Post date: Apr 4, 2018 12:45:54 AM

• The microscopist uses blue light rather than white light to illuminate objects being viewed. The reason behind is more details can be observed with less diffraction which is the case with short-wavelength blue light.

• The dolphin employs long-wavelength sound to get an overall image of objects in its environment. But emitting short-wavelength sound enables it to examine finer details.

• Electron microscopes can see and examine tiny objects owing to the fact that electron beams have extremely short wavelengths compared to light waves. Electron microscopes work thanks to the wave nature of electrons. Focusing of an electron beam is achieved by guiding and deflecting the beam using electromagnetic fields.

• The wavelengths of AM radio waves range from 180 m to 550 m, and this size range allows these waves to bend around buildings and obstacles. In contrast, diffraction is bad for FM radio waves with wavelengths range from 2.8 m to 3.4 m, because these waves ``see” the obstacles.

• Remember, when light passes through a wide window, it casts a sharp shadow. When the object’s size is larger than the wavelength of light, a sharper shadow is cast.

• If the object’s size is smaller than the wavelength of light, no structure can be seen. Diffraction is more pronounced through a small opening than through a large opening.

References

1. P. G. Hewitt, Conceptual Physics, 10th ed., Pearson Addison Wesley, San Francisco, USA, 2006.

2. P. A. Tipler and G. Mosca, Physics for Scientists and Engineers, 5th ed., W. H. Freeman and Company, New York, USA, 2004.

Post date: Mar 4, 2014 1:35:35 PM

Squeezing...compressing...expanding...

Meticulous operational definition must also be given to ``pressure". That is, describe exactly what we do and then assign numerical values to the concept. Idea first, name afterward.

Scenarios to imagine or demonstrated if possible:

I. We analyze the forces acting on a solid rectangular block lying at rest on a table. The upward normal force due to the table is equal to the downward pull of the earth. The downward pull is the sum of the little downward pulls integrated over the whole block. Likewise, the upward normal force is the result of adding up the little upward forces acting on little areas.

The value of total-force-divided-by-the-area differs for each face of the block in contact with the table. We notice that largest value occurs for the smallest area. 

II. A film of water and a sheet of plywood on a table. A solid rectangular block is placed over the water and plywood. The water squirts to the sides, the plywood does not.

III. Tall cylinder of fluid is acted on by the gravitational pull of the earth. Then, imagine horizontal planes through this column at different levels. The FBD of the upper fluid is: upward force exerted by the (lower) fluid below the horizontal plane, and weight of the upper fluid. 

IV. People live at the bottom of a ``sea of air".

V. Suppose you press your hand against a surface, pressure will be greater if someone else's hand presses against your hand in the same direction.

The following are the concepts that must be clarified:

1. Force and pressure are different. Whatever face of the block is in contact with the table, the total force acting on the block is the same. But, the pressure differs as the area differs.

2. Pressure depends on depth, weight density, and gravity.

Reference:

Arons, A. B. Teaching  Introductory Physics. Wiley, New York (1997)

Post date: Apr 4, 2018 12:45:54 AM

Recall the experience in beam balancing and seesaw. Child 1 of weight W1 is at a distance L1 from the turning point, while child 2 of weight W2 at L2. To achieve balance, it is likely to apply the ratio W1/W2 = L2/L1. 

In this form of ratio, the forces and lever arms (both subscripts) appear on the left and right sides of the equation.

If we restate the balance condition in the form: W1L1 = W2L2, we see can see that -  

the product W1L1 (force x lever arm) is the "effect" intrinsic to the left of the turning point (and W2L2 is the "effect" intrinsic to the right).

If W1L1 > W2L2, there is a "turning effect" at the left, a counterclockwise rotation.

Furthermore, the ratio form can no longer be used to express the balance condition if Child 3 joins in the seesaw. This is now the balance condition: W1L1 = W2L2 + W3L3

The "turning effect" is additive.

Now, if forces are applied, maximum "turning effect" occurs when the force and lever arm are mutually perpendicular. A component of the force that is parallel to the lever arm (and passing through the turning point) produces zero "turning effect". 

This "turning effect" is named torque. 

Post date: Feb 16, 2014 5:43:58 AM

Here are suggested topics and activities from Aron's book that may possibly be incorporated in lectures.

1. Students may be asked to sketch the possible force-versus-time graph for each body during the interval of contact.

2. Definition of "interval of contact". When does the interaction begin (when does it end)?

3. Consider carts equipped with magnets for repulsion. What is the time interval of contact? What is the possible force-versus-time graph? 

Post date: Feb 15, 2014 8:30:58 PM

What changes in motion take place when two particles collide? Simple experiments may be conducted using carts equipped with springs, or using steel balls.

Students are asked to make purely qualitative observations. They are invited to test how good their powers of observations are. 

I. Elastic collisions. 

(a) Body B is initially stationary (vB1 = 0). Body A is moving to the right with velocity vA1. Assume positive direction is toward the right. After collision, the following are possible scenarios:

    (1) If mA = mB, body A stops (vA1 = 0) and body B moves with velocity vB2 = vA1.

    (2) If mA > mB, both A and B move to the right. B moves faster than A, vB2 > vA2. Body A moves slower than             before collision, vA2 < vA1. 

    (3) If mA < mB, body A bounces and moves to the left while body B moves to the right. 

The velocity equations after collision are then derived from the conservation of kinetic energy and conservation of momentum. The above observations are examined using the velocity equations.

II. Perfectly inelastic collisions. Instead of bumper springs or steel balls, we use carts equipped with sticky masking tapes.

(a) Body B is initially stationary. Body A is moving to the right. After collision, they move off as one unit to the right at slower speed than initial speed of A. v2 < vA1. 

(b) Bodies A and B are moving toward each other with equal speed. If mA = mB, the bodies stick together and come to a dead stop.