Педагогические науки/5.Современные методы преподавания

V. Novikov,

associate professor of HAC;

G.Kalakova,

a senior lecturer of computer science and mathematics of Kostanay State University named by A.Baitursynov;

B.Kalakov,

assistant professor of physics and GTD Kostanay State Pedagogical Institute

Pseudodecisions of tasks on physics

In methodical literature it is accepted PseudoDecisions to count such decisions that at seeming loyalty, result in incorrect results. Such decisions are mostly made by students in considering of combined non-standard tasks, especially the tasks of increased difficulty, offered to them on different competitions, Olympiads of different levels.

Consider the following examples said.

Task 1 Two sources of EMF with 5V and 3V and internal resistance of 1 ohm and 3 ohm respectively are connected in series and close the external resistance at 12 ohms. Determine the potential difference across the first source.

Decision of student

Ohm¢s law for a clozed circuit

I=E/(R + r), (1)

Where E = E₁ + E₂ – EMF sources connected in series; r = r₁ + r₂ – their general internal resistance.

We find the numerical values of these quantities and put them in a formula(1) : we get

I = 0,5 A.

Sought after difference of potentials

U₁ = Ir₁ = 0,5 A*1 Ом = 0,5 V.

And answer to the task - 4,5 V.

Student isn’t defined sought value, and the voltage drop within the first current source (work performed shock - or rather, an electric field to move a unit of positive charge inside it).

Potential difference between the first source, that is, voltage at its terminals (in other words, the voltage in the external circuit of the first source)

U₁ = E₁ – Ir₁ = 5В – 0,5В = 4,5V.

Task 2 For the test of equipment under conditions of weightlessness a container up is thrown upwards by a pneumatic piston device, located at the bottom of the mine evacuated. The piston acts on container during the time Δt with a force F = nmg, where m - mass of the container with the equipment. How long does the container will fall to the bottom of the mine? During what time does last the state of weightlessness for an equipment, if ∆t = 0,04 with, and n = 125?

We will show a decision resulting in a right answer at once, paying attention to the mistakes assumed by students.

During a given time, the equipment was moving with acceleration

(1)

And acquired speed

(2),

rising to a height

(3)

To this part of the decision pupils make the mistake, assuming that the container is moving with the equipment only under the force and therefore acceleration. And therefore,, .

The container moves under the action only of gravity, and rises to a height H (see fig. 1)

(4)

Lifting time of the container to this height is determined from the equation:

(5)

And time of descent to the bottom of the mine container (also under the influence of gravity only) from the equation:

(6)

Many students here believe that, assuming that the piston does not return to its original position (on the bottom level of the mine)

From equalization(6) we have: .And taking into account equalizations (3) (4) we get:

The time during which the state of weightlessness lasts for an equipment, equal:

And all the time motions of container :

The decisions of these two problems have occurred due to the substitution mistakes of physical quantities.

Task 3. Pile weight of 100 kg is driven into the ground pile-driver, whose mass is 400 kg. Pile-driver falls freely from a height of 5 m, and at each stroke pile descends to a depth of 5 cm. Determine the average force of resistance of the soil.

True answer is 325 kN, students received – 405kN