MCAT Physical : MCAT Physical Sciences

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

Example Question #1 : Density

What is the specific gravity of a boat that has a mass of 6000kg and a volume of 10m3?

Possible Answers:

100

6

10

0.6

1

Correct answer:

0.6

Explanation:

Specific gravity is the density of the substance over the density of water.

\(\displaystyle SG= \rho _{substance}/\rho _{water}\)

\(\displaystyle \rho _{water}= 1000 kg/m^{3}\)

Density is given by mass over unit volume.

\(\displaystyle \rho =m/V\)

A boat with a mass of 6000kg and a volume of 10m3 will have a density of 600 kg/m3.

\(\displaystyle \rho=\frac{6000kg}{10m^3}=600\frac{kg}{m^3}\)

\(\displaystyle SG=\frac{600\frac{kg}{m^3}}{1000\frac{kg}{m^3}}=0.6\)

Example Question #1 : Density

A 200-gram object is placed into a bucket of water and floats so that its top edge is just below the surface of the water. Which best describes the density of this object?

Density of water is 1000kg/m3.

Possible Answers:

\(\displaystyle \rho_{object} > \rho_{water}\)

\(\displaystyle \rho_{object}\) may be different depending on the volume of water in the bucket

\(\displaystyle \rho_{object}\) varies depending on the depth of the object below the surface

\(\displaystyle \rho_{object} < \rho_{water}\)

\(\displaystyle \rho_{object} = \rho_{water}\)

Correct answer:

\(\displaystyle \rho_{object} = \rho_{water}\)

Explanation:

If the object is at rest, the net force on it must equal 0. At any point in the liquid, the total downwards force is described as the difference between the gravitational force and bouyant force: \(\displaystyle F_{net} = F_{g} - F_{b}\). When the object is not accelerating, this reduces to \(\displaystyle F_{g} = F_{b}\).

Plugging in the equations for these forces respectively yields \(\displaystyle m_{object}g = \rho_{water}V_{displaced}g\).

Since mass is the product of density and volume, \(\displaystyle \rho_{object}V_{object}g = \rho_{water}V_{displaced}g\).

Note that in this case the object's volume equals the volume of liquid displaced, since the object is completely submerged. So, the volumes on either side cancel, as does gravity, leaving just \(\displaystyle \rho_{object} = \rho_{water}\).

Example Question #3 : Density

An ice cube is placed into a glass containing an unknown liquid. If the ice cube sinks to the bottom, which of the following conclusions could be made?

Possible Answers:

The liquid has a density less than that of water

No conclusions can be made unless the temperature of the liquid is known

The density of the liquid is equal to the density of water

No conclusions can be made unless the mass of the ice cube is known

The liquid has a density greater than that of water

Correct answer:

The liquid has a density less than that of water

Explanation:

The density of ice is less than that of water (density of ice = 0.92 g/cm3, density of water = 1 g/cm3). Ice will only sink in liquids that are less dense than 0.92 g/cm3. Since ice will only sink in liquids that are less dense than water, the unknown liquid must have a lower density than that of water.

Example Question #3 : Density

Phase diagrams are used to depict changes in the properties of a solution at different temperatures and pressures. Below is a phase diagram of a polar solution.

Phase_diagram_ps

The density of the solution in section 1 is __________ the density of the solution in section 2.

Possible Answers:

less than

greater than

equal to

Correct answer:

less than

Explanation:

The slope of the solid/liquid phase transition line can predict the comparative density of the solution in each section. A negative slope indicates that the solid phase is less dense than the liquid phase. A positive slope indicates that the solid is more dense. In the above phase diagram, the slope is negative, indicating that the solid is less dense than the liquid. 

Example Question #1 : Density

An object is at equilibrium when of the total volume is submerged in gasoline. Find the density of the object.

\(\displaystyle \small \rho_{gasoline} = 0.8\frac{g}{mL}\)

Possible Answers:

\(\displaystyle 0.264\frac{g}{L}\)

\(\displaystyle 0.264\frac{g}{mL}\)

\(\displaystyle 0.736\frac{g}{mL}\)

\(\displaystyle 0.736\frac{g}{L}\)

\(\displaystyle 0.8\frac{g}{mL}\)

Correct answer:

\(\displaystyle 0.264\frac{g}{mL}\)

Explanation:

To solve, we can equate the volume of the gasoline displaced to the volume of the portion of the object that is submerged.

\(\displaystyle V_{gas}=V_{obj}\)

\(\displaystyle V=\frac{m}{\rho}\)

We know that of the object is submerged, thus of the object's total volume will equal the volume of water displaced.

\(\displaystyle \frac{m_{gas}}{\rho_{gas}}=0.33*\frac{m_{obj}}{\rho_{obj}}\)

\(\displaystyle \frac{\rho_{obj}}{\rho_{gas}}=0.33\frac{m_{obj}}{m_{gas}}\)

The displaced masses are equal.

\(\displaystyle \frac{\rho_{obj}}{\rho_{gas}}=0.33\)

 \(\displaystyle \rho_{obj}=(0.33)(0.8\frac{g}{mL})=0.264\frac{g}{mL}\)

Example Question #441 : Mcat Physical Sciences

Water has a density of \(\displaystyle \small 1.00\frac{g}{cm^3}\) at a temperature of \(\displaystyle \small 4^oC\). An certain oil has a specific gravity of \(\displaystyle \small 0.882\) at this temperature. What is the mass of \(\displaystyle \small 1L\) of the oil?

Possible Answers:

\(\displaystyle 882 g\)

\(\displaystyle 88.2g\)

\(\displaystyle 8.82g\)

\(\displaystyle 1134g\)

\(\displaystyle 1000g\)

Correct answer:

\(\displaystyle 882 g\)

Explanation:

The formula for density is:

\(\displaystyle \rho=\frac{\text{mass}}{\text{volume}}=\frac{m}{V}\)

Specific gravity is the density of a material relative to the density of water.

\(\displaystyle SG_{oil}=\frac{\rho_{oil}}{\rho_{water}}\)

We are given the specific gravity of the oil and the density of water, allowing us to calculate the density of the oil.

\(\displaystyle 0.882=\frac{\rho_{oil}}{1.00\frac{g}{cm^3}}\)

\(\displaystyle \rho_{oil}=(0.882)(1.00\frac{g}{cm^3})=0.882\frac{g}{cm^3}\)

Returning to the equation for density, we can use the density of the oil to find the mass on one liter.

\(\displaystyle \rho_{oil}=\frac{m}{V}\)

\(\displaystyle 0.882\frac{g}{cm^3}=\frac{m}{1000cm^3}\)

\(\displaystyle m=(0.882\frac{g}{cm^3})(1000cm^3)=882g\)

Example Question #87 : Ap Physics 2

A given iceberg floats such that 90% of its volume is below water. Supposing that the iceberg is composed of pure water, what is the density of the saltwater in which the iceberg floats?

\(\displaystyle \rho_{ice}=0.917\frac{g}{mL}\)

Possible Answers:

\(\displaystyle 1.019\frac{g}{mL}\)

\(\displaystyle 0.7063\frac{g}{mL}\)

\(\displaystyle 0.9017\frac{g}{mL}\)

\(\displaystyle 1.112\frac{g}{mL}\)

\(\displaystyle 1.000\frac{g}{mL}\)

Correct answer:

\(\displaystyle 1.019\frac{g}{mL}\)

Explanation:

We can solve this question by using a density ratio, given by the equation:

\(\displaystyle X=\frac{\rho_{iceberg}}{\rho_{saltwater}}\)

We know the density of ice, and we can find the ratio of the densities by using buoyancy. If the iceberg were completely submerged, then we could conclude that the density of the ice was equal to the density of the salt water. Since the iceberg is 90% submerged, we can conclude that the density of the ice is 90% of the density of the salt water.

\(\displaystyle \frac{\rho_{iceberg}}{\rho_{saltwater}}=0.9\)

Use this ratio and the density of ice to solve for the density of the salt water.

\(\displaystyle 0.9=\frac{0.917\frac{g}{mL}}{\rho_{saltwater}}\)

\(\displaystyle \rho_{saltwater}=1.019\frac{g}{mL}\)

Example Question #11 : Fluids And Gases

A researcher performs an elemental analysis on a compound. He finds that the compound is made up of only carbon, hydrogen, and oxygen atoms. He isolates a pure sample of the compound and finds that this sample contains \(\displaystyle 144g\) of carbon, \(\displaystyle 24g\) of hydrogen, and \(\displaystyle 32g\) of oxygen. The researcher wants to perform further analysis on this compound the next day. Before leaving the lab the researcher creates three stock solutions of varying concentrations of this compound: \(\displaystyle 1M\) (solution A), \(\displaystyle 2M\) (solution B), and \(\displaystyle 3M\) (solution C). He stores these solutions overnight at a temperature of \(\displaystyle 15^oC\).

Molecular weight of this compound = \(\displaystyle 300.486\frac{g}{mol}\)

Which of the following is true regarding the densities of the three stock solutions?

Possible Answers:

\(\displaystyle \rho_A>\rho_B>\rho_C\)

\(\displaystyle \rho_A=\rho_B< \rho_C\)

\(\displaystyle \rho_A=\rho_B=\rho_C\)

\(\displaystyle \rho_A< \rho_B< \rho_C\)

Correct answer:

\(\displaystyle \rho_A< \rho_B< \rho_C\)

Explanation:

Density is a measure of concentration, defined as mass per unit volume.

\(\displaystyle \rho=\frac{\text{mass}}{\text{volume}}\)

Since the stock solutions contain different concentrations, the density of each solution must be different. The solution with the highest concentration will contain the highest density, and the solution with lowest concentration will contain the lowest density. The order of increasing density is solution A, solution B, and solution C. Solution C is the most concentrated and will contain the greatest amount of mass per unit volume, followed by solution B and solution A.

\(\displaystyle \rho_A< \rho_B< \rho_C\)

Remember that concentration can be defined in various ways: molarity, molality, mass percent, density, etc. Increasing concentration means you are increasing all of these quantities. Also remember that the density of the compound remains the same in each solution; only the solutions, as a whole, have different densities.

Example Question #1 : Buoyancy And Displacement

Use the following information to answer questions 1-6:

The circulatory system of humans is a closed system consisting of a pump that moves blood throughout the body through arteries, capillaries, and veins. The capillaries are small and thin, allowing blood to easily perfuse the organ systems. Being a closed system, we can model the human circulatory system like an electrical circuit, making modifications for the use of a fluid rather than electrons. The heart acts as the primary force for movement of the fluid, the fluid moves through arteries and veins, and resistance to blood flow occurs depending on perfusion rates.

To model the behavior of fluids in the circulatory system, we can modify Ohm’s law of V = IR to ∆P = FR where ∆P is the change in pressure (mmHg), F is the rate of flow (ml/min), and R is resistance to flow (mm Hg/ml/min). Resistance to fluid flow in a tube is described by Poiseuille’s law: R = 8hl/πr4 where l is the length of the tube, h is the viscosity of the fluid, and r is the radius of the tube. Viscosity of blood is higher than water due to the presence of blood cells such as erythrocytes, leukocytes, and thrombocytes.

The above equations hold true for smooth, laminar flow. Deviations occur, however, when turbulent flow is present. Turbulent flow can be described as nonlinear or tumultuous, with whirling, glugging or otherwise unpredictable flow rates. Turbulence can occur when the anatomy of the tube deviates, for example during sharp bends or compressions. We can also get turbulent flow when the velocity exceeds critical velocity vc, defined below.

 vc = NRh/ρD

NR is Reynold’s constant, h is the viscosity of the fluid, ρ is the density of the fluid, and D is the diameter of the tube. The density of blood is measured to be 1060 kg/m3.

Another key feature of the circulatory system is that it is set up such that the organ systems act in parallel rather than in series. This allows the body to modify how much blood is flowing to each organ system, which would not be possible under a serial construction. This setup is represented in Figure 1.

Circulatory_system_circuit

 A red blood cell floating in the body weighs 27 picograms and has a volume of 40.0 * 10-12cm3. What is the buoyant force on the red blood cell?

Possible Answers:

2.7N

4 * 10-7N

2.7 * 10-10N

4 * 10-13N

Correct answer:

4 * 10-13N

Explanation:

To answer this question, we use Fb = Vgρ, where V is volume, g is gravity, and ρ is the density of the fluid.

In this case we can use the density of blood, since it is given in the passage as 1060kg/m3. Since the density is in cubic meters, we have to make sure that the volume is also converted to cubic meters.

(4.0 * 10-11)x (1 * 10-6m/ 1cm) = 4 * 10-17m3

Fb = (4.0 x 10-17m3) (10 m/s2) (1.06 x 103kg/m3) = 4 * 10-13N

Example Question #2 : Buoyancy And Displacement

Two identical balloons, balloon A and balloon B, are filled with gas A and gas B, respectively. In which described scenario will the balloon rise the fastest?

\(\displaystyle \small \rho_{gasA} = 100\frac{mg}{mL}\)

\(\displaystyle \small \rho_{gasB} = 50\frac{mg}{mL}\)

Possible Answers:

Balloon A at \(\displaystyle \small 50^oC\)

Balloon B at \(\displaystyle \small 50^oC\)

Balloon B at \(\displaystyle \small 100^oC\)

Balloon A at \(\displaystyle \small 100^oC\)

All scenarios describe equal rising rates

Correct answer:

Balloon B at \(\displaystyle \small 100^oC\)

Explanation:

As temperature increases, density decreases.

In a free body diagram of each balloon, the only forces acting in the vertical direction are gravity and buoyant force.

\(\displaystyle F_g =mg= (\rho_{obj})(V_{obj})g\)

\(\displaystyle F_B = (\rho_{medium})(V_{obj})g\)

These forces act in opposite directions, with the buoyant force pulling up and gravity pulling down.

As gas B is heated to the higher temperature, the density decreases. Gas B already has a lower density than gas A; heating it simply intensifies this discrepancy. As the density decreases, the force of gravity on the balloon decreases as well, allowing a greater net force in the upward direction due to the buoyant force.

The net upward force experienced by balloon B at the highest temperature will be greater than any of the other scenarios listed, resulting in the greatest upward acceleration.

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