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The Jerk Vector in Projectile Motion

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The Jerk Vector in Projectile Motion The jerk vector in projectile motion A. Tan and M. E. Edwards Department of Physics, Alabama A & M University, Normal, Alabama 35762, U.S.A. E-mail: [email protected] (Received 12 February 2011, accepted 29 June 2011) Abstract The existence of the jerk vector is investigated in projectile motion under gravity. The jerk vector is zero in the absence of air resistance, but comes into life when velocity-dependent air drag is present. The jerk vector is calculated when the air resistance is proportional to the velocity. It is found that the jerk vector maintains a constant sense in the upward and forward direction, with its magnitude attenuated exponentially as a function of time. Keywords: Jerk vector, Projectile motion, Air resistance. Resumen La existencia del vector reflejo es investigada en el movimiento de proyectiles en virtud de la gravedad. El vector reflejo es cero en ausencia de la resistencia del aire, pero vuelve a la vida cuando la resistencia del aire dependiente de la velocidad está presente. El vector reflejo es calculado cuando la resistencia del aire es proporcional a la velocidad. Se ha encontrado que el vector reflejo mantiene una sensación constante en la dirección hacia arriba y hacia delante, con su magnitud atenuada exponencialmente en función del tiempo. Palabras clave: Vector reflejo, Movimiento de proyectiles, Resistencia del aire. PACS: 45.50.-j, 45.50.Dd, 45.40.Gj, 45.40.Aa ISSN 1870-9095 I. INTRODUCTION projectile of mass m is projected with an initial velocity v0 making an angle α with the horizontal plane. In a Cartesian The vast majority of physical laws are represented by coordinate system x-y with the launch point at the origin, second order differential equations. This is because the first the equations of motion are given by and second derivatives of the pertinent physical quantities give, respectively, the slope and curvature of these dvx quantities, which are sufficient for the solution of the m 0 , (1) dt problem at hand. In kinematics, for example, the velocity and acceleration are defined as the first and second and derivatives, respectively, of the position vector with respect dv y m mg . (2) to time. The third derivative is normally overlooked in the dt vast majority of the textbooks in undergraduate and graduate physics curricula. Yet the existence of this The initial conditions at time t = 0 are given by derivative has been known for a long time. Formerly known v v cos ; v v sin ; x = 0; y = 0. as the second acceleration, this derivative is now x 0 y 0 commonly referred to as the jerk [1]. The jerk vector has Eqs. (1) and (2) are uncoupled equations which can be been studied in simple harmonic motion [2], uniform integrated separately. Integrating them twice with respect to circular motion [3] and Keplerian motion [2, 4]. In this t and applying the initial conditions, we obtain paper, we investigate the existence of the jerk vector in projectile motion. vx v0 cos , (3) v y v0 sin gt , (4) II. PROJECTILE MOTION WITHOUT AIR RESISTANCE x v0 cost , (5) and The problem of projectile motion under gravity in the 1 2 absence of air resistance forms one of the basis topics in y v0 sint gt . (6) elementary physics courses [5]. In the standard treatment, a 2 Lat. Am. J. Phys. Educ. Vol. 5, No. 2, June 2011 344 http://www.lajpe.org The jerk vector in projectile motion Eqs. (3) – (6) furnish the position and velocity vectors of da j x k 2v cosekt . (14) the projectile as functions of time: x dt 0 1 2 The vertical motion can likewise be solved, giving r v0 gt yˆ , (7) g and g kv sin g v 0 ekt . (15) a gyˆ . (8) y k k A further differentiation of Eq. (8) supplies the jerk vector: From Eq. (11), j 0 . (9) dv y kt a y kv 0 sin ge , (16) dt Thus, for projectile motion under gravity in the absence of air resistance, the jerk vector is zero. This is to be expected whence since the force (and hence the acceleration) of the projectile is a constant vector in the downward direction. We shall da y kt next show that the jerk vector will not be a null vector in j y kkv 0 sin ge . (17) projectile motion if velocity-dependent drag forces are dt present. The velocity, acceleration and jerk vectors of the projectile can be constructed from Eqs. (12 - 17): III. PROJECTILE MOTION WITH LINEAR AIR kt g kt RESISTANCE v vx xˆ v y yˆ e v0 1 e yˆ , (18) k Two types of air resistance are commonly found in the kt kt literature: (1) proportional to the velocity (linear drag); and a ax xˆ a y yˆ ke v0 ge yˆ , (19) (2) proportional to the square of the velocity (quadratic and drag) [6]. The general effects of both types of drag forces 2 kt kt on the projectile motion are similar. However, exact j jx xˆ j y yˆ k e v0 kge yˆ . (20) solutions of the problem in time are found only in the linear case [7]. Hence, we shall restrict ourselves to the linear Eqs. (19) and (20) can be rewritten as drag force in this paper. Also, it is advantageous to express the drag force as kmv , where k is the drag coefficient per a kv gyˆekt , (21) unit mass [6]. The equations of motion are then written as 0 and dv j kkv gyˆekt . (22) m x kmv , (10) 0 x dt and By inspection, kv0 gyˆ is a constant vector whose sense is -kt dv y between the forward and upward directions. Also, e is an m mg kmv . (11) dt y attenuation factor which diminishes exponentially in time. Consequently, the jerk vector maintains a constant direction Eqs. (10) and (11) are still uncoupled and thus can be (between the forward and upward directions) while its integrated separately. We assume the same initial conditions magnitude diminished exponentially in time. The constant as before. slope angle θ of this vector is given by The horizontal motion is solved by separating the kvsin g g variables vx and t and integrating both sides from the initial tan110 tan tan sec . (23) conditions to give kv00cos kv kt vx v0 cose . (12) Further, from Eqs. (21) and (22), we have Thus, from Eq. (10), one gets j ka , (24) dv a x kv cosekt , (13) x dt 0 which indicates that the jerk vector or second acceleration is anti-parallel to the acceleration vector (which is between whence the downward and backward directions). It may be mentioned that in uniform circular motion, the jerk vector is Lat. Am. J. Phys. Educ. Vol. 5, No. 2, June 2011 345 http://www.lajpe.org A. Tan and M. E. Edwards anti-parallel to the velocity vector and perpendicular to the time of descent follows from the above: τ2 = 49.6s. We have acceleration vector [3]. τ2 > τ1 as expected. It is instructive to calculate the times of ascent (τ1) and The initial magnitude of the jerk vector is obtained by descent (τ2) of the projectile under gravity subject to linear setting t = 0 in Eqs. (14) and (17) and applying Pythagoras’ drag force. At the apex of the trajectory, vy = 0. Thus, from theorem: Eq. (15): j k k2 v 2 2 kv g sin g 2 , (30) 1 g 0 0 0 1 ln . (25) k kv sin g 3 0 giving j0 = .153m/s . The slope angle of the jerk vector as given by Eq. (23) is θ = 76.4o. Fig. 2 shows the trajectory of To obtain the time of flight τ (= τ1 + τ2), we integrate Eq. the projectile. Also displayed in the figure are the jerk (15) with respect to t and apply the initial conditions getting vectors at intervals of 10s. As noted earlier, the vector maintains a fixed direction while its amplitude diminished gt kv sin g steadily in time. y 0 1 ekt . (26) k k 2 14000 The time of flight is obtained when y 0 . One arrives at a 12000 transcendental equation 10000 45s 35s 55s 25s f (t) f (t) , (27) 8000 1 2 65s with y, m y, 6000 15s f1(t) t , (28) 75s and 4000 5s 85s kv 0 sin g kt 2000 f 2 (t) 1 e . (29) gk 0 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 Eq. (27) can be solved by the graphical method giving τ. In x, m Fig.1, the intersection of the curves f1(t) and f2(t) furnishes FIGURE 2. The jerk vector in projectile motion with launch the value of τ. The time of descent τ is then obtained by o 2 speed v0 = 600m/s; launch angle α = 60 ; and drag coefficient k = subtracting τ1 from τ. .01/s. f1(t) V. DISCUSSION f(t) The existence of the jerk vector in projectile has been investigated. The absence of air resistance ensures the f2(t) absence of the jerk vector, since the constant gravitational force cannot produce jerk. It is air resistance which is responsible for creating jerk in projectile motion. The jerk τ is quite a common phenomenon, even though it is not a part of of the physics curricula. A variable force will always t produce a variable acceleration, and hence jerk. The jek vector in projectile motion with air drag is only another FIGURE 1. Graphical solution of Eq. (27). The intersection of example of this common but ignored phenomenon.
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