The subatomic particles in an atom's nucleus, protons and neutrons, are held together by their own force called the strong nuclear force. Strong nuclear forces act in a somewhat counterintuitive way, and only over very short distances. The strong force is what allows protons to overcome their electrical repulsion to one another and exist together in the nucleus.

The binding energy of atoms is due in large part to the nuclear force, or strong force, which is the most powerful of the four universal forces.

Because the magnitude of the strong force is so large, it is able to overcome the electromagnetic force that is generated by the repulsion of like charges. Atoms exist in other parts of the universe beyond Earth, so the gravitational force is clearly not responsible. Neutrons have "neutral" charge, but that does not mean they are able to "neutralize" positive or negative charges.

Nuclear attraction is a force that holds together the molecules in the nucleus of an atom. Remember that there are a total of three subatomic particles in an atom: protons, neutrons, and electrons. Protons are positively charged, neutrons are neutral, and electrons are negatively charged. Of the three subatomic particles, only protons and neutrons are found inside the nucleus. Nuclear attraction must occur between a neutron and a proton. This force counteracts repulsion between protons to hold the nucleus together. Electrons are found in electron shells outside the nucleus and do not participate in nuclear attraction.

Nuclear forces occur between protons and neutrons in the nucleus of an atom. Recall that quarks are elementary particles that make up composite particles called hadrons. Protons and neutrons are types of hadrons; therefore, nuclear forces occur between two hadrons.

There are four major types of fundamental forces: gravitational force, electromagnetic force, weak force, and strong force. Gravitational force is the attractive force between two bodies, electromagnetic force is a combination of the force between charges and magnetic force, weak force is the force between a W boson and a Z boson, and strong force is the force that occurs between two quarks. Nuclear forces between protons and neutrons are an effect created by the strong force between two quarks; however, nuclear forces are not classified as a type of strong force.

Since the half-life of the sample is 3 days long, throughout the course of 9 days, the sample will undergo a half-life cycle 3 times. This means we have to half the mass of the original sample 3 times.

We originally start with 

After 3 days: 

After 6 days: 

After 9 days: 

Another way to solve these types of problems is using this formula:  where  is the number of half lives. 

thus  or  of the original sample remains. 

Therefore after 9 days, there will be  of the original sample remaining.

This question asks us to determine what would happen to the intensity of a sound wave if a variable could be changed. First, we need to remember the equation for intensity: , where A is the wave amplitude, rho is the density of the medium, f is the frequency of the wave, and v is the velocity of the wave.

This formula is important to understand qualitatively. For the MCAT, it is important to understand how the various factors impact the intensity of a wave. If we wanted to quadruple the intensity, we can see that we could double amplitude or double frequency. Seeing that we want the sound pitch to remain the same, we would want to double the amplitude and not the frequency. The relationship between intensity, frequency, and amplitude is important to understand.

This question asks us to determine what would happen to the intensity of a sound wave if a variable could be changed. First, we need to remember the equation for intensity, , where P is the power of the wave, A is the area,  is the wave amplitude, rho is the density of the medium, f is the frequency of the wave, and v is the velocity of the wave.

This formula is important to understand qualitatively. For the MCAT, it is important to understand how the various factors impact the intensity of a wave. If we wanted to halve the intensity, we can see that we could halve the power of the wave or double the area the wave is hitting; thus, we could double the size of the room, allowing the sound wave to have more room to spread out and increasing the area the wave interacts with.

This question asks us to determine the intensity of a wave, given to us as a comparison of the wave intensity to the intensity of the limit of human hearing. In math terms, we know that the decibel scale is calculated as shown below.

I₀ is the limit of human hearing (10^-12 W/m²).

Regardless of the units measured, a ten-fold decrease in volume equals a  decrease in intensity level.

Here, there is a twenty-fold decrease in sound intensity.

A ten-fold decrease would be a  change, and a hundred-fold decrease would be a  change. A  decrease is closer to a ten-fold decrease than a hundred-fold decrease, hence the decrease in intensity level should be closer to .

When a sound wave contacts a surface, it transfers longitudinal vibration into the solid. Because of the increased density in the solid, these longitudinal compressions actually speed up and increase in wavelength. The frequency of the sound remains constant.

Only some of the sound energy is transferred to the solid, however. When the wave impacts the solid, some of the energy is bounced of the surface and reflected back to the source as an echo. This reduction of energy accounts for the lessened intensity experienced by a listener on the opposite side of the wall. Energy is also lost to internal reflection within the wall.