What do forces act on

A car doesn't have rubber tires that rotate, six-way adjustable seats, ample cup holders, and a rear window defogger; it's a box. A person doesn't have two arms, two legs, and a head; they aren't made of bone, muscle, skin, and hair; they're a box.

This is the beginning of a type of drawing used by physicists and engineers called a free body diagram. Physics is built on the logical process of analysis — breaking complex situations down into a set of simpler ones.

This is how we generate our initial understanding of a situation. In many cases this first approximation of reality is good enough. When it isn't, we add another layer to our analysis. We keep repeating the process until we reach a level of understanding that suits our needs.

Just drawing a box is not going to tell us anything. Objects don't exist in isolation. They interact with the world around them. A force is one type of interaction. The forces acting on an object are represented by arrows coming out of the box — out of the center of the box. This means that in essence, every object is a point — a thing with no dimensions whatsoever. The box we initially drew is just a place to put a dot and the dot is just a place to start the arrows.

This process is called point approximation and results in the simplest type of free body diagram. First example: Let's start with the archetypal example that all physics teachers begin with — a demonstration so simple it requires no preparation.

Reach into the drawer, pull out the textbook, and lay it on top in a manner befitting its importance. A book lying on a level table.

Is there anything more grand? Now watch as we reduce it to its essence. Draw a box to represent the book. Draw a horizontal line under the box to represent the table if you're feeling bold. Then identify the forces acting on it.

Something keeps the book down. We need to draw an arrow coming out of the center pointing down to represent that force. Thousands of years ago, there was no name for that force. We now have a more sophisticated understanding of the world. Books lie on tables because gravity pulls them down. We could label this arrow F g for "force of gravity" or W for it's more prosaic name, weight. Prosaic means non-poetic, by the way.

Prosaic is a poetic way to say common. Prosaic is a non-prosaic word. Back to the diagram. Gravity pulls the book down, but it doesn't fall down. Therefore there has to be some force that also pushes the book up. What do we call this force? The "table force"? No that sounds silly and besides, it's not the act of being a table that makes the force. It's some characteristic the table has.

Place a book in water or in the air and down it goes. The thing about a table that makes it work is that it's solid. So what do we call this force? The "solid force"? That actually doesn't sound half bad, but it's not the name that's used.

Think about it this way. Rest on a table and there's an upward force. Lean against a wall and there's a sideways force. Jump on a trampoline high enough to hit your head on the ceiling and you'll feel a downward force.

The direction of the force always seems to be coming out of the solid surface. A direction which is perpendicular to the plane of a surface is said to be normal. The force that a solid surface exerts on anything in the normal direction is called the normal force.

Calling a force "normal" may seem a little odd since we generally think of the word normal as meaning ordinary, usual, or expected. If there's a normal force, shoudn't there also be an abnormal force? The origin of the Modern English word normal is the Latin word for a carpenter's square — norma. The word didn't acquire its current meaning until the 19th century. Normal force is closer to the original meaning of the word normal than normal behavior behavior at a right angle?

Are we done? Well in terms of identifying forces, yes we are. This is a pretty simple problem. You've got a book, a table, and the Earth. The Earth exerts a force on the book called gravity or weight. The table exerts a force on the book called normal or the normal force. What else is there? Forces come from the interaction between things. When you run out of things, you run out of forces.

The last word for this simple problem is about length. How long should we draw the arrow representing each force. There are two ways to answer this question. One is, "Who cares? This is a reasonable reply. Directions are what really matter since they determine the algebraic sign when we start combining forces.

The algebra really will take care of it all. The second answer is, "Who cares is not an acceptable answer. Knowing the relative size of the forces may tell us something interesting or useful and help us understand what's going on. So what is going on?

In essence, a whole lot of nothing. Our book isn't going anywhere or doing anything physically interesting. Wait long enough and the paper will decompose that's chemistry and decomposers will help decompose it that's biology. Given the lack of any activity, I think it's safe to say that the downward gravitational force is balanced by the upward normal force. In summary, draw a box with two arrows of equal lengths coming out of the center, one pointing up and one pointing down. Label the one pointing down weight or use the symbol W or F g and label the one pointing up normal or use the symbol N or F n.

It may seem like I've said a lot for such a simple question, but I rambled with a reason. There were quite a few concepts that needed to be explained: identifying the forces of weight and normal, determining their directions and relative sizes, knowing when to quit drawing, and knowing when to quit adding forces. Second example: a person floating in still water.

We could draw a stick figure, but that has too much unnecessary detail. Remember, analysis is about breaking up complex situations into a set of simple things.

Draw a box to represent the person. Draw a wavy line to represent water if you feel like being fancy. Identify the forces acting on the person. They're on Earth and they have mass, therefore they have weight. But we all know what it's like to float in water. You feel weightless.

There must be a second force to counteract the weight. The force experienced by objects immersed in a fluid is called buoyancy. The person is pulled down by gravity and buoyed up by buoyancy. Stories from Physics Newton's Second Law. Appears in these Collections Thinking About Teaching Collection Non-zero force changes speed - Physics narrative A Physics Narrative presents a storyline, showing a coherent path through a topic Newton's First Law Forces and Motion.

Force Forces and Motion. We've won an award! Learn more. Close Physics Links Explorer Explore the links between different physics concepts. All objects upon earth experience a force of gravity that is directed "downward" towards the center of the earth.

The force of gravity on earth is always equal to the weight of the object as found by the equation:. Caution: do not confuse weight with mass. The normal force is the support force exerted upon an object that is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book.

On occasions, a normal force is exerted horizontally between two objects that are in contact with each other. For instance, if a person leans against a wall, the wall pushes horizontally on the person. The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. There are at least two types of friction force - sliding and static friction.

Though it is not always the case, the friction force often opposes the motion of an object. For example, if a book slides across the surface of a desk, then the desk exerts a friction force in the opposite direction of its motion.

Friction results from the two surfaces being pressed together closely, causing intermolecular attractive forces between molecules of different surfaces. As such, friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together. The maximum amount of friction force that a surface can exert upon an object can be calculated using the formula below:.

The friction force is discussed in more detail later on this page. The air resistance is a special type of frictional force that acts upon objects as they travel through the air. The force of air resistance is often observed to oppose the motion of an object. This force will frequently be neglected due to its negligible magnitude and due to the fact that it is mathematically difficult to predict its value.

It is most noticeable for objects that travel at high speeds e. Air resistance will be discussed in more detail in Lesson 3. The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends.

The tension force is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire. The spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it. An object that compresses or stretches a spring is always acted upon by a force that restores the object to its rest or equilibrium position.

For most springs specifically, for those that are said to obey " Hooke's Law " , the magnitude of the force is directly proportional to the amount of stretch or compression of the spring. A few further comments should be added about the single force that is a source of much confusion to many students of physics - the force of gravity.

As mentioned above , the force of gravity acting upon an object is sometimes referred to as the weight of the object. Many students of physics confuse weight with mass. The mass of an object refers to the amount of matter that is contained by the object; the weight of an object is the force of gravity acting upon that object.

Mass is related to how much stuff is there and weight is related to the pull of the Earth or any other planet upon that stuff. The mass of an object measured in kg will be the same no matter where in the universe that object is located. Mass is never altered by location, the pull of gravity, speed or even the existence of other forces. For example, a 2-kg object will have a mass of 2 kg whether it is located on Earth, the moon, or Jupiter; its mass will be 2 kg whether it is moving or not at least for purposes of our study ; and its mass will be 2 kg whether it is being pushed upon or not.

On the other hand, the weight of an object measured in Newton will vary according to where in the universe the object is. Weight depends upon which planet is exerting the force and the distance the object is from the planet. Weight, being equivalent to the force of gravity, is dependent upon the value of g - the gravitational field strength. On earth's surface g is 9.

On the moon's surface, g is 1. Go to another planet, and there will be another g value. Furthermore, the g value is inversely proportional to the distance from the center of the planet.