the world in a different light
I was reading up on tides this morning as I have to relieve a colleague’s IP2 class on forces, when I stumbled upon this video. It explains tides in a way that differs from most textbooks that clarified for me why the oceans bulge at both ends along the Moon-Earth line at the 2’30” mark onwards.
I bought a simple beta Stirling engine online at dx.com recently and it came in the mail today. It works well with a cup of hot water placed under it, although it might take a little push to get it started due to the initial static friction. However, once it starts spinning, the wheel goes on and on for a very long time.
From the video, you can observe the expansion of the air within the main piston cylinder as the heat below raises the temperature and pressure. This forms the power stroke. When the piston rises, it pushes air into a secondary piston, which also helps to provide torque to the wheel. When the air in both pistons expand, it cools down. An understanding of the 1st law of Thermodynamics (JC syllabus) is necessary to appreciate why that happens. Upon cooling, pressure decreases and the pistons fall. The cycle repeats itself.
A very good explanation of standing waves on Chladni plates. Watch out for the 3-Dimensional standing wave at 3’11”.
This is the Pendulum Clock from the LEGO Education Simple and Powered Machines Set. It can be used to demonstrate the variation of period with length of pendulum and is a very good visual representation of the escapement mechanism.
There are many other models that one can build using this set, including a weighing scale, elastic energy powered car, etc. All with potential for class demonstrations.
You can buy a set from Duck Learning in Singapore at (S$329.75), an exclusive distributor of LEGO Education products in Singapore. If you are purchasing in bulk for your school, you may want to contact them directly for a package deal. You can also purchase them from overseas sites such as Bricklink.com if you can find them at a better price.
For my students: To download the file and video for analysis using Tracker, right-click the file here…
To verify the equation F = ma, where F is the resultant force on an object, m is the mass of the object and a is the acceleration, this is one of the ways to do so:
Equipment:
1. Motion Sensor
2. Datalogger
3. Cart with variable mass
4. End Stop
5. Pulley with clamp
6. Hanger Mass Set
7. String (about 1.2 m)
For a system of a cart of mass M on a horizontal track that is connected to a hanging mass m with a string over a pulley, the net force F on the entire system (cart and hanging mass) is the weight of hanging mass. F = mg (no friction assumed).
According to Newton’s Second Law, mg = (M+ m)a. We will try to prove experimentally that this is true in the video below.
The following is a question (of a more challenging nature) posed to JC1 students when they are studying the topic of kinematics.
A gun is aimed in such a way that the initial direction of the velocity of its bullet lies along a straight line that points toward a coconut on a tree. When the gun is fired, a monkey in the tree drops the coconut simultaneously. Neglecting air resistance, will the bullet hit the coconut?
It is probably safe to say that if the bullet hits the coconut, the sum of the downward displacement of coconut $$s_{yc}$$ and the upward displacement of the bullet $$s_{yb}$$ must be equal to the initial vertical separation between them, i.e. $$s_{yc}+s_{yb}=H$$
This is what we need to prove.
Since $$s_{yc}=\frac{1}{2}gt^2$$
$$s_{yb}=u\text{sin}\theta{t}-\frac{1}{2}gt^2$$ and $$s_{xb}=u\text{cos}\theta t$$
$$s_{yc}+s_{yb}=u\text{sin}\theta{t}=u\text{sin}\theta\times \frac{s_{xb}}{u\text{cos}\theta}=s_{xb}\times{\text{tan}\theta}$$
At the same time, the relationship between $$H$$ and the horizontal displacement of the bullet $$s_{xb}$$ before it reaches the same horizontal position of the coconut is $$\text{tan}\theta=\frac{H}{s_{xb}}$$
Hence, $$s_{yc}+s_{yb}=H$$