MCAT Physics Practice Question

A common concern students have in physics is being able to memorize all of the equations that are on the MCAT. My previous post talked about a technique for making study sheets as a way to memorize them.

However, another very important MCAT skill is interpreting the equations that are on the test. And when the MCAT wants to test your ability to interpret an equation, often it will simply give the equation to you. Here’s an example of how the MCAT will test this crucial skill:

Item 13

Escape velocity is the speed an object must reach to break free of a planet’s gravitational field regardless of the trajectory the object is following. Escape velocity is given by the equation:

where v is velocity, G is the gravitational constant, M is the mass of the planet, and r is the distance between the center of gravity of the planet and the object.

An object’s escape velocity will increase if:

A) the mass of the object increases.

B) the radius between the object and the planet increases.

C) the acceleration due to gravity decreases.

D) the radius between the object and the planet decreases.

Explanation

In addition to knowing what matters in an equation, we must also know what doesn’t matter. The MCAT does this a lot with gravity. When we’re solving questions that are set on the surface of the earth, the acceleration due to gravity, g, is usually just rounded off to 10  and this doesn’t change when the mass of the object changes.

In our example question, g doesn’t even show up. So changes in g won’t affect anything. That lets us eliminate (C). Similarly, the mass of the object isn’t in the equation. Since the mass of the object isn’t in the equation, changes in the object’s mass won’t have any effect. (A) is gone.

We’re now down to choices (B) and (D). Since the equation shows that there’s an inverse relationship between v and r (because r is in the denominator), to increase v we must decrease r, making (D) the correct answer.

This question would be considered an easy one on the MCAT, although the real MCAT does have a fair number of straightforward questions like this. The goal on Test Day is to make sure you don’t make careless mistakes, that you have a grasp on the basic concept, and that you can get it done quickly and confidently before moving on to more challenging material.