MCAT Question of the Day

Correct Answer: D. Electric potential

Fields are vectors because they cause forces, and force is a vector. Current is a vector because charge can flow in either direction in a wire. However, electric potential is related to potential energy (PE = qV), and all forms of energy are scalar quantities.

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Correct Answer: D. continue moving with constant velocity.

For something to change direction or accelerate there must be a net force acting on it (Newton’s 2nd Law).

Also, Newton’s 1st Law states that an object in motion will stay in motion at constant velocity (a form of inertia) unless acted upon by a net (an unbalanced) force.

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Correct Answer: B. B

While in the area between the plates, the negatively charged electron is attracted to the positive plate and so it would bend towards the positive plate, or “upwards” in the diagram. But after leaving the plates, there is no more net force acting on the electron. Therefore, the electron would continue in motion in a straight line according to Newton’s first law (inertia).

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Correct Answer: D. 11 tons.

This is the kind of reasoning question that you can expect as a result of an MCAT passage.

The difference in pressure between inside and outside the cabin at 10,000 meters is:
75 – 20 = 55 kPa = 55,000 Pa or N/m² of pressure pushing on the door (keep in mind that the SI unit for pressure is the pascal, Pa).

Pressure is defined as force per unit area: P = F/A
The area of the door is 2 x 1 = 2m²

So F = PA = (55,000)(2) = 110,000 N
Thus weight mg = 110,000, and since g = 10 m/s², then m = 11,000 kg or 11 metric tons.

Solving this problem requires reasoning, dimensional analysis and one very basic equation. List of physics equations to memorize: www.goldstandard-mcat.com/physics-equation-lists/

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Correct Answer: C. It is moving up and slowing down.

Let’s randomly assign the upwards direction as positive. If up is positive, keeping in mind that her weight is directed down (i.e. towards the center of the Earth), the scale pushes up so that she does not go through the scale, and we know that the sum of forces is equal to ma from Newton’s 2nd Law, we get:

Fscale – weight = ma

Examine the preceding equation again keeping in mind the following interpretation: The scale reading is less than her normal weight, so ma and thus the acceleration must be negative; i.e., down. Downward acceleration means either: (1) something speeds up while moving down, or (2) something slows down while moving up (i.e. upward deceleration).

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Correct Answer: A. decreases nonlinearly.

The only forces that act perpendicular to the incline are the normal force and the perpendicular component of weight. There is no acceleration perpendicular to the incline for this stationary object. The normal force is equal to the perpendicular component of the weight, which is given by mg cos. As the angle increases, the cosine of the angle must decrease. The graph of the normal force N vs. would show a curve, not a line and so the change is nonlinear.

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Correct Answer: C. 25 N

This is a ‘Double Block tackle’ and as there are four ropes lifting the 10 kg (100 N weight), each has a tension of 100/4 = 25 N.

The only catch is he needs to pull through four times the rope than would be required to simply lift the weight.

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Correct Answer: C. Newton’s Third Law of Motion

Newton’s Third Law of Motion states that: “When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction to that of the first body.”
Newton’s First Law of Motion deals with unbalanced forces or inertia and his second law with force, mass and acceleration, F = ma.
Note: Newton only had three Laws of Motion.

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Correct Answer: B. 6,667 km

The Universal Law of Gravity is an “Inverse Square Law”: F a (m1.m2)/d²
So if Planet = m1 = 4 and the moon = m2 = 1
The force due to the planet is: F1 = M1/d1² and the force due to the moon is: F2 = M2/d2²
At the point where the gravitational forces are equal F1 = F2.
So M1/d1² = M2/d2² and M1/ M2 = 4/1 = d1²/d2²
Therefore d1 = sqrt4 = 2 and d2 = sqrt1 = 1
The distance from the planet = 2/3 x 10,000 = 6,667 km.

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Correct Answer: D. 25 m/s

F = ma
a = F/m = (10 N/ 2.0kg) = 5.0 m/s²

V = V₀ + at
V = at
V = (5 m/s²)(5.0 sec) = 25 m/s

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