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Physics Questions People Ask During Job Interviews

The question from left field. I was ready for this interview. I studied the company, industry, technology, people, and FOF articles about Salesforce interview questions. They make microcontrollers for swimming pool filtration systems. They need an Admin to support a vibrant growing business. First question, before we took a seat: “Explain what pH all about”.

Left field questions I have experienced. Everyone has experienced interview questions from left field. Technical screens have their own special variety of curve balls. Here are some I have experienced:

  • A projectile is fired horizontally at the same time as a marble is dropped from the same height. Which object hits the ground first? 
  • Why does the moon orbit the earth?
  • Twirl a ball on a string. Release it. In what direction does the ball travel? Describe the trajectory.
  • A boat floats in a pond. Throw a rock from the boat into the water. Does the pond waterline go up, down, or remain unchanged? Why?
  • Melt an ice cube in a glass of water. Does the waterline go up, down, or remain unchanged?
  • Why is the sky blue?
  • Why does the sound of a horn from an approaching vehicle seem to rise in pitch, and seem to descend in pitch as the vehicle travels away?

There is no mystery to the rationale for such questions. Employers want to see you think on your feet, explain things clearly, react to a surprise, and show that you are well rounded as a software engineer. 

Projectile fired horizontally

The objects hit the ground at the same time. A projectile fired horizontally will descend at the same rate as an object simultaneously dropped from the same height and falling directly down. The initial trajectories are at right angles to each other. Use ‘orthogonal’ instead of perpendicular if you want to really sound technical. Vertical acceleration due to gravity is independent of horizontal acceleration.

This assumes the marble is dropped directly down, the projectile travels horizontally and at a right angle to the trajectory of the marble. It ignores air resistance, curvature of the earth, projectile shape, aerodynamic lift, air density, humidity, weather, the stock market, the wrath of god, the will of Zeus, whether Benioff shaved his beard, and other environmental factors. Accelerations that are orthogonal by definition have no influence on each other. This can be proven mathematically using cross product.

Gravity and orbiting planets

The force causing a celestial body to orbit another celestial body is gravity. This is best explained with Newton’s first law of motion, often called the ‘inertia law’. Orbiting bodies such as the moon want to travel in a straight line. Gravitational forces between moon and earth bend that line of travel. In physics class this is called centripetal (‘center seeking’) acceleration.

Gravity and orbiting planets

Force is a push or a pull. There are four forces, and only four, in our universe: gravity, magnetism, strong nuclear, and weak nuclear. Five if you wish to include my wife’s persistent will. Six if you wish to include Gemma Blezard. There is no such thing as centrifugal force. This is an illusion created by a rotating inertial reference frame.

Technically the moon does not orbit the earth, and planets in our solar system do not orbit the sun. They all orbit around their combined center of gravity. The combined center of gravity between the earth and moon is positioned very close to the center of the earth, creating an appearance of the moon orbiting earth.

Twirl a ball on a string. Release it. The ball and string will travel in a straight line at a perpendicular to the radial position where it was released.

The next two water related questions all come from Archimedes principles.

Rock thrown from a boat in a pond. The water level goes down.

Imagine a boat floating in a pond. There is a rock in the boat. Throw the rock into the water and the waterline of the pond will go down. In the boat, the rock displaces its mass. In the water the rock displaces its volume. Assuming the rock is denser than water, and there are exceptions, a pound of rock occupies less volume than a pound of water. This can also be proven mathematically.

Melting ice cube. Water level does not change.

Melt an ice cube in a glass of water and the water level will not change. The water line will not go up or down. Ice is marginally less dense than water. Ice cubes and icebergs float. The floating ice cube displaces a volume of water that weighs the same as the ice cube.

Similarly, picture two identical buckets filled with water to the brim. One bucket has a piece of wood floating in it. Which bucket is heavier? Why? Both buckets and contents weigh the same. The floating object displaces a volume of water that weighs the same as the floating object. Take the one-pound piece of wood out of the bucket, replace it with a pound of water, and the bucket will be filled to the brim.

Blue sky caused by atmospheric particle reflection

If you get this question, it may behoove you to first look outside and make sure the sky is blue. The weather may be foggy, overcast, sunset, evening; in which case the sky may not be blue. On a clear day, with the sun high in the sky, it is most likely blue.

There are several reasons for blue sky but most of it is from the scattering of light waves traveling through earth’s atmosphere and reflecting off atmospheric particles. Shorter wavelength light is more prone to scattering and is therefore more visible.

Light from the sun is energy. It travels in the form of a sine wave. Yellow, orange, and red light waves have a relatively long wavelength. This is the distance from peak to peak of the wave. Blue and Purple have a relatively short wavelength. Short wavelength light is more prone to bouncing and reflecting off atmospheric particles. This makes the light more visible. The result is more visible light, especially blue.

Light on earth travels from the sun through the atmosphere. Our atmosphere is loaded with particles such as oxygen, water vapor, helium, hydrogen, nitrogen, ozone, other greenhouse gasses, natural particulates, industrial particulates, vehicle emissions, and other molecules and particulates. Light with longer wavelengths can more easily travel through the atmosphere without colliding and scattering in the atmosphere. Short wavelength light is more likely to bounce or reflect off atmospheric particles. The result is more visible light of short wavelength.

In the evening the path of light from the sun to you travels through the atmosphere at an acute angle. That path travels through more atmosphere, allowing more bounce and reflection from a range of shorter wavelength light bands. This is why sunsets appear to have red and purple hues.

Approaching sound waves are compressed

The sound of a horn from an approaching vehicle seems to rise in pitch, and seems to fall in pitch as the vehicle gets farther away. Sound and light are forms of energy that travel in waves. Wavelength is the distance between consecutive peaks. Higher pitched sound has a shorter wavelength. Lower pitched sound has a longer wavelength. The sound wave emitted from a vehicle traveling towards you is continuously compressed because the sound wave and the  source of the sound are moving towards you.

A compressed sound wave has a continuously shortened wavelength and therefore sounds higher in pitch. If the sound source is moving away from you the sound wave is continuously being stretched, causing a progressively longer wavelength. The sound seems continuously lower until it is no longer audible.

Compressed sound wave

This can also be visualized using a water hose. Stand in your backyard holding an outstretched garden hose. Shake it up and down and it will start to resemble a sound wave. Walk away from the hose while shaking it. The wave stretches out. The wavelength increases. Walk towards the hose while shaking it and the wave length shortens. This is what happens to a sound wave from a moving vehicle.

pH is the negative base ten logarithm of the molar concentration of hydronium [H3O+] in an aqueous, or water based, solution. It indicates the relative concentrations of hydronium [H30+] and hydroxide [OH-] ions, indicating the acidity of the solution. A pH value of seven is considered neutral and means there are 1.0 * 10^-7 moles per liter of hydronium present in the solution. There are also 1.0 * 10^-7 moles per liter of hydronium.

That interview, and frustration with not remembering what pH is, motivated me to take a few chemistry classes at the local community college. Like many exogenous and odd questions, it will most likely never be seen again, but it is good to know such things.


We all know there are good reasons for a rigorous technical screen. The smart Admin will embrace the experience. Employers want to know what you know. They want to observe how you navigate challenging and new problems you do not know the answer to.

For you the candidate, this is a good opportunity to learn about your prospective employer. Some interviewers are professional, polite, gentle, and patient about asking such questions. Some will offer gentle guidance if you are struggling. Other interviewers seem to relish the experience of making a candidate uncomfortable, or they enjoy telling you how smart they are. By weathering a rigorous technical screen you learn a lot about a prospective employer and whether you would really want to work there.

Who else?

The orbiting planet question degenerated into an argument with the interviewer who believed ‘centrifugal force’ is a force. It isn’t. I lost the argument. He was a real pompous horse's ass with an inflated ego and limited tolerance for dissenting points of view. Nobody wants to work for someone like that. Things degenerated further when I started addressing him as Copernicus. For some odd reason I never heard back after that first interview.

These sorts of seemingly odd and exogenous questions from a Salesforce technical screen seem to stick with me. I would be interested in knowing from other readers of this post what kinds of similarly interesting questions you have experienced.

Physics Questions People Ask During Job Interviews

Note on sources

pH is explained in most chemistry textbooks. I used Chemistry and Chemical Reactivity, by Kotz, Treichel., and Townsend, seventh edition. Forces, centripetal acceleration, Archimedes’ principles, gravity, light, sound, and related and very interesting topics are explained in most first semester text books on mechanical physics. Physics, by Haliday and Resnik is my favorite. It is something of a classic in academic circles. I used the fourth and tenth editions for this article.

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Dave Ge

Dave Ge

Sailing Instructor, 11x certified Salesforce Administrator, Math Tutor.