CHAPTER 2 LINEAR MOTION

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1 0 CHAPTER LINEAR MOTION

2 HAPTER LINEAR MOTION 1 Motion o an object is the continuous change in the position o that object. In this chapter we shall consider the motion o a particle in a straight line, which will be taken to be one o the coordinate axes..1 AVERAGE VELOCITY x x v = slope Q x i x origin P Q (a) x Figure.1 (a) A particle moves along the x-axis. It starts at point P at t=t i and inishes at point Q at a later time t=t. (b) The position-time graph or the same particle. The average velocity is the slope o the straight line connecting the initial and the inal points. Consider a particle moving along a straight line (x-axis) as in Figure.1(a). It starts at point P, with position x i, at time t i, and inishes at point Q, with position x, at time t. In the time interval t = t ti the displacement o the particle is x = x xi. The average velocity, v, o the particle during the time interval t is now deined as the ratio o x to t, or x x i P t i t (b) t t x v = = t x t xi t i (.1) From this relation, it is clear that the velocity has a dimension o length divided by time (L/T) with a unit o m/s, cm/s, and t/s according to the SI, Gaussian, and British system, respectively. The average velocity, as it is clear rom Equation.1, depends only on the initial and the inal positions o the particle. This means that i a

3 particle starts rom a point and return back to the same point, its displacement, and so its average velocity is zero. Fig.1(b) shows the variation o x with t where the average velocity is given by the slope o the straight line between points P and Q. Remark: There is a dierence between distance and displacement. Distance, a scalar quantity, is the actual long o the path traveled by a particle, but displacement, a vector quantity, is the shortest distance between the initial and the inal positions o the particle. Speed is the magnitude o the velocity. This means that the speed can never be negative. The average speed diers rom average velocity in that it covers the total distance rather than the total displacement, that is total distance average speed = (.) t Although the velocity and the displacement are vectors, we do not need to write them as vectors since all vectors o this chapter have only one component.. INSTANTANEOUS VELOCITY The instantaneous velocity, v, is deined as the value o v when approaches zero (becomes instant), that is t x v = lim = t 0 t dx dt (.3) In the position-time graph (Figure.), v at some instant is the slope o the tangent at that instant.

4 HAPTER LINEAR MOTION 3 x A slope=v at A t A t Figure. Position time graph or a particle moving along the x- axis. The instantaneous velocity v at point A is deined as the slope o the tangent line at t A. Example.1 The position o a particle varies with time according to x = t + 3 t with x in m and t in s. a) Find v or the interval t=0 to t= s b) Find v at t=1.5 s Solution a) to ind x i and x, we have to substitute or t i and t in the x-t relation, that is, and x = t ) + 3( t ) 0, i ( i i = x = t ) + 3( t ) = m. ( = Now, rom Equation.1, we have x xi 10 0 v = = = 5 m/s. t t 0 b) From Equation.3, we get dx v = = t + 3. dt i

5 4 The instantaneous velocity at t= 1.5 s is obtained by substituting or t= 1.5 s in the last equation v = (1.5) + 3 = 6 m/s.3 ACCELERATION When the velocity o a moving body changes with time, we say that the body has acceleration. The average acceleration a, during a time interval t, is the ratio o the change in velocity, v, to the time interval t, i.e., a v t v t i = = (.4) v t i The unit o the acceleration is m/s in SI system. In analog with the instantaneous velocity, the instantaneous acceleration, a, is deined as the limit o the average acceleration as t approaches zero, i.e. a v dv lim = (.5) t 0 t dt = From Equation.5, it is clear that the acceleration is the slope o dx the velocity-time graph. As v = (rom Equation.3), then dt Equation.5 can be rewritten as dv d dx d x a = = ( ) = (.6) dt dt dt dt

6 5 Example. The velocity o an object moving along the x-axis varies with time according to the relation v=5t-3 with v in m/s and t in s. a) Find a during the interval t=1 s to t= s b)find a at t=s Solution a) v = ( t ) 3 = 5(1) 3 m/s, i 5 i = and v = 5( t ) 3 = 5() 3 = 7 m/s. So, rom Equation.4 v vi 7 a = = = 5m/s. t t 1 b) Using Equation.5, we obtain a dv = = 5 m/s. dt i Example.3 The position-time graph o a particle moving along the x-axis is given in Figure.3. Find a) v during the interval t= s to t=5 s. b)a during the interval t=0.5 s to t=.5 s Solution a) As it is clear rom the graph, at t = s x i = 3 m, and at t = 5 s x (m) t (s) Figure.3 Example.3

7 6 x = 0. Now rom Equation.1, we get v = x t xi t i 0 3 = = 1m/s 5 b) Since v at any point is the slope o the x-t graph at that point, we have at t=0.5 s v= 6m/s, and at t=.5 s v=0, so a v = t vi t i 0 6 = = 3 m/s ٢.٤ LINEAR MOTION WITH CONSTANT ACCELERATION The simplest type o linear motion is the uniorm motion in which the acceleration is constant. In such a case a =a, so rom Equation.4, we obtain a v vi v vo = = (.7) t t t i where we denote v i by v o, v by v, and t by t, and take t i =0. The above equation now is v = v 0 + at (.8) Also, because a is constant, we can write v + v v = o (.9) Using Equations.1 and.9, we obtain

8 HAPTER LINEAR MOTION 7 x x v + = t o v o (.10) Substituting or v rom Equation.8 and rearrange, we obtain o o 1 x x = v t + at (.11) Now substituting or t rom Equation.8 into Equation.10 we get v o = v + a( x x ) (.1) o When the velocity is constant, (a=0) it is clear rom Equations.8 and.11 that v = and x x o = vt (.13) v o For simplicity, the origin o the coordinates is oten chosen to be coincident with x o (i.e. x o =0). Equations.8,.11, and.1 are the undamental three equations that govern the linear motion with constant acceleration. Strategy or solving problems with constant acceleration: (i) Choose your coordinates such that the particle begins its motion rom the origin (x o =0). (ii) Decide the sense o the positive direction. (iii) Make a list o the known quantities. Do not orget to write any vector quantity (x, v, v o, a) that have a direction opposite to your positive sense as a negative quantity. (iv) Make sure that all the quantities have the same system o units.

9 8 (v) According to what is given and what is requested, you can easily decide which equation or equations rom equations.8,.11, and.1 you need to solve or the unknowns. Example.4 A car starts rom rest and moves with constant acceleration. Ater 1 s its velocity becomes 10 m/s. Find, a) the acceleration o the car b) the distance the car travels in the 1 s Solution Let the direction o motion be along the positive x-axis, where the car starts rom the origin at t=0. Now v o =0, v= 10 m/s, t=1 s. a) Using Equation.8, namely v = v 0 + at, we have a v v = t o 10 0 = = 10m / s 1. b) Equation.11 reads yields x + 1 = v0t at, substituting or v o, t, and a x = 0 + (0.5)(10)(1) = 70m..5 FREELY FALLING BODIES A reely alling body is any body moving reely under the inluence o gravity regardless o its initial motion. Neglecting the air resistance and assuming that the gravitational acceleration, denoted by g, is constant, we can consider the motion o a reely body as a linear motion with constant acceleration. Taking your axis to be the y-axis with the positive sense upward, Equations.8,.11, and.1 will apply with the substitutions, x y and a -g, i.e.,

10 HAPTER LINEAR MOTION 9 v = v 0 gt (.14) y 1 y0 = v0t gt (.15) v = v g( y y ) (.16) o o The irst substitution is because the motion is now vertical and the negative sign in the second substitution indicates that the acceleration is downward. Remark: Remember that the gravitational acceleration, g, is constant in magnitude and in direction and this means that the negative sign o g in Equations will not be changed unless you change your positive sense, regardless o the direction o motion. Example.5 An object is thrown vertically upward with an initial speed o 5 m/s. a) How long does it take to reach its maximum height? b) What is the maximum height? c) How long does it take to return to the ground? d) What is its velocity just beore striking the ground? h 5 m/s v=? Figure.5 Example.5. Solution The track o the object is shown in Figure.5. a) At the maximum point v= 0. From Equation.14 we have

11 30 vo v 5 0 t = = =.55s. g 9.8 b) Using Equation.16, i.e. v = v gy we have o h = y = v o v g = (5) 0 (9.8) = 31.9m c) When returning to the ground, the displacement o the object is zero (y=0). So rom Equation.15, we have 0 gt 1 = v o t. Solving or t we get v 5 t = o = = 5.1s. g 4.9 d) From Equation (.14) we have v = v gt = 5 9.8(5.1) = 0 5m/s The minus sign indicates that v is directed downward. Example.6 A student, stands at the edge o the roo o a building, throws a ball vertically upward with an initial speed o 0 m/s. The building is 50 m high, and the ball just missed the edge o the roo in its way down, (Figure.6). Find, a) the time needed or the ball to return to the level o the roo

12 HAPTER LINEAR MOTION 31 b) the velocity and the position o the ball at t =5 s c) the velocity o he ball just beore hitting the ground Solution a) When the ball returns to the level o the roo, its displacement, y, is zero. Substituting in Equation.15 we get 0 t 1 = 0t (9.8). v o = 0 m/s 50 m Figure.6 Example.6. Now solving or t we get 40 t = = 4.08s. 9.8 b) Using Equation.14, v = v 0 gt, we obtain v = 0 9.8(5) = 9 m/s, and using Equation.15, y o = v t 1 gt, we have y = 0(5) 4.9(5) =.5m. c) From the equation v = v gy, (Equation.16), we obtain o v = (0) (9.8)( 50) = = 1380 m /s, rom which we ind v = m/s

13 3 The positive solution is rejected because the ball hits the ground while alling. ٢.٦ RELATIVE MOTION E A x AE x PA x PE P Figure.7 Omer (rame E) and Ahmed (rame A) observe particle P. Omer is stationary while Ahmed is moving with constant speed relative to Omer. x The displacement, velocity, and acceleration o a particle are always described with respect to a speciic coordinate system or rame o reerence. Usually, this rame o reerence is taken to be ixed, but what happen i this rame o reerence is in motion with respect to another rame o reerence. Suppose that two persons want to observe the motion o a particle P. The irst person Omer is stationary with respect to the earth, while the second person, Ahmed is moving with constant speed relative to the earth. Let the reerence o rame o Omer be reerred by rame E (rame o the earth) and the reerence o rame o Ahmed by rame A. The displacement o the particle with respect to the earth (Omer) is denoted by x PE, and x pa is the displacement o the particle with respect to Ahmed. From Figure.7 we can write

14 HAPTER LINEAR MOTION 33 x = x + x.17 PE pa AE where x AE reers to the displacement o Ahmed with respect to the earth. I we dierentiate Equation.17 with respect to time, we get v = v + v (.18) PE pa AE In the last equation v PE is the velocity o P relative to E, v PA is the velocity o P relative to A, and v AE is the velocity o A relative to E. Example.7 A man, in a car, is driving on a straight highway at constant speed o 70 km/h relative to the earth. Suddenly, he spots a truck traveling in the same direction with constant speed o 60 km/h relative to the earth. a) What is the velocity o the truck relative to the car? b) What is the velocity o the car relative to the truck? c) I the car spots the truck when they are 1.5 km apart, how long does it take the car to overtake the truck? Solution a) Let v TC denotes the velocity o the truck relative to the car, and v TE and v CE denote, respectively, the velocities o the truck and the car relative to the earth. Now rom Equation.18 we have or v = v + v v TE TC TC CE = vte vce = = 10 km/h The minus sign indicates that the car is moving in the opposite direction relative to the car. b) Again rom Equation.18, the velocity o the car relative to the truck v CT is

15 34 or v = v + v v CE CT CT TE = vce vte = = 10 km/h Note that vct = vtc as one should expect. c) The time needed or the car to overpass the truck is 1.5 t = v CT 1.5 = = 0.15h = 9minutes 10

16 HAPTER LINEAR MOTION 35 PROBLEMS ٢.١ An automobile travels on a straight track at 80 km/h or 30 minutes. It then travels at 60 km/h or another 0 minutes. a) What is the average velocity o the automobile during the 50 minutes trip? b) What is the average speed o the automobile during the whole trip.. A particle moves according to the equation x = 8t +, where x in meters and t in seconds. a) Find the average velocity during the time interval rom s to 4 s. b) Find the instantaneous velocity at t=.5 s..3 Figure.8 is a plot o the displacement x(t) or a particle that is initially at rest, then moves orward, and then stops. a) Find the average velocity o the particle during the interval t=1s to t=10 s. b) Find the instantaneous velocity at t=4 s. x (m) t (s) Figure.8 Problem.3.4 A particle moves along the x-axis according to x = 5t + 30t, where x in meters and t in seconds. Calculate a) the instantaneous velocity at t=3.0 s, b) the average acceleration during the irst 3.0 s, c) The instantaneous acceleration at t=3.0 s.

17 36.5 A car traveling initially at a speed o 80 km/h is accelerated uniormly to a speed o 105 m/s in 15 s. What is the distance traveled by the car during this 15 s interval..6 The initial speed o a body is 6.4 m/s. I its acceleration is 3.5 m/s, ind its speed ater.8 s..7 An automobile moving with constant acceleration travels a distance o 60 m in 6 s. I its inal speed is 15 m/s, a) what is its acceleration? b) what is its initial speed?.8 A train starts rom rest at a station and accelerates at a rate o 600 km/h or 10 minutes. Ater that it travels at constant speed or 1 h, and then slows down at -800 km/h until it stops at another station. Find the total distance covered by the train..9 A car traveling at constant speed o 50 km/hr is 60 m rom a stationary lorry when the driver slams on the brakes. I the car just missed the impact, a) ind the acceleration o the car, b) ind the time interval rom the instant the driver slams on the brakes to the instant the car stopped..10 At the instant the traic light turns green, a car starts rom rest with constant acceleration o m/s. At the same instant a truck, traveling with constant speed o 10 m/s overtakes and passes the car. a) How ar beyond the traic light will the car overtake the truck?. b) What is the speed o the car at that instant..11 A car traveling at a constant speed o 60 km/h passes a trooper hidden behind a tree. Two second ater the car passes the tree, the trooper sets in chase ater the car with constant

18 37 CHAPTER LINEAR MOTION acceleration o 3.5 m/s. How long does it take the trooper to overtake the car? v(m/s) t(s) Figure.9 problem.1.1 The graph in Figure.9 depicts the motion o an automobile in a street. a) Find the displacement o the automobile during the ollowing intervals: 0-4s, 4s-1s, and 1s-16s. b) Find the velocity o the automobile at t=s, t=8s, and at t=14s. c) Find the acceleration o the automobile during 0-4s, 4s-1s, and 1s-16s..13 A small ball is thrown vertically upward and rises to a maximum height o 0 m. a) With what velocity the ball is thrown. b) How long will it be in the air..14 A small ball is released rom a height o 45 m. a) Find the velocity o the ball just beore it hits the ground. b) How long does it take the ball to hit the ground?

19 38.15 A body is thrown vertically upward rom the edge o a building 1m high. In its way down it misses the building and hits the ground below with a speed o 4 m/s, as in Figure.10. a) With what velocity the body is thrown. b) What is the time o light? v o =? 1 m v=4 m/s Figure.10problem Reer again to Figure.10. Now the height o the building is 30m and the body is thrown with a speed o 16 m/s. a) What is the average velocity o the body during the whole light? b) What is the average speed o the body during the same interval?.17a bird is traveling vertically upward at a constant speed o.5m/s with a piece o bread in his mouth. When it is 150 m above the ground the bread piece is released rom the bird s mouth. a) Find the speed with which the bread hits the ground. b) How long does the bread take to reach the ground? c) What is the average speed o the bread? d)what is the average velocity o the bread? v o =0.18 A alling object starting rom rest requires 1.5 s to travel the last 30m beore hitting the ground. From what height above the ground did it all. H=? 30 m t = 15.s Figure.11 (Problem.18)

20 39 CHAPTER LINEAR MOTION.19 A ball is thrown vertically upward. In its way up it passes a point P with speed v, and another point Q,.5 m higher than P with a speed 0.5v. Find the value o v. ٢.٢٠ A child releases a ball rom the balkone o his house that is 7 m high. The ball hits a man walking with constant speed. The man was 0 m rom the point o impact when the ball was released. What was the speed o the walking man? ٢.٢١ Two objects released rom the same height 0.5 s apart. How long ater the irst object begins to all will the two objects be 8.0 m apart?. A parachutist jumps horizontally to the air rom a height o 000 m and alls reely 400m beore the parachute opens. The parachutist hits the ground with a speed o 5 m/s. a) Calculate the acceleration o the parachute while it is open. b) How long was the parachutist in the air..3 A student in his way back to the home is traveling at constant speed o 50 km/h. What would be the velocity o the trees in the roadside as observed by the student?.4 A river has a steady speed o 0. m/s. A man swims upstream a distance o 500 m and returns to the starting point. I the man can swim at a speed o 1.0 m/s in still water, how long does the trip take?.5 A parachutist throws a stone vertically upward with a speed o 1 m/s relative to himsel. At the instant he throws the stone he is 10 m high and alling at 4m/s. How long does it take the stone to reach the ground?

Chapter 3. Motion in One Dimension

Chapter 3. Motion in One Dimension Chapter 3 Motion in One Dimension Outline 3.1 Position, Velocity and Speed 3.2 Instantaneous Velocity and Speed 3.3 Acceleration 3.4 Motion Diagrams 3.5 One-Dimensional Motion with Constant Acceleration