling ball: http://www.youtube.com/watch?

v=i2GdY1OlDpA3. One way to secure the mass is to attach a ring clamp to a support for the

suspended ceiling in the physics room.

Specific Expectations SPH4C

A1.1 formulate relevant scientific questions about observed relationships, ideas, problems, or issues, make informed predictions, and/or formulate educated hypotheses to focus inquiries or research

A1.8 synthesize, analyse, interpret, and evaluate qualitative and/or quantitative data; solve problems using quantitative data; determine whether the evidence supports or refutes the initial prediction or hypothesis and whether it is consistent with scientific theory; identify sources of bias and/or error; and suggest improvements to the inquiry to reduce the likelihood of error

E3.1 describe and compare various types of energy and energy transformations (e.g., transformations related to kinetic, sound, electric, chemical, potential, mechanical, nuclear, and thermal energy)

E3.2 explain the energy transformations in a system (e.g., a toy, an amusement park ride, a skydiver suspended from a parachute), using principles related to kinetic energy, gravitational potential energy, conservation of energy, and efficiency

Return to top