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2.2.4.1: Free Fall

  • Page ID
    84583
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    If we raise a body above the Earth surface at a distance \( H\), and let it go -- then, no matter what the value of \( H\) is, one meter or one million meters, the body starts "falling freely" because it is attracted towards the Earth center by the force of gravity. And, if we forget about the air drag force (which is relevant only for \( H\) values of a few tens of kilometers), then we certainly have the right to say: "A free fall is the motion under the influence of the force of gravity, and no other force".

    Now, suppose that the in initial velocity of the body considered is not zero, but we give it a "push" in the direction perpendicular to the line drawn between the initial body's position, and the Earth center. Let's call this line "the \( z\) direction", and the speed along this line as "the vertical velocity \( v_{\rm z}\)". Let's call the direction perpendicular to the \( z\) line as"the \( x\) axis", and the speed along the \( x\) axis as "the horizontal velocity \( v_{\rm x}\)". So, at the initial moment \( v_{\rm z}=0\), and \( v_{\rm x}\ne 0\). What happens next? The body starts moving downwards, with constant acceleration \( g\), under the influence of gravity. But there is no horizontal force acting on it, so the initial \( v_{\rm y}\) remains unchanged. The resultant body's trajectory is a parabola, and it hits the Earth surface some distance from the point right below the initial body's position.

    In introductory-level physics textbooks the type of motion described above is usually referred to as "projectile motion"}. Three different sub-types are discussed, with the initial "push" directed strictly horizontally, at some angle upwards, or at some angle downwards. Each sub-type has to be treated slightly differently. It's a good approach, instructive and pedagogical: it helps young students to develop an ability called "physical insight", in which a correct "conceptual understanding" of a physical phenomenon considered is combined with a correct choice of "mathematical tools" needed to analyze it in quantitative terms. However, in more advanced textbooks on mechanics one may not find such a distinction: all the types of motion listed above are simply classified as a free fall. How comes? Well, as was said, a free fall is the motion of a body under the influence of gravitational forces only. And are there forces other than the "gravitational pull" involved in a vertical downward fall, and in the "projectile motion"? No, definitely not!

    For obtaining a mathematical description of the motion of a free-falling body, in each case one has to do the same -- namely, to solve the so-called "equation of motion", which always has the familiar form: \( F = m\cdot a\), i.e., it's nothing else that what we call the "Newton's Second Law of Dynamics". The only trick is that the force and the acceleration have to be taken in a vector form:
    \[
    \vec{F} = m\cdot \vec{a}
    \]
    In the most general case an object moves in a three-dimensional space. Hence, we write both
    \)\vec{F}\) and \( \vec{a}\) as three-component vectors:
    \[
    (F_{\rm x}, F_{\rm y}, F_{\rm z}) = m\cdot (a_{\rm x}, a_{\rm y}, a_{\rm z})
    \]
    which can be rewritten as a set of three equations for individual components of the force and acceleration vectors:
    \[\begin{array}{l}
    F_{\rm x}&=&m\cdot a_{\rm x} \\
    F_{\rm y}&=&m\cdot a_{\rm y} \\
    F_{\rm z}&=&m\cdot a_{\rm z}
    \end{array}\]
    Now, recall that the acceleration, by definition, is the second time-derivative of the time-dependent position of the object. We can then rewrite the above set as3:
    \[\begin{array}{l}
    \dfrac{d^2 x}{dt^2}&=&\dfrac{F_{\rm x}}{m} \\
    \dfrac{d^2 y}{dt^2}&=&\dfrac{F_{\rm y}}{m} \\
    \dfrac{d^2 z}{dt^2}&=&\dfrac{F_{\rm z}}{m}
    \end{array}\]
    This is a set of three differential equations. You may not know how to solve such equations -- many students, who are interested in "Energy Alternatives", never take math courses at such level, and those who do, take it towards the end of their Sophomore year, or at the Junior
    year. So, why are the equations mentioned in this text? Well, to advertise how mighty they are! One can use the very same set of equations to solve a vertical fall, all types of projectile motions -- and this is not the end of the list: also, the motion of satellites, on circular orbits as well as on elongated elliptic orbits, the motion of planets, moons, asteroids and comets. The very same set of equations for all problems listed!1 How can it be? It's simple, because all these types of motion have one thing in common -- the only force acting on the moving object is the force of gravity!

    Presenting the solution procedures for specific problems listed above would take too much space in this chapter. However, the Author of this text does not like to make statements, and then ask: "Please accept without proof that the said statement is true". So, several solution procedures will be presented in detail, but in the Appendices. Yet, the Appendices are added to the text with the intention of satisfy the curiosity of students who like math -- but, definitely, reading them is not necessary for students whose goal is only to attain a good \textbf{conceptual} understanding of the course material.

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    1. It’s an average value – Earth is not a perfect sphere, it is somewhat “flattened” at the poles, so that the weight of a body may be slightly different, depending on the geographical region where it is measured; also, the spinning of Earth contributes to a gradual decrease of the body’s weight when it is moved from a pole towards the Equator. Bur the “weight loss” of a body between a pole and the Equator is only about 0.3%, so that with a good approximation one can neglect such effects.

    2. \footnote{Earth is not a perfect sphere. The value of the radius we use here, 6371 km, is an average value. But the distance from the Earth center at different geographical regions may be slightly larger or slightly smaller than the average value, so that the actual values may slightly differ from the result obtained from the Eq. 2.15.

    3. These equations are a powerful tool -- solving motions resulting from the action of gravity only is just one application, the equations are generally valid, no matter what's the origin of the


    2.2.4.1: Free Fall is shared under a CC BY 1.3 license and was authored, remixed, and/or curated by Tom Giebultowicz.

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