Consider inviting students to make predictions about how the shape of the ellipse will change in response to changing the foci in different ways. This can be related to a variety of mathematical and astronomical phenomena.

Place the lead brick gently on your fingers and strike the lead brick sharply with the hammer. The inertia of the lead brick prevents damage to your fingers.

Engagement Suggestion

If a plastic straw (like those from McDonald's) is pushed slowly into a raw potato, the rigid potato will cause the softer plastic straw to bend and break. However, if a significant velocity is given to the straw it can be pushed clean through the potato. This is what often happens in a tornado, where apparently softer objects are accelerated to high speeds by the wind and driven through apparently harder objects.

CAUTION: Be sure that the hose fitting is securely attached to the tube and that the plastic shield is in place before firing. The shield should be latched in place, with no debris blocking its edge from meeting the baseplate

Engagement Suggestions

• • Before using, encourage your students to predict what will happen to the pencil.
• • For advanced students, discuss the energy involved in the problem and where the kinetic energy of the pencil went after the collision.

Background

This demonstration can be presented in multiple ways. It has been offered classically as an illustration of the principle of inertia – the pencil is in motion at a high velocity, and continues in motion despite the intervening wood until arrested by a greater force. Alternatively, consider the high velocity and high momentum of the pencil. The abrupt deceleration at the plywood means a high impulse. The pointed pencil has a very small cross-sectional area, resulting in force applied over a small area leading to a high momentary pressure.

Linked below is a slow-motion video of the collision, shot at 600 frames per second. A fun class activity could be to use the video to measure the motion of the pencil and estimate its momentum and kinetic energy, based on what you see in the video and by measuring typical lengths and masses for wooden pencils.

Operation:

• • Turn the tube vertically while still filled with air; show that the disc drops rapidly to the bottom end, and the feather flutters down slowly.
• • Invite students to predict how this behaviour will change when the air is removed.
• • Connect the pump and pump out most of the air. There will be an audible change in pitch when the tube is sufficiently evacuated, after 1-2 minutes.
• • Turn the tube vertically again, and let the students see that both now fall at the same rate.
• • CAUTION: The tube is thick glass; please handle with care.

Background:

The key physics in play here is twofold. Absent other forces, the two objects undergo the exact same acceleration in free fall, and so will fall at the same rate. With no air in the tube, the only force acting on them is gravity, which pulls downward on each object proportional to its mass.

However, when air is in the tube, there is a second force involved: air resistance.

The force of air resistance pushes upwards on the falling objects. It depends on two factors: the surface area of the falling object, and its velocity. So the faster they fall, the more resistance they face from the air. But recall that the force of gravity is proportional to the mass of the object, and the net acceleration of an object is the result of the sum of the forces acting on it. So if two objects have similar surface area, but one has a higher mass, then the higher mass one experiences a larger downward force than the other, while air resistance will exert close to the same upward force on both, and so the heavier object then has a greater acceleration. And that’s what we see when the tube is full of air – the more massive disc falls faster than the less massive feather. Take away the air and the force of air resistance, and they fall together!

Background

Turning the crank causes the elastic rings to rotate about the vertical axis. The rotation mechanism here uses the mechanical advantage of a large cranked wheel driving a smaller pulley to give the rotating rings a very high angular velocity.

Engagement Suggestion

Background

This effect is seen in astronomy and geography, as rotating planets, stars, and other bodies take on similarly oblate spheroidal forms.

Please take care when using this device, especially when accelerating. You can gain a significant increase in rotational speed, so hold on! And it is best not to have a person push the chair around very much, as it is very easy to hit them with a weight by accident.

Engagement Suggestions

Background

To show this in a different way, a single user with a single weight can move themself in a circle by swinging their arm wide holding the weight from front to back, then drawing it inwards before extending their arm forwards again and repeating the motion. This is essentially a rotational analogue of pumping a swing.

Invite students to predict how changing the ball’s starting velocity (in magnitude or direction) will affect its path. This is a good opportunity for one or more student volunteers to participate.

Background

The surface of this “potential well" is shaped so as to model an inverse square gravitational force. When a ball enters the well enters the well, it is attracted to the center; if it has no initial velocity, it will fall directly to the center. But if it enters with some velocity tangential to the center, it will fall into an elliptical orbit that gradually decays to the center as the ball rolls around the well.

When you roll the ball across the surface, you use some initial force to start it moving. Once it is rolling on its own, though, the only forces acting on it are the force of gravity, pulling downwards, and the normal force and frictional force of the surface holding it up. So the ball accelerates as it rolls down the surface, exchanging potential energy for kinetic energy, until it falls into the hole.

Drop a steel ball onto a dish of sand. The ball becomes partially buried and a crater forms.

Bury the end of the hose in the sand using the plastic strip attached to the end of the hose, and smooth out the sand. Using lung power, blow in a blast of air and notice the crater that forms.

A generation ago there was a debate among geologists and astronomers as to the origin of lunar and terrestrial craters. This demonstration illustrates two ways in which craters can form.

Let the magnets snap together as you discuss how they do work and in the process become slightly less massive, as in E=mc^2. Then discuss the analogy to protons combining to form deuterium and then helium while releasing energy.

Three light bulbs are mounted along a rod: two 150 watt bulbs with a 15 watt bulb between them. The lower-power center bulb appears darker, compared to the two brighter bulbs, because it has a lower temperature.

Sunspots appear darker than the surrounding area because they are regions of lower temperature.