The Physics of Everyday Objects

STEMgineer’s Guide to Problem Solving Workbooks

Physics is everywhere in our lives, even though we don’t often realize how or why everyday objects actually work. From doorknobs to straws to phones, physics is what keeps everything in our world functioning. Understanding the science behind these questions can help us see the world from a completely different perspective.

Why Are Doorknobs Always Placed in the Same Position?

Think about the last door you opened: where was the doorknob? Nearly every doorknob is placed on the side opposite the hinges, just about halfway up the door. The reason behind this placement is torque. If you pulled or pushed on the door next to the hinges, it wouldn’t move. Torque is the force required to rotate an object around an axis or fulcrum. A door is essentially a large lever, and the hinge is a fulcrum. Torque depends on the direction of the force and how far the force is from the axis, so with a door, you have more torque when you apply force farther from the hinge.

How Do Straws Work?

Straws are everywhere, but you probably can’t remember learning to use one. While straws may seem simple to understand, did you know that you can’t use a straw in space? Straws work because of atmospheric pressure. When you drink using a straw, you’re reducing the air pressure inside the straw. When you lower the pressure inside the straw, atmospheric pressure pushes down on the liquid and forces it up. In space, there is no atmospheric pressure, meaning that the liquid won’t rise in the straw, no matter how hard you try. You can actually test this idea for yourself with this experiment from Forbes. First, grab a cup with a lid and straw. Fill the cup with a liquid of your choice. Using aquarium caulk, seal the edges around the lid and around the straw. The caulk will prevent air from escaping, preventing atmospheric pressure from acting on the cup. As long as there are no gaps, you won’t be able to use the straw.

Why Does Ice Float?

In nearly every substance, the solid form is denser than the liquid form, causing the solid to sink. Water is the one exception, allowing ice to float on the top of the water. Ice is actually less dense than water. Archimedes’s Principle explains that something only floats if it is less dense than water because of buoyancy. This is why rocks sink, but ships can float. Similarly, ice only floats because it’s less dense than water. This unique property comes from water’s molecular composition. Water is made of two positively charged hydrogen atoms and one negatively charged oxygen atom. The hydrogen atoms form weak bonds with the hydrogen atoms from other molecules because these atoms share the same charge. These hydrogen bonds cause the molecules to form a hexagonal pattern, creating space between the molecules that makes ice less dense than water. This simple rule of the universe allows life on Earth to work. If ice sank, it would kill all the fish and other aquatic life that live at the bottom of bodies of water.

How Does Your Phone’s Touchscreen Work?

Have you ever wondered why you can just use a finger to click something on a phone, but you need a mouse and keyboard to use anything on a computer? Capacitive touchscreens have special technology inside them that can read the electrical signals from your body. Your body actually conducts electricity through your central nervous system. The touchscreen can identify where your finger is based on the electricity and update the display accordingly. This is why wearing gloves or having wet hands can make a touchscreen work less accurately. 

Why Can We Hear Things That We Cannot See?

If light and sound travel in waves, why can we talk to someone around the corner whom we cannot see? While both light and sound are waves, they are different types of waves with different wavelengths. Sound is a mechanical wave, meaning that it needs a medium to move through. Sound moves through the air, water, or even walls, but it cannot travel without a medium. There’s no sound in space because there is no air for it to travel through; it’s a mechanical wave. On the other hand, light is an electromagnetic wave. These waves travel through electric and magnetic fields, which is why we can see things from space that we cannot hear. There is a broad spectrum of electromagnetic waves, and light is only a small part of the larger spectrum. According to NASA, visible light wavelengths range from 380 nanometers to 700 nanometers. Sound has much larger wavelengths, which allow it to diffract, or bend and spread around obstacles, more easily. This is why you can hear someone around a corner but can’t see them. Because visible light has much shorter wavelengths, it doesn’t diffract around large objects like walls as noticeably as sound does.

Thinking About the World Scientifically

Everything in our world is fueled by science. Although we can easily overlook aspects of our world that seem ordinary, asking questions about how our everyday objects work helps students build curiosity and think critically about the world around them. From opening a door to using a phone, physics is constantly at work. Encouraging students to explore the science behind everyday experiences can make complex concepts easier to understand.

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