what could be the best physics demo that a 4th yr stud. could do? just a simple one yet amazing?

Question

a simple yet an amazing one and the needed materials are readily available

Answer 1

Has your class talked about torques and center of mass yet?

One surprising demo is to hang a hammer from a ruler that has one edge barely hanging on to a desk.

About a quarter inch of the ruler rests on the desk (you can get this down to 2 or 3 millimeters if you try). The handle of a hammer hangs from a loop of string about 2 inches from the other end of the ruler. The very end of the handle has to rest against the bottom of the ruler. The loop of string should be short enough that the hammer hangs from the ruler with around a 15 or 20 degree angle relative to the ruler. The hammer is longer than the ruler, so the head of the hammer hangs free under the desk.

At first glance, an observer would think the hammer and ruler have to fall off of the table. You really have two pivot points: one at the edge of the table and ruler and another where the hammer hangs from the string. The weight of the hammer pushes up on the bottom of ruler with enough torque to hold the ruler on the table.

The key is that the center of mass for the entire system (ruler and hammer) lies under the table in the head of the hammer. The center of mass will move to its lowest point possible. To determine this point, you have to ignore the distractions and focus on where the center of mass is relative to the first pivot point - the edge of the table and the ruler. If you drew a line from the edge of the table to the head of the hammer and looked at it as a simple pendulum, it would be obvious which direction the head of the hammer should move and why the ruler stays on the table.

By the way, focusing on where the center of mass is relative to the edge of the table is very important. You wouldn't believe how many grade school and junior high science teachers explain this experiment incorrectly. There's such a shortage of science teachers that many schools resort to using non-science majors for their lower level science classes.

Answer 2

Well if you are talking about school then you can show that light will bend in certain cases... all u need is a laser light - one that is available in shops(battery operated) a glass box and a bucket. Make a tiny hole on one side of the glass box. Fill the box with water and when water starts flowing out u can point the laser from the opposite side such that the light beam falls on the hole. u can notice the light beam bending along the path of water. But the hole has to be tiny.

Answer 3

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