1.5
Estimativa das Grandezas Físicas
Em muitas ocasiões, físicos, outros cientistas e engenheiros precisam fazer estimativas de uma quantidade específica. Isso às vezes é chamado de estim…
Problemas envolvendo a estimativa de quantidades físicas são frequentemente chamados de problema de Fermi, em homenagem ao físico Enrico Fermi, conhecido por fazer estimativas rápidas e aproximadas com base em cálculos e suposições simples.
A determinação do número de folhas de grama em um campo de futebol é um exemplo de um problema de Fermi.
A resposta para qualquer problema de Fermi, chamada de estimativa de Fermi, é alcançada dividindo o problema em partes menores e usando as informações disponíveis para estimar aproximadamente as incógnitas.
Agora, o número de folhas de grama em um campo de futebol pode ser estimado considerando as dimensões do campo e estimando que existem 20 folhas de grama em um centímetro quadrado.
A partir do comprimento e largura, a área do campo de futebol é calculada.
Em seguida, convertendo a área do campo em centímetros e multiplicando a área pelo número de folhas de grama por unidade de área, obtém-se a estimativa de Fermi do número total de folhas de grama.
View the full transcript and gain access to JoVE Core videos
Q1: What is a Fermi problem and who was it named after?
A Fermi problem is named after physicist Enrico Fermi, who was famous for making quick, rough estimates based on simple calculations and assumptions. These problems involve breaking complex estimation tasks into smaller, manageable parts using available information to estimate unknowns. Fermi problems develop physical intuition and help scientists and engineers make reasonable approximations of quantities.
Q2: How do you estimate the number of blades of grass on a soccer field?
To estimate grass blades on a soccer field, first calculate the field's area using its length and width. Then estimate the number of grass blades per unit area, such as 20 blades per square centimeter. Convert the field's area to centimeters and multiply by the blades per unit area to obtain the total Fermi estimate.
Q3: What strategies help when estimating physical quantities?
Key estimation strategies include obtaining big lengths from smaller lengths by breaking objects into manageable parts, deriving areas and volumes from linear dimensions using geometric formulas, and estimating masses from volumes and densities. When dealing with completely unknown quantities, bound the estimate by determining what it must be larger and smaller than. One significant figure is typically sufficient for estimation calculations.
Q4: Why is estimation useful in physics and engineering?
Estimation develops physical intuition and allows scientists to perform sanity checks on calculations and policy proposals by ruling out unrealistic scenarios. Estimates help identify correct physical principles and relevant variables. Using prior experience and sound physical reasoning, estimation provides rough approximations that guide further investigation and validate whether results are reasonable.
Q5: How do you estimate the volume or area of a complex object?
When estimating volume or area of complex objects, introduce a simple geometric model such as a sphere or box. Estimate the linear dimensions first, then use standard geometric formulas to calculate the volume or area from those estimates. This approach simplifies calculations and provides reasonable approximations for irregular shapes.
Q6: What should you check after making an estimation calculation?
After completing an estimation, ask whether the answer makes sense by checking if it is reasonable. Compare the result with values of other known quantities having the same dimensions. This sanity check helps verify that your estimate is physically plausible and within an expected range, catching errors in calculation or assumptions.
Q7: What is the difference between estimation and random guessing?
Estimation uses prior experience and sound physical reasoning to determine a quantity's value, not random guessing. Effective estimation requires identifying correct physical principles and making informed guesses about relevant variables. Many estimates rely on formulas where input quantities are known only to limited precision, making systematic reasoning essential for obtaining meaningful approximations.