The discipline of Physics in high school is one of the most feared among students and at Enem this is no different.
Although the questions often do not require elaborate calculations, applying the concepts and laws associated with everyday life is often not trivial.
The biggest difficulties of students in physics questions are:
- Difficulty in interpreting the statements of the questions.
- Difficulty in applying Physical laws, especially those that hurt common sense.
- Difficulty in identifying the quantities involved in the problem, knowledge and correct application of formulas and adequacy of related units.
- Lack of knowledge of the scientific vocabulary used.
- Lack of mastery in elementary calculations.
- Difficulty interpreting data in tables and graphs.
1. Understand the physical concepts
If you are one of those students who think Physics is just memorizing formulas, it is high time to forget about this idea!
At Enem, the Physics questions seek to recognize in the participant skills and competences acquired throughout his school life.
Within this context, you should try to master the concepts involved in a given phenomenon, trying to understand what is happening, how and why.
You must also be able to associate theoretical content with practical situations, interpreting causes and effects, according to the problem presented.
Paying attention, mainly, to concepts that hurt our common sense, because, even knowing the theory, our convictions often lead us to error.
For this not to happen, these concepts need to be very well understood and grounded. As such, it will be important to see examples and do exercises that explore different contexts in which these laws apply.
As an example, we present a question below, which assesses whether the participant has correctly mastered the concept of heat and temperature.
Note that the student can easily be misled as these are terms used in everyday life differently from the physical concept.
Therefore, to correctly answer this question, it is essential to have these concepts well consolidated.
Read too: Studying at home: essential tips for studying.
Question from Enem - 2nd application/2016
On cold days, it is common to hear expressions such as: “This clothing is warm” or “Close the window so the cold does not enter”. The common sense expressions used are at odds with the thermodynamic concept of heat. Clothes are not "warm", much less the cold "enters" through the window.
The use of the expressions "clothing is warm" and "so the cold doesn't come in" is inappropriate, as the (a)
a) clothes absorb the person's body temperature, and the cold does not enter through the window, the heat leaves through it.
b) clothing does not provide heat as it is a thermal insulator, and the cold does not enter through the window, as it is the temperature of the room that exits through it.
c) clothes are not a source of temperature, and the cold cannot enter through the window, as the heat is contained in the room, so the heat leaves through it.
d) heat is not contained in a body, being a form of energy in transit from a higher temperature body to a lower temperature one.
e) heat is contained in the person's body, not clothing, being a form of temperature in transit from a warmer body to a cooler body.
Correct alternative: d) heat is not contained in a body, being a form of energy in transit from a higher temperature body to a lower temperature one.
Heat is defined in physics as energy in transit and temperature is a measure of the degree of agitation of molecules.
This way, the temperature will not be absorbed by the clothes, much less the temperature will go out the window. Therefore, items "a" and "b" are not true.
Items "c" and "e" indicate that heat is contained in the room or in the person's body, which is not correct, as the concept is linked to energy transport. Furthermore, the "e" item still puts the wrong idea of temperature in transit.
2. Learn the relationship between magnitudes
The Enem questions give great importance to the concepts, however, this does not mean that there is no need to know the basic formulas.
Questions often appear where it will be necessary to perform calculations, and applying the formula correctly can reduce the time it takes to resolve the question.
However, it's no use memorizing a bunch of formulas and not knowing what each letter means!
Therefore, our suggestion is that before worrying about memorizing the formulas, you learn to dialogue with them.
For this, when studying, your main objective should be to know the physical quantities related to a phenomenon and identify their relationships.
To fix the studied relationships you must ask questions involving calculations. In this way, you will naturally end up saving the formulas.
Below is an example of a question that explores this type of knowledge.
Question from Enem/2018
A designer wants to build a toy that launches a small cube along a horizontal rail, and the device needs to offer the option to change the launch speed. For this, it uses a spring and a rail where friction can be neglected, as shown in the figure.
In order for the cube's launch speed to be increased four times, the designer must
a) keep the same spring and increase its deformation twice.
b) keep the same spring and increase its deformation four times.
c) keep the same spring and increase its deformation sixteen times.
d) exchange the spring for another with an elastic constant twice as great and maintain the deformation.
e) exchange the spring for another with an elastic constant four times greater and maintain the deformation.
Correct alternative: b) keep the same spring and increase its deformation four times.
In this question, we have that the elastic potential energy of the spring will be transferred to the cube in the form of kinetic energy. Upon receiving this energy, the cube will come out of rest.
Considering that friction on the rail can be neglected, the mechanical energy will be conserved, that is:
ANDpotential = ANDkinetics
The elastic potential energy is directly proportional to the product of the spring's elastic constant (k) by the square of its deformation (x) divided by 2.
We also have that the kinetic energy is equal to the product of the mass (m) by the square of the velocity (v) also divided by 2.
Replacing these expressions in the equality above, we find:
Thus, isolating the speed, we have:
Therefore, we conclude that if we keep the same spring, the value of k will be the same and if we quadruple the deformation, the velocity will also be quadrupled, as requested in the problem.
3. Optimize reading time for questions
Many of the questions in Physics address technological innovations and being aware of these new technologies can help to get some questions right.
An interesting strategy is to get used to reading news about scientific discoveries and their applications. This will help you to become familiar with the scientific language, which will make reading and interpreting statements easier and faster.
As it is contextualized, the test usually presents very large statements. To avoid reading the same question over and over, make it a habit to underline important information as you read.
Another important observation is not to be scared by the texts. Often, issues that initially seem too difficult or that are too big can be resolved just by looking at a chart, for example.
Below you can check a question of this type.
Question from Enem/2017
the epilation to laser (popularly known as hair removal the laser) consists in the application of a light source to heat and cause a localized and controlled lesion in the hair follicles. To prevent other tissues from being damaged, wavelengths are selected that are absorbed by melanin present in the hairs, but not affecting the blood oxy-hemoglobin and tissue water in the region where the treatment will be applied. The figure shows what is the absorption of different wavelengths by melanin, oxy-hemoglobin and water.
What is the ideal wavelength, in nm, for epilation a laser?
a) 400
b) 700
c) 1 100
d) 900
e) 500
Correct alternative: b) 700
Note that the question addresses a technological application related to electromagnetic waves, which at first appears to be a complex issue.
However, to resolve the issue, it was only necessary to correctly analyze the information contained in the statement itself and in the graph presented.
The statement indicates that the chosen laser wavelength should be the one that is absorbed by melanin and which does not affect either the oxy-hemoglobin of the blood or the water of the tissues where it will be applied.
The graph indicates the absorption of radiation by these substances at different wavelengths.
Thus, it is enough to identify in the graph which wavelength is more absorbed by melanin while it has a reduced absorption for the other two substances.
We see then that this occurs when the wavelength is equal to 700 nm, as it has a high level of absorption by melanin and zero for oxy-hemoglobin and water.
4. Master the interpretation of graphs, tables and elementary calculations
Questions involving graphs and tables fall very frequently not only in the Physics test, but also in other areas. Therefore, knowing how to interpret the information contained in these resources is essential.
For this type of question, it is always important to pay attention to the quantities indicated. Often, the student comes to wrong conclusions by looking at the axes of the graph.
Also, you should pay special attention to the units of measure, as you might need to do conversions to find the correct result.
An interesting point is that sometimes, when you are unsure of the relationship between the quantities involved in a proposed situation, the units of measure can give you a clue.
In Enem the use of calculators is not allowed. So when you're studying, resist the temptation and get used to doing the math without this resource.
Also try to learn ways to simplify the calculations. The more you train, the faster you'll be able to get it right. With practice, this will earn you precious minutes.
Follow the resolution of the question below, how to simplify the calculations.
Question from Enem/2017
Electronic devices that use low-cost materials, such as semiconductor polymers, have been developed to monitor the concentration of ammonia (toxic and colorless gas) in farms poultry. Polyaniline is a semiconductor polymer that has its nominal electrical resistance value quadrupled when exposed to high concentrations of ammonia. In the absence of ammonia, polyaniline behaves like an ohmic resistor and its electrical response is shown in the graph.
The electrical resistance value of polyaniline in the presence of high concentrations of ammonia, in ohm, is equal to
a) 0.5 × 100 .
b) 2.0 × 100 .
c) 2.5 × 105 .
d) 5.0 × 105 .
e) 2.0 × 106 .
Correct alternative: e) 2.0 × 106.
To start the question, it is important to note that the graph represents the relationship between current (i) and d.d.p (U).
We see that the two quantities are directly proportional, because when the potential difference increases, the current increases in the same proportion.
It should also be noted that the current value is multiplied by 10-6. Therefore, it will be important that you master the calculations with powers of ten.
Even questions that do not have a power of ten, but have numbers with many zeros or many digits, it is interesting to use this feature, as it speeds up calculations.
The first step is to find the resistance value for low ammonia concentrations using the graph.
For this, we can choose any point on the graph, but always try to choose the point that is easier to solve the calculations.
We choose the point (0.5 ,1.0. 10-6) and we replace in the list:
To make the calculation easier, we can also transform the 0.5 to a power of ten:
Now, just multiply this value by 4, as the resistance in the presence of high concentrations of ammonia has its value quadrupled.
5. control the time
You may already know that the correction of the Enem test takes into account the consistency of the answers, that is, who gets it right more difficult questions and easy mistakes have their final grade decreased because the system considers that the student got it right by "kick".
This often happens, with some students who spend a long time on a particular question that is more difficult and at the end of the test they no longer have time to read the other questions.
So that this doesn't happen to you, learn to control time!
Students should spend an average of 2 minutes on each question. If you find that one issue is taking much longer than that, move on to another and if you have time, try to resolve it at the end.
A tip is when solving physics questions write down how many minutes it takes for each question and keep trying to reduce this time.
Taking simulations and tests from previous years, using a stopwatch, is also a good option. In addition to getting used to the style of the test, you will learn to manage time.
Remember: time is your biggest enemy in Enem!
Don't stop there. There are more texts very useful for you:
- Physics in Enem: subjects that fall the most (with exercises)
- Enem: Everything you need to know
- Natural Sciences and its Technologies
- Enem Study Plan
- How to Study for Enem: valuable tips
- News that may fall into Enem and Vestibular
- Physics Formulas
- Simulated Enem: questions that fell on the test
- Enem's questions that fell on the test
