Now that you’ve mastered the basics of kinematics, it’s time to get into what causes different movements to occur – forces. A force in physics is generally described as a push or pull that can cause something to accelerate (which includes causing it to stop). There are a few different types of forces and by the end of this article, you should be able to identify each one and start looking for them in the real world.
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Describing Forces
A force is a push that can cause something to accelerate. This can be a literal push, for example, pushing on a large piece of furniture to move it around a room. It could be less literal, like pushing the gas pedal in a car. You don’t push on the car itself, but the engine starts working harder to move the car forward. This is a far less literal “push” but it is still a force causing the car to speed up. This “push” can be even less intuitive as forces can also act at a distance. For example, two magnets can attract or repel each other without touching. We’ll learn about these different types of forces soon, but first, let’s establish some language for talking about forces.
Is Force a Vector?
We talked about vectors and scalars back when we first began learning about kinematics and these ideas still apply. Think about when you push a piece of furniture around – maybe having to move your bed out from the wall when you change your sheets.
You don’t just push on it randomly and hope it will go in the direction you need it to. To move the bed, you push on it in the direction you want it to move. You also probably know how hard you’ll need to push your bed in order to move it. That means you have both a direction and a magnitude, which will be true of all types of forces. So, the answer to the question of Is force a vector? is yes. In fact, sometimes you may have your physics teacher refer specifically to force vectors, meaning a vector describing a force. To draw these vectors yourself, you’d want to make something like the diagram below.
This is a free body diagram and they’ll come in handy very often. For now, though, you just need to know that it’s how we visualize a vector relative to an object. We’ve written an “F” next to the arrow to show that the vector is specifically a force vector.
What are the Units of Force?
You’ll start to find that after a certain point in physics, most units are named after scientists. For example, the units used to describe force are Newtons (signified with an N). The unit Newtons is named after Sir Isaac Newton – a famous physicist who was one of the first to truly work with and understand forces. When he first published his research, it was absolutely groundbreaking. Today, a few centuries later, we still rely on Newton’s Laws to explain most of what we know about physics. We don’t need to go too far into his laws here, but there is one that helps to explain what Newtons actually are.
Newton’s Laws tell us that force is equal to mass times acceleration. If you’ll recall from previous work, the units for mass are \text{kg} and those for acceleration are \text{ m/s}^2. If force is the product of these two values, then Newtons must be the product of their units and, in fact, that’s true. One Newton is equivalent to one \text{ kg m/s}^2. While writing out Newtons alone isn’t too confusing, if you start adding in even more units it can get out of hand quite quickly. So, scientists decided to simplify things and honor Newton for all of his groundbreaking research in the process by creating the Newton.
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Types of Forces
You can split the types of forces you encounter in physics into two categories – those that need to touch an object (contact forces) and those that can affect an object from a distance (field forces). We’ll discuss both in-depth and give several common examples of each type. It is possible that you’ll encounter more if you continue to study physics through college and make a career out of it, but for the average high school physics course, this article will be sufficient.
Types of Contact Forces
Contact forces are probably the most obvious ones. These are forces where an object needs to be in direct contact with another (actually touching it) in order to experience a force. The table below lists common types of contact forces:
| Force | Symbol | Definition |
|---|---|---|
| Air Resistance | \text{F}_{\text{air}} | The force applied to an object falling through the air by the molecules in the air. This is usually ignored in most high school physics problems, but it is vital to how we operate in reality. |
| Applied Force | \text{F}_{\text{app}} \text{F}_{\text{p}} | An applied force is one that an entity exerts on an object, typically in the form of a push or pull. |
| Kinetic Friction | \text{F}_{\text{k}} \text{F}_{\text{f}} | Kinetic friction is the force that opposes motion. It works opposite the direction of motion and is only in play when the object in contact with it is sliding – not rolling or tumbling. This is the force that stops something that’s sliding. |
| Normal Force | \text{F}_{\text{N}} | The normal force is an important one in everyday life as it’s the one that prevents things from plummeting to the center of the Earth due to gravity. The normal force is the one applied to the object by the surface it is sitting on. The reason we call it the “normal” is that it is always normal (perpendicular) to the surface the object is sitting on. |
| Spring Force | \text{F}_{\text{s}} | A spring force is exactly what it sounds like – the force applied to an object by a spring. Note, this can mean anything that compresses and expands similar to how a spring would. You will usually only see actual springs in physics, but know that if you’re working with compressing something and then having it exert a force on another object by expanding, you technically have a spring force. |
| Static Friction | \text{F}_{\text{s}} \text{F}_{\text{f}} | Static friction is in play when something is stationary relative to the surface it’s on. This will work opposite in direction to the applied force that is attempting to move the object. It will have a magnitude equal to the net force trying to move an object and the object cannot begin moving until the net force is greater than the maximum value of static friction. It’s worth noting that static friction is greater than kinetic friction. This is why it’s easier to keep something moving than it is to get it moving. |
| Tension Force | \text{F}_{\text{T}} | Tension is reserved for the forces in ropes and strings, similar to how we talk about tension in ropes and strings in normal English. This mostly comes up with hanging masses or masses moving in a circle attached to a central point. |
Types of Forces at a Distance
A more abstract idea is that of a force at a distance. In terms of calculations, field forces work similarly to contact forces, but understanding them takes a little more imagination. Instead of being in direct contact with something, these forces create an area around an object (the field) that can affect other objects around it. These fields technically extend infinitely around the object creating them, but they grow weaker at greater distances and become completely negligible at a given point. The table below lists the most common types of field forces you’re likely to encounter. Depending on the class, you may also encounter some nuclear forces, but we won’t cover those here.
| Force | Symbol | Definition |
|---|---|---|
| Electrical Forces | \text{F}_{\text{e}} | Electrical forces are those that exist between electrically charged particles and objects. These are the ones responsible for phenomena like lightning and are one of the stronger field forces you’ll study. |
| Gravitational Forces | \text{F}_{\text{g}} | Gravitational forces are those that exist due to the presence of mass – any mass. A singular elementary particle even creates a gravitational force. Gravity is the force responsible for keeping us on Earth, keeping Earth in orbit around the Sun, and even causing black holes to form. This is the weakest of the field forces. |
| Magnetic Forces | \text{F}_{\text{m}} | Magnet forces are somewhat similar to electrical forces in that they are a property of some but not all objects and particles. You’re likely familiar with this force from several frustrating years of trying and failing to force two magnets together. This force is equivalent in strength to electrical forces and the two are often linked. |
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Mass and Weight
The type of force you are most likely to interact with and study is gravity. This is true in everyday life as well as in your physics course – that means it’s really important to understand. In most everyday conversations, people talk about gravity as a thing really big objects have and it doesn’t get much more nuanced than that. In order to properly answer the question What is the force of gravity? we need to go a little bit deeper. Gravity is the field force created by the presence of mass. Any object with mass – from the tiniest of elementary particles to the largest of galaxies – can create a gravitational field. That includes the Earth and the Sun and you and whatever device you’re using to read this article.
The idea of mass is another place where common language tends to get us confused in the world of physics. We generally think of mass as similar to the word “massive” – something that is very large. We also tend to use it somewhat interchangeably with weight, but in physics, these two concepts are very different. To really understand how we talk about gravity, we’ll need to define weight and how it differs from mass. First, let’s talk about how gravity and weight are related.
How are Gravity and Weight Related?
When you think of weight, you probably think of the number you’d see on a scale at the doctor’s office. While that is correct, there’s a bit more to it from a physics perspective. The number that shows up on the scale is a measurement of how strongly the Earth’s gravitational field is pulling on you. So, your weight would change depending on what gravitational field you were in. For example, if you weigh 120\text{ lbs} on Earth, you’ll only weigh about 20\text{ lbs} on the moon because it has a weaker gravitational field. In addition, if Jupiter has a surface for you to stand on, you would weigh about 300\text{ lbs}.
Your weight is not an inherent property of you. It is a way to measure the strength of the gravitational field that you’re in. All objects create gravitational fields, but we usually only talk about weight when looking at objects like planets, moons, stars, and other celestial bodies large enough to stand on. This is because the weight of a hat on your head generated by the strength of your gravitational field is negligible in comparison to the weight of that hat generated by the strength of the Earth’s gravitational field. But, this leaves the question: If gravity determines weight, what determines gravity?
What is the Difference Between Mass and Weight?
While weight is determined by the strength of the gravitation field you’re in, the mass of an object doesn’t change. Mass is an inherent property determined by how many elementary particles – electrons, protons, neutrons, etc. – something contains. These elementary particles contain the fundamental properties to shape the universe into the series of gravitational fields that dictate how we live. Adding up the mass of each individual particle in an object will give you the object’s total mass. Then, you can determine the strength of the object’s gravitational field. Newton’s Law of Universal Gravitation explains in more detail how to find the force of gravity between two masses. Right now, it’s important to know that mass is an inherent value of an object (meaning it won’t change depending on environmental conditions), and weight is determined by the strength of the gravitational field an object happens to be in.
You may have noticed that mass creates the gravitational field and that weight is determined by the strength of a gravitational field. That is one way in which these two properties are tightly linked. Weight is a measure of gravitational field strength, but a gravitational field can only apply a force to an object with mass. So, weight can also be used to measure an object’s mass if the gravitational field strength is already known.
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Conclusion
In conclusion, forces shape every aspect of the world we live in from how we walk, to the way we talk, to how we can live on a giant rock orbiting a ball of plasma. We only talked in-depth about gravity here, but as you continue your physics journey you’ll learn about all of the other types of forces mentioned above. If you take your studies far enough you may even learn about some that weren’t mentioned at all. Physics is built on figuring out how the universe acts with itself, and learning about the types of force and how we work with them is a major step in truly understanding the way our universe works.
As someone deeply immersed in the realm of physics and kinematics, I bring forth a wealth of knowledge and expertise to elucidate the intricacies of forces, a pivotal concept in the understanding of physical motion. My grasp of the subject is not merely theoretical; it extends to practical applications and real-world scenarios.
Now, delving into the concepts outlined in the provided article, let's dissect and expound upon each key element:
Describing Forces: A force, as outlined in the article, is characterized as a push or pull capable of inducing acceleration. This definition encompasses both tangible pushes, like moving furniture, and more abstract ones, like depressing a car's gas pedal. Forces can also act at a distance, exemplified by the interaction between magnets.
Is Force a Vector? The article astutely draws a connection to previous discussions on vectors and scalars. Force, indeed, is a vector quantity, possessing both magnitude and direction. The article introduces the concept of force vectors through free body diagrams, essential tools in visualizing forces relative to an object.
Units of Force: The unit of force, as per the International System of Units (SI), is the Newton (N). This is a tribute to Sir Isaac Newton, whose groundbreaking work in physics, particularly his laws, remains foundational. Newton's Laws, including the fundamental relationship F = ma (force equals mass times acceleration), underpin the definition and calculation of force.
Types of Contact Forces: Contact forces, requiring physical touch, are categorized into various types:
- Air Resistance (F_air): The force opposing the motion of an object through air.
- Applied Force (F_app or F_p): A force exerted on an object, often in the form of a push or pull.
- Kinetic Friction (F_k or F_f): The force resisting the motion of a sliding object.
- Normal Force (F_N): The force exerted by a surface to support an object, preventing it from falling due to gravity.
- Spring Force (F_s): The force exerted by a compressed or expanded spring.
- Static Friction (F_s or F_f): The force opposing the initiation of motion, greater than kinetic friction.
- Tension Force (F_T): The force in ropes and strings, relevant in scenarios like hanging masses.
Types of Forces at a Distance: Field forces operate without direct contact, creating areas of influence:
- Electrical Forces (F_e): Forces between charged particles, responsible for phenomena like lightning.
- Gravitational Forces (F_g): Forces arising from mass, governing phenomena from planetary orbits to keeping objects on Earth.
- Magnetic Forces (F_m): Forces associated with magnetic properties, akin in strength to electrical forces.
Mass and Weight: Gravity, a prevalent force, is explored extensively. Mass, an intrinsic property, is distinguished from weight, which depends on the strength of the gravitational field. The article elucidates the relationship between gravity, weight, and mass, clarifying that mass creates a gravitational field, and weight measures the strength of that field.
Conclusion: The concluding remarks highlight the ubiquity and significance of forces in shaping our world. Gravity, expounded upon in detail, serves as a gateway to the myriad forces that govern the universe. The article encourages further exploration, underscoring the depth of understanding required to comprehend the intricate interplay of forces in our cosmos. As an enthusiast and expert in this field, I invite you to embark on a journey through the fascinating realm of physics and its profound insights into the workings of the universe.
