Notes 3-1 - AP Physics C Notes 3-1 Newtons Laws Reading Ch...

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Unformatted text preview: AP Physics C Notes 3-­‐1 Newton’s Laws Reading: Ch. 5 Sect. 1-­‐6 & Ch. 7 Sect. 3 of text Isaac Newton collated the results of experimental observations into three statements that completely describe the motion and change in motion of objects. These statements are now known as Newton’s 3 Laws of Motion. We now know that Newton’s Laws do not correctly describe the motion of objects that move too fast, close to the speed of light (relativity), or objects that are too small, atomic sizes (quantum mechanics). However, the motion of most objects that we experience on an everyday basis (i.e. walking, moving vehicles, sending a rocket to the Moon) is well described by Newton’s Laws. Newton’s 1st Law was developed by Galileo, and is also referred to as the Law of Inertia. Inertia is a property of an object that resists a change in motion, which as we will see a little later, is a good definition for mass as well. The 1st Law says: An object at rest (or in motion with a constant velocity) remains at rest (or in motion with a constant velocity) unless acted upon by a non-­zero net force. A force is something that can act on an object to change its motion – a push or a pull for example. Some of the types of forces we will look at include the force of gravity (weight), applied force (push or pull), friction, normal force (support force), air resistance, tension, force due to a spring, electrostatic force (2nd semester), and magnetic force (2nd semester). The net force is the vector sum (a force has both a magnitude and a direction – it is a vector) of all the forces acting on an object. Based on this, the 1st law tells us that: • If a single force acts on an object than the objects motion will change – it will speed up, slow down, and/or change its direction of motion. • If multiple forces act on an object we can not say for certain that the motion of the object will change, since the forces might cancel each other out. • If an object is at rest, there might still be forces acting on it as long as those forces cancel each other out. • Just because an object is moving does not mean there is a force acting on it, since the object could be moving at constant velocity (constant speed in a straight line). Newton’s 2nd Law is a mathematical relation between the net force and the change in an objects motion. If the mass of the object does not change, we can state the 2nd Law as: Fnet = ma In this equation, Fnet is the net force acting on the object, a is the acceleration of the object, and m is the mass of the object, which we will define as a measure of an objects resistance to acceleration. Using this equation, we can determine the units for force as: kg⋅ m = N s2 where “N” stands for a “Newton” in honor of Isaac Newton and is equivalent to a kilogram meter per second squared. As mentioned above, this equation will only hold true if the mass of the object remains the same. 1 AP Physics C Notes 3-­‐1 We can assume this occurs in a moving car (since the change in mass as fuel is burned up is very small compared to the mass of the car), but it is not true when you launch a rocket into space. For these cases, one would have to revert to the original version of Newton’s 2nd Law, which will be discussed in the next unit. For the majority of cases we will discuss, the net force acting on the object in question will be constant. This means that the acceleration is also constant, which means we can use the kinematics equations developed in the previous unit. Newton’s 3rd Law is typically shortened to some along the line of for every action there is a reaction, but this statement overly simplifies the 3rd law, and it is much more useful to look at it in its entirety. If object A exerts a force on object B, then object B must exert a force of equal magnitude that acts in the opposite direction on object A. There are three key elements to the 3rd law: • The “action-­‐reaction” force pair have the same magnitude and act in opposite directions. • The forces act on different objects. • Both forces are of the same type. While these forces are often referred to as an action-­‐reaction force pair, that wording implies causality, which is not necessarily the case. For example, if you have a box sitting on a table, the box exerts a (normal) force downward on the table, and the table exerts a (normal) force of equal magnitude upward on the box. Both forces show up simultaneously once the box is placed on the table, so it doesn’t really matter which is called the action force and which is the reaction. Going back to the box example, it turns out that there is another force acting on the box – the force of gravity due to the Earth. We know that the force of gravity on the box and the normal force on the box must be equal (since the box is at rest and remains so the 1st law tells us the net force must be zero therefore the force of gravity must cancel the normal force). However, these two forces are not “action-­‐reaction” force pairs since they violate 2 of the 3 elements of the 3rd law – both forces act on the same object, the box, and they are different types of forces. The reaction force to the force of gravity on the box by the Earth (commonly called the Weight of the box) is the force of gravity on the Earth by the box. It turn out that the box pulls on the Earth the same amount that the Earth pulls on the box. You might then ask why the box falls to the Earth when dropped, but the Earth doesn’t move up to the box. The fact that the box moves is explained by the 2nd law. Since the mass of the box is much much less than the mass of the Earth, the acceleration (change in motion) of the box must be much much greater than the acceleration of the Earth as the forces are equal. 2 ...
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