SUCK

Too many teachers of science talk about milk being sucked up a straw and/or passengers being sucked out of an aeroplane. Unfortunately this generates a false picture in the minds of their students.

The dictionary indicates that "suck" means to draw into the mouth. The milk is in fact forced into the mouth in accordance with Newton's first law of motion. Again, the passengers are forced out of the plane in accordance with the first law.

Pressure is defined as Force divided by Area. P = F/A. If the area is fixed then Pressure will vary directly with Force.

You place your lips over the straw. The pressure of the air inside the straw above the milk is balanced by the pressure of the air above the milk outside of the straw. Hence, the force per unit area is the same. You consciously lower your diaphragm and your lung capacity increases. A greater volume with the same number of air particles must result in a drop in pressure. Hence, the pressure of the air inside the straw drops which means that the force per unit area drops. We now have a greater force per unit area acting on the surface of the milk, outside of the straw, and hence milk is forced into the straw.

You are sitting in an aeroplane, at sixty thousand feet, next to a window. The air pressure acting on the inside of the window is far greater than that on the outside. Hence, the force per unit area is greater inside. The window shatters and in accordance with Newton's first law the pieces of glass move out and away from the plane. In general the net movement of air particles will be towards the hole where the window used to be. The air pressure on the side of your body closest to the window will drop very quickly and hence so will the force per unit area acting on that part of your body. The greater air pressure and force per unit area on the other side of your body will cause you to move in accordance with Newton's first law. You are forced out of the plane. In simple terms the air molecules move into regions of low pressure and if you are in the way then bad luck!

EINSTEIN / NEWTON

Following on from Einstein's theory of relativity some teachers of science have suggested that Newton got it wrong. He did not!

Newton's second law of motion is mathematically represented by the equation F = d(mv)/dt.
 Not F = ma. Newton assumed that if an object is subjected to an unbalanced force its inertial mass m would not change and that F = d(mv)/dt would simplify down to F = ma. For velocities of say 100m/s changes in inertial mass are not measurable.

Einstein took Newton's second law from one inertial reference frame (which is one in which Newton's second law holds) to another such frame using Lorentz transformations. When this was done he found that for the law to hold (Galilean Invariance) the objects inertial mass changed. The new mass is represented by the equation m = m' /w, where w = square root (1 - v.v/c,c) and m' is the objects inertial mass in the rest frame.

This is Einstein's only modification of Newton's laws of motion. Newton's first law remains unchanged. Newton's third law still holds but the modified mass must now be taken into account when considering conservation of momentum.

GONSKI

When politicians talk about improving educational outcomes they seem to mainly focus on the amount of money they are prepared to invest. I suggest that there is only a very small positive  correlation between money spent and improved results. Politicians often talk about improving school resources.

I suggest that a school's greatest resource is its staff (mainly its face to face teaching staff). What is "Gonski" going to do about the thousands of teachers who are poorly motivated and struggle to understand the subject matter they are directed to teach? Very little I think.

 What is "Gonski" going to do about the increasing levels of violence in our schools? Next to nothing I think.

Why waste billions of dollars on educational systems of work and processes which are continually failing?

NAPLAN / VIOLENCE

Some academics have suggested that an indication of the level of violence in a school be published within the publication of a schools NAPLAN results. I suggest that this is already being done. It is my experience, over many years, that as violence in a school increases the student attendance rate drops. Hence, I'm suggesting that a school's student attendance rate gives a good indication of the level of violence within that school. Personally I would be reluctant to enroll a child in a school with an attendance rate below 90%.

I further suggest that publishing a staff attendance rate would be a very useful guide to a school's culture and general well being.

Finally it is my opinion that student attendance rates, as published in the NAPLAN results, are in many cases underestimated.

WORK DONE

The definition of work done, in some science books, is written as force x distance. This is incorrect.
The true definition is Work Done = Force x Displacement (W.D = F x s).

Suppose we move an object of mass m = 6kg (resting on the earth) against gravity g (approximately 10 m/s/s).

Let the distance moved = 20m and assume the object comes to rest 5m above the earth.
Then the total Work Done = F x s = mg x s = 6 x 10 x 5 = 300 J (joules).

Notice that this is also equal to the increase in gravitational potential energy of the object. Also note that force and displacement are vector quantities whilst work done is a scalar quantity.

High School Science Text Books

Often when I have taught physics to high school students there has arisen  conflict between what I am telling the students and what their issued text states. I just bite the bullet and point out why the text is incorrect. "Obviously" those teachers of science who rely on the text being correct may be  inadvertently disadvantaging their students. Not a nice thought!

NEWTON'S SECOND LAW OF MOTION

It is most unfortunate that a significant number of teachers of science put forward Newton's Second Law as being Force = Mass x Acceleration.

If an unbalanced force acts on an object its momentum will change with time. Newton's Second Law states that the unbalanced force is directly proportional to the rate of change of momentum with time and is in the same direction as the force.

Momentum  p = mv and is a vector quantity. Hence, from the above definition F = dp/dt = d(mv)/dt.
Upon expansion we get  F = mdv/dt + vdm/dt.   Now, dv/dt = a.

Hence, F = ma + vdm/dt. This is the correct mathematically presentation of Newton's Second Law.
If we assume that the inertial mass of the object does not change then dm/dt = 0 , and Newton's Second Law reduces to F = ma + 0 = ma.

Consider an object of mass m having an initial velocity of v'. Its momentum (p) is mv'. Suppose an unbalanced force  F acts on the object for t seconds and it's new velocity is v''. Hence, its new momentum is mv''. Note, I have made the assumption that its mass has not changed. Let the change in velocity be v. Then, v = v'' - v,'

From Newton's Second Law we get  F = (Change in momentum) / (Change in time).

Hence, F = (mv'' - mv')/t = m(v''-v')/t = m(v/t) = ma.