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Introducing Newton's First Law Defining and Identifying Forces

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Introducing Newton's First Law Defining and Identifying Forces Mr. Joanis’ Science Class! ~ Physics ~ WEEK 13 Name: ________________ Period: ____ Introducing Newton’s First Law Defining and Identifying Forces So far, we’ve discussed a number of different ways to describe and quantify the movement of physical objects. With kinematics and the energy equations, you’ve also learned how to predict certain things about the movement of objects (so long as there is enough data to make those predictions). But what makes stuff move, anyway? We’ve talked a little bit about gravity at this point – it shows up in the equation for gravitational potential energy, and in some scenarios where we can use the kinematic equations. You probably know that gravity makes objects move in certain ways, but what even is gravity? Let’s take a step back and look at the first of a series of scientific laws that we will be investigating, confirming, and explaining throughout this next unit of the course: Newton’s First Law. Newton’s First Law states that… • An object at rest will stay at rest unless acted upon by an outside force. • An object moving in a straight line will continue moving at a constant velocity along that straight line unless acted upon by an outside force. What does all of this mean? Here are a couple of example scenarios that illustrate the consequences of Newton’s First Law, as we understand it so far: Take the example of a rock, which has been placed on the floor. The rock isn’t moving, meaning it is “at rest.” According to Newton’s First Law, the rock should continue to stay at rest unless some force acts upon it. This should make intuitive sense to us, too. I’ve certainly never seen a resting rock spontaneously start to roll away on a flat surface. But…what about gravity? Gravity is pulling the rock toward the ground, as gravity does to all things on Earth. Why, then doesn’t the rock sink into the ground? What about the example of a ball being launched from a slingshot? When the ball launches, will it fly in a straight line forever? Newton’s First Law tells us that, unless the ball is acted upon by an outside force, that ball should continue to move at a constant velocity. We know however, based on our observations of what happens when balls are thrown, that they should actually change velocity as they travel. Instead of flying forward forever, real balls will start to fall as they fly through the air until they hit the ground. Since there velocity of a ball changes as it moves, if it is thrown or flung forward by a slingshot, then there must be some sort of force acting on that ball. These two scenarios reveal that there’s a fundamental concept in physics that we don’t yet understand: What is a force? By defining what a force is, and covering some of the basic and most important types of forces that exist, we should be able to fully explain what is happening in both the “resting rock” and “flying ball” example, and how those two examples both follow Newton’s First Law. δ A force is… 1. A push or a pull. a. A force either pulls, or it pushes. A force cannot do both at the same time, and there can be multiple forces that “pull” or multiple forces that “push” in a given scenario. 2. Something that acts on an object. a. In other words, the “pushing” or “pulling” that a force does must be applied to an object in order for it to be considered a force. 3. Something that requires an agent that acts or exerts the force. a. A force cannot exist “because” of nothing. All forces must have a specific, identifiable cause. 4. A vector a. This means that a force has a direction (what direction is the push or pull being applied in) as well as a magnitude (how much pushing or pulling is being done). A vector, in general, is just the combination of a magnitude and a direction. We will discuss and practice with vectors in a future lesson. These are the properties of a “generic force.” All forces follow those four rules. Beyond that, there are multiple different types of forces that exist. These forces are: Gravitational Force (퐹⃑⃑⃑⃑퐺 ) A gravitational force is the pull of a planet on an object that is on or near that surface of that planet. In this scenario, the planet is the agent that enacts the force. The gravitational force acts on all objects, whether they are moving or at rest. The direction of a gravitational force (part of what makes up a force vector) always points vertically downward; in other words, it points toward the center of mass of the planet enacting that gravitational force. The drawing to the right demonstrates the way that a gravitational force would be illustrated. In this drawing of a box falling to the ground, the only force acting on the box is the gravitational force of the Earth. Normal Force (푁⃑⃑ 표푟 퐹⃑⃑⃑푁⃑ ) A normal force is the push of a surface against an object that is touching that surface. In a scenario where any two objects are touching (such as a box laying on the floor, or a person leaning against a wall), the surface being touched is the agent that enacts the force. Much like how a gravitational force acts on all objects under a certain criterion, a normal force acts on all objects which touch a surface, regardless of whether the object touching the surface is moving or at rest. The normal force always points perpendicular to and away from the surface enacting the normal force. What causes a normal force, though? Consider the drawings in this section. First, the drawing of a box sitting motionless on δ the flat ground. If the gravitational force acting on the box was the only force acting on it, then the box would sink straight into the floor. We know, however, from our long lives watching boxes not sink into floors, that boxes don’t behave that way. There must, therefore, be some sort of a force that pushes the box up to prevent it from sinking into the floor. We know that the gravitational force exists at all times, even when the box is at rest, so we cannot simply remove it from our model. The reason for the normal force existing is the resistance of the atoms that make up the floor to having their “space” invaded by the atoms that make up the box. The floor’s atoms, as we learned in chemistry class, are surrounded by a cloud of negatively charged electrons. The box’s atoms look the same way. Negative things repel (push away) other negative things, which is why the box’s atoms (and therefore the box itself) is pushed away from the floor and the floor’s atoms. This is our simple explanation of the origin of the normal force. In the other example of a normal force, where a man is leaning with one hand against a solid wall, a normal force is exerted in three different places. The floor exerts two normal forces on the man, one at each point where his feet touch the ground, and the wall exerts one normal force on al the man at the point that his hand touches the wall. This keeps the man from both sinking into the ground, and from falling into the wall. Quick Aside – Contact Forces vs. Long Range Forces Before we continue on to the two other forces we will discuss in this lesson, we need to draw another distinction between the two forces that have been discussed so far. In addition to the four rules about forces that we laid out near the beginning of this chapter, any force that exists must either be… • A contact force or • A long-range force A contact force is a force that can only be exerted on an object when physical contact is made between the agent exerting the force and the object to which the force is applied. Normal force is a contact force. A normal force only acts on the box above when it is in contact with the floor (agent of the force). A long-range force is a force that acts upon an object without physical contact. Gravity is an example of a long-range force, as it is always acting on the box whether it is making physical with contact with something or not. The agent of the long-range force in the case of gravitational force – the Earth – does not need to make contact with the object that it acts upon. This makes gravitational force an example of a long-range force. δ Tension Force (푇⃑ ) When a string or rope or wire pulls on an object, it exerts a contact force that we call the tension force. The direction of a tension force is always in the direction of the string or rope that pulls the object. In an example like the one represented by the illustration of a sled being pulled along a surface, the rope is the agent that enacts the tension force on the sled. Also note that, in this illustration, the gravitational force is still pointed straight down despite the fact that the surface is sloped. In addition, the normal force (as was discussed in the section on normal force), points perpendicular to the surface enacting it. Because the surface is not flat, the normal force doesn’t point straight up.
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