Note: The fan spins quite fast. Don't let it hit your fingers! To connect and disconnect power, use the alligator clip wire on the rear; clip it to the fan support frame for safety when not in use.

Consider inviting students to make predictions about the cart's behaviour with and without the sail. Invite them to discuss the forces involved.

The "putt-putt" or "pop-pop" boat is a classic toy rarely seen in the 21st century, and a fascinating example of a heat engine at work. A small heater using vegetable oil boils water, forcing out water and steam from the tailpipe of the boat. After the steam and water are expelled water is pulled back into the boiler. This process produces a net force in the forward direction, propelling the boat forward. Refer to the geometry of the boat boiler system (above) to further understand this phenomenon.

This is similar to the processes by which the inverse sprinkler is driven in the forward and the reverse directions to cause the direction of rotation to reverse for the inverse sprinkler.

Note: Be sure to completely fill the boiler chamber with water before heating it. Practice lighting the wick.

A sail is attached as shown to an air track glider. Wind from a fan blows the sailboat in the direction from which the wind is coming if the angle between the wind and the sail is correct.

Click your mouse on the link below to see a video of the action. After the video begins, (a) the air cushion is turned on, then (b) the air gun is started, creating the force situation shown in the drawings below.

An hourglass with its sand has a weight W when at rest on a scale as photographed. Before time t=0 the sand is held in the top of the hourglass by an invisible massless membrane. At time t=0 the membrane is removed by a massless demon, allowing the sand to fall into the bottom of the hourglass. At time t=T the sand is all in the bottom section. If the original and the final weight of the hourglass with sand is W, what is the force (or weight) read by the scale during the time interval from t=0 to t=T.

The answer involves two parts: (1) the start and stop of the sand flow, and (2) the steady-state flow. At the start, because there is some sand in the air, not being weighed, the scale momentarily falls. During the steady-state sand fall the extra force of sand hitting the bottom very nearly cancels the loss of weight of the sand in the air, so the scale reads very nearly W (see below). When the sand column is ending, the force of the sand hitting the bottom exceeds the loss of weight of the shrinking sand column, so the scale momentarily rises. This can be seen in an mpeg video by clicking on the link below above.

During the steady state fall, the downward frictional force of the sand on the inner surface of the funnel is accompanied by an upward reaction force exerted by the funnel on the sand. This force provides a very small additional "weight" seen by the balance, causing the steady-state reading to be slightly higher than the actual weight before or after the sand falls. This can be observed using an electronic balance that has been zeroed with the container and sand at rest, seen in the photograph at the left below. The picture, taken during the time when the sand was falling, shows the small reaction force created by the sand sliding through the funnel. Clicking on the link, below, starts an mpeg video of the action, showing the entire sequence: a momentary negative pulse at the start, the slightly increase in weight during the period when the sand is falling, and the positive pulse at the end.

A subject not sensitive about his or her weight stands very quietly on the scale. Observers will notice that the measurement is not extremely stable, but rather has a periodic short dip in the reading lasting a small fraction of the time between heartbeats.

The blood flows out of the heart in the upward direction. The blood to the head continues upward through the carotid artery. However, the blood servicing the lower part of the body flows upward out of the heart through the aorta, which the has a sharp bend that deflects the blood downward. The aorta exerts a downward flow on each burst of blood (called a "bolus"), which in turn exerts an upward reaction force on the aorta. This upward force is transferred to the body, which experiences a short period during which the body weight is decreased by the amount of force the bolus exerts on the aorta. This is seen in the practice as a series of small downward blips in the scale reading. Click your mouse on the link, below, to see a slow motion rendering of the weight as a function of time.

The study of this and related effects is known as "ballistocardiography." Ballistocardiography is a useful tool in determining the strength of heart muscles by directly measuring the blood flow; an electrocardiogram (ECG) measures the activity of the muscles in the heart less directly.

A box sits on an adjustable inclined plane. Masses can be placed in the box to change its weight, and thus the normal force exerted by the inclined plane.

Set the empty box on the incline and increase the angle until sliding ensues. Add weights to the box and repeat the experiment. The weighted box begins to slide at the same angle.

(Optionally, a string and pulley can be used to add add an additional force to the system.)

With no air pressure on the tilted air track and an appropriate counterweight, the glider will be held in place by friction. Start the blower and, if the counterweight is sufficient, the glider will move up the incline.

The pulley end of the air track is raised on one of the large wooden blocks. Using a small glider, as photographed, a 10-gram weight is insufficient to pull the glider up the track with the air on, and the glider moves down the incline. Adding the 20-gram weight (total of 30 grams) causes the glider to move up the air track with the air on.

Compare to K2-40: Magnetic Accelerator