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Proportionality Basics

Understanding proportionality is crucial for excelling in various sections of the Medical College Admission Test (MCAT). Proportionality questions are common and span across multiple subjects, including physics, chemistry, and biology. These questions test your ability to understand and manipulate relationships between different variables. Mastering this concept not only aids in solving direct proportionality problems but also enhances your overall problem-solving skills, a vital aspect of the MCAT.

Identifying Proportionality Questions

Recognizing proportionality questions in the MCAT is key to tackling them efficiently. Here’s how you can spot them:

  1. Key Terms and Phrases: Look for words like “directly proportional,” “inversely proportional,” “varies as,” “increases,” “decreases,” and discussions of factors of increase or decrease. These terms often hint at a proportionality relationship.
  2. Comparison Format: Questions that ask about the effect of changing one variable on another are common. They typically follow a format like, “What happens to X if Y is doubled?”
  3. Graphical Data Analysis: Examine graphs for linear relationships (straight lines passing through the origin) or hyperbolic patterns, which may indicate proportionality.
  4. One Missing Variable Setup: Often, these questions present a scenario with two sets of variables. For example, “If the osmotic pressure (variable A1) is 5 atm at a 5M concentration (variable B1), what is the osmotic pressure (variable A2) under the same conditions if the concentration is 6 M?” These setups are designed to test your understanding of how changes in one variable affect another.

Identifying these types of questions quickly is crucial, as it allows you to apply the right problem-solving strategy efficiently.

Methodology for Solving Proportionality Questions

1. Determine the Proportionality Relationship

  • Isolating the Variable of Interest: The first step in solving a proportionality question is to isolate the variable you need to find out about or are given changes for. This can be done by rearranging the equation or, in questions without a specific equation, by focusing on how the variables are described in relation to each other in the scenario.
  • Identifying Direct or Inverse Relationships:
    • Direct Proportionality: This is when the relationship can be described as A = kB, where k is a constant. In this case, as one variable increases, the other also increases proportionally, and vice versa.
    • Inverse Proportionality: This occurs when the relationship is of the form A = k/B. Here, an increase in one variable results in a proportional decrease in the other, and vice versa. It’s crucial to recognize this relationship as it fundamentally changes how you predict the effect of variable changes.

2. Predict Value Changes

  • Apply the Identified Relationship: Based on whether the relationship is direct or inverse, predict how a change in one variable will affect the other. Remember that in direct proportionality, variables change in the same direction, while in inverse proportionality, they change in opposite directions.

3. Understand the Role of Exponents

  • Exponential Relationships: If the relationship involves an exponent (e.g., Y ∝ X²), changes in one variable have a more dramatic effect on the other. Doubling X in this example would quadruple Y.

4. Apply to Specific Situations

  • Use the Relationship: With the relationship and its nature (direct or inverse) established, apply this understanding to solve the question.
  • I know this step is a bit vague at present. We will be expanding upon it in further lessons for now go ahead and look through the examples to see how we use these steps to answer the problems.

Example Problems

1. Physics Example – Circuits:

Question: In a simple electric circuit, if the current (I) is inversely proportional to the resistance (R), and the current is 2 A when the resistance is 4 Ω, what is the current when the resistance is 8 Ω?

Original Equation: \( V = IR \) (Ohm’s Law).

Rearranging for Proportionality: \( I = \frac{V}{R} = \frac{k}{R} \), where \( k \) is a constant representing \( V \).


  • Step 1: Determine the Direction: Inverse relationship; increasing \( R \) decreases \( I \).
  • Step 2: Determine the Magnitude of Change: Doubling \( R \) should halve \( I \).
  • Step 3: Calculate the Change: \( \frac{1}{2} \times 2 \, \text{A} = 1 \, \text{A} \). The new current is 1 A.

2. Biology Example – Osmotic Pressure:

Question: If the osmotic pressure (π) of a solution is 10 atm at a concentration (C) of 2M, what is the osmotic pressure at a concentration of 3M?

Original Equation: \( \pi = iMRT \) (Van’t Hoff’s Law), where \( i \) is the ionization factor, \( M \) is molarity, \( R \) is the gas constant, and \( T \) is temperature.

Rearranging for Proportionality: \( \pi = k \times C \), where \( k = iRT \).


  • Step 1: Determine the Direction: Direct relationship; increasing \( C \) increases \( \pi \).
  • Step 2: Determine the Magnitude of Change: Concentration increases by 1.5x.
  • Step 3: Calculate the Change: \( 10 \, \text{atm} \times 1.5 = 15 \, \text{atm} \). The new osmotic pressure is 15 atm.

3. Physics Example – Kinetic Energy:

Question: A car of mass \( m \) (1000 kg) is traveling at a velocity \( v \) (10 m/s). If the velocity is doubled, what is the new kinetic energy \( KE \) of the car?

Original Equation: \( KE = \frac{1}{2} mv^2 \).

Introducing \( k \) for Proportionality: \( KE = k \times v^2 \), where \( k = \frac{1}{2} m \).


  • Step 1: Determine the Direction: Since \( KE \) is proportional to \( v^2 \), increasing \( v \) will significantly increase \( KE \).
  • Step 2: Determine the Magnitude of Change: Doubling \( v \) quadruples \( KE \) (since \( KE \) is proportional to the square of \( v \)).
  • Step 3: Calculate the Change: Original KE = \( \frac{1}{2} \times 1000 \, \text{kg} \times (10 \, \text{m/s})^2 = 50,000 \, \text{J} \). The new KE, with \( v \) doubled, would be \( 50,000 \, \text{J} \times 2^2 = 200,000 \, \text{J} \).