Chapter 2 Solutions. = 16.1 m/s. = 11.5 m/s m. 180 km = ( ) h. = 2.5 m/s. = 3.3 m/s

Transcription

1 Chapter Solutions *.1 (a) v.30 m/s v x 57.5 m 9.0 m 3.00 s 16.1 m/s (c) v x 57.5 m 0 m 5.00 s 11.5 m/s. (a) Displacement ( m/h) h m x ( ) 10 3 m 180 km Average velocity displacement time 180 km ( ) h 63.4 km/h.3 (a) v av x 10 m s 5 m/s v av 5 m 4 s 1. m/s (c) v av x x 1 t t 1 (d) v av x x 1 t t 1 5 m 10 m 4 s s 5 m 5 m 7 s 4 s.5 m/s 3.3 m/s (e) v av x x 1 t t m/s.4 x 10t For t(s) x(m) (a) v x v x 50 m 1.0 s 4.1 m 0.1 s 50.0 m/s 41.0 m/s

2 Chapter Solutions.5 (a) Let d represent the distance between A and B. Let t 1 be the time for which the walker has the higher speed in 5.00 m/s d t 1. Let t represent the longer time for the return trip in 3.00 m/s d t. Then the times are t 1 speed is: d (5.00 m/s) d and t (3.00 m/s). The average Total distance v Total time d + d d d (5.00 m/s) + d (8.00 m/s)d (3.00 m/s) (15.0 m /s ) v (15.0 m /s ) 8.00 m/s 3.75 m/s She starts and finishes at the same point A. With total displacement 0, average velocity 0.6 (a) Total distance v Total time Let d be the distance from A to B. Then the time required is d v 1 + d v. And the average speed is d v 1 v v d + d v 1 + v v 1 v With total displacement zero, her average velocity is 0..7 (a) x (m) t (s) 5 v slope 5.00 m ( 3.00 m) (6.00 s 1.00 s) 8.00 m 5.00 s 1.60 m/s

3 Chapter Solutions 3.8 (a) At any time, t, the displacement is given by x (3.00 m/s )t. Thus, at t i 3.00 s: x i (3.00 m/s )(3.00 s) 7.0 m At t f 3.00 s + :x f (3.00 m/s )(3.00 s + ), or x f 7.0 m + (18.0 m/s) + (3.00 m/s )() (c) The instantaneous velocity at t 3.00 s is: v lim x f x i o lim 0 [(18.0 m/s) + (3.00 m/s )], or v 18.0 m/s.9 (a) at t i 1.5 s, x i 8.0 m (Point A) at t f 4.0 s, x f.0 m (Point B) x f x i v t f t i (.0 8.0) m (4 1.5) s 6.0 m.5 s.4 m/s The slope of the tangent line is found from points C and D. (t C 1.0 s, x C 9.5 m) and (t D 3.5 s, x D 0), x (m) C A 4 B D t (s) v 3.8 m/s (c) The velocity is zero when x is a minimum. This is at t 4 s. x (m).10 At t 5.0 s, the slope is v 58 m.5 s At t 4.0 s, the slope is v 54 m 3 s At t 3.0 s, the slope is v 49 m 3.4 s 3 m/s 18 m/s 14 m/s At t.0 s, the slope is v 36 m 4.0 s 9.0 m/s t (s) (c) a v 3 m/s 5.0 s 4.6 m/s 0 v (m/s) (d) Initial velocity of the car was zero t (s)

4 4 Chapter Solutions.11 (a) v (5 0) m (1 0) s 5 m/s x (m) 10 v (c) v (d) v (5 10) m (4 ) s (5 m 5 m) (5 s 4 s) 0 ( 5 m) (8 s 7 s).5 m/s 0 +5 m/s t (s).1 a v f v i t f t i m/s 15.0 s m/s The negative sign in the result shows that the acceleration is in the negative x direction. *.13 Choose the positive direction to be the outward perpendicular to the wall. v v i + at a v.0 m/s ( 5.0 m/s) s m/s.14 (a) Acceleration is constant over the first ten seconds, so at the end v v i + at 0 + (.00 m/s )(10.0 s) 0.0 m/s Then a 0 so v is constant from t 10.0 s to t 15.0 s. And over the last five seconds the velocity changes to v v i + at 0.0 m/s (3.00 m/s )(5.00 s) 5.00 m/s In the first ten seconds x x i + v i t + 1 at (.00 m/s )(10.0 s) 100 m Over the next five seconds the position changes to x x i + v i t + 1 at 100 m m/s (5.00 s) m And at t 0.0 s x x i + v i t + 1 at 00 m m/s (5.00 s) + 1 ( 3.00 m/s )(5.00 s) 6 m

5 Chapter Solutions 5 *.15 (a) Acceleration is the slope of the graph of v vs t. For 0 < t < 5.00 s, a 0 For 15.0 s < t < 0.0 s, a a (m/s ) For 5.0 s < t < 15.0 s, a v f v i t f t i 1.0 a 8.00 ( 8.00) m/s 0.0 t (s) We can plot a(t) as shown. a v f v i t f t i (i) For 5.00 s < t < 15.0 s, t i 5.00 s, v i 8.00 m/s t f 15.0 s, v f 8.00 m/s; a v f v i 8.00 ( 8.00) t f t i m/s (ii) t i 0, v i 8.00 m/s, t f 0.0 s, v f 8.00 m/s a v f v i 8.00 ( 8.00) t f t i m/s

6 6 Chapter Solutions.16 (a) See the Graphs at the right. Choose x 0 at t 0 40 x (m) At t 3 s, x 1 (8 m/s)(3 s) 1 m 0 At t 5 s, x 1 m + (8 m/s)( s) 8 m t (s) At t 7 s, x 8 m + 1 (8 m/s)( s) 36 m For 0 < t < 3 s, a (8 m/s)/3 s.67 m/s v (m/s) 10 For 3 < t < 5 s, a 0 (c) For 5 s < t < 9 s, a (16 m/s)/4 s 4 m/s t (s) (d) At t 6 s, x 8 m + (6 m/s)(1 s) 34 m 10 (e) At t 9 s, x 36 m + 1 ( 8 m/s) s 8 m a (m/s ) t (s) 5.17 x t t, v dx t, a dv At t 3.00 s: (a) x ( ) m.00 m v ( ) m/s 3.00 m/s (c) a.00 m/s.18 (a) At t.00 s, x [3.00(.00).00(.00) ] m 11.0 m At t 3.00 s, x [3.00(9.00).00(3.00) ] m 4.0 m so v x 4.0 m 11.0 m 3.00 s.00 s 13.0 m/s

7 Chapter Solutions 7 At all times the instantaneous velocity is v d (3.00t.00t ) (6.00t.00) m/s At t.00 s, v [6.00(.00).00] m/s 10.0 m/s At t 3.00 s, v [6.00(3.00).00] m/s 16.0 m/s (c) a v 16.0 m/s 10.0 m/s 3.00 s.00 s 6.00 m/s (d) At all times a d ( ) 6.00 m/s (This includes both t.00 s and t 3.00 s)..19 (a) a v 8.00 m/s 6.00 s 4 3 m/s Maximum positive acceleration is at t 3 s, and is approximately m/s (c) (d) a 0, at t 6 s, and also for t > 10 s Maximum negative acceleration is at t 8 s, and is approximately 1.5 m/s

8 8 Chapter Solutions *.0 reading order velocity acceleration a b c d e f One way of phrasing the answer: The spacing of the successive positions would change with less regularity. Another way: The object would move with some combination of the kinds of motion shown in (a) through (e). Within one drawing, the acceleration vectors would vary in magnitude and direction. *.1 From v f v i + ax, we have ( m/s) 0 + a(0 m), so that a m/s which is times g. (a) Assuming a constant acceleration: a v f v i t 4.0 m/s 8.00 s 5.5 m/s Taking the origin at the original position of the car, x 1 (v i + v f) t 1 (4.0 m/s)(8.00 s) 168 m (c) From v f v i + at, the velocity 10.0 s after the car starts from rest is: v f 0 + (5.5 m/s )(10.0 s) 5.5 m/s

9 Chapter Solutions 9 *.3 (a) x x i 1 (v i + v) t becomes 40 m 1 (v i +.80 m/s)(8.50 s) which yields v i 6.61 m/s a v v i t.80 m/s 6.61 m/s 8.50 s m/s.4 Suppose the unknown acceleration is constant as a car moving at v i 35.0 mi/h comes to a v 0 stop in x x i 40.0 ft. We find its acceleration from v v i + a(x x i ) a (v v i) (x x i ) 0 (35.0 mi/h) 580 ft (40.0 ft) 1 mi 1 h 3.9 ft/s 3600 s Now consider a car moving at v i 70.0 mi/h and stopping to v 0 with a 3.9 ft/s. From the same equation its stopping distance is x x i v v i a 0 (70.0 mi/h) 580 ft ( 3.9 ft/s ) 1 mi 1 h 3600 s 160 ft.5 Given v i 1.0 cm/s when x i 3.00 cm (t 0), and at t.00 s, x 5.00 cm x v i t + 1 at ; x x i v i t + 1 at ; (.00) + 1 a (.00) ; a a cm/s

10 10 Chapter Solutions Goal Solution A body moving with uniform acceleration has a velocity of 1.0 cm/s in the positive x direction when its x coordinate is 3.00 cm. If its x coordinate.00s later is 5.00 cm, what is the magnitude of its acceleration? G: Since the object must slow down as it moves to the right and then speeds up to the left, the acceleration must be negative and should have units of cm/s. O: First we should sketch the problem to see what is happening: x (cm) initial final Here we can see that the object travels along the x-axis, first to the right, slowing down, and then speeding up as it travels to the left in the negative x direction. We can show the position as a function of time with the notation: x(t) x(0) 3.00 cm, x(.00) 5.00 cm, and v(0) 1.0 cm/s A: Use the kinematic equation x x i v i t + 1 at, and solve for a. a (x x i v i t) t a [ 5.00 cm 3.00 cm (1.0 cm/s)(.00 s)] (.00 s) a 16.0 cm/s L: The acceleration is negative as expected and it has the correct units of cm/s. It also makes sense that the magnitude of the acceleration must be greater than 1 cm/s since this is the acceleration that would cause the object to stop after 1 second and just return the object to its starting point after seconds.

11 Chapter Solutions 11.6 (a) Total displacement area under the (v, t) curve from t 0 to 50 s. x 1 (50 m/s)(15 s) + (50 m/s)(40 15)s + 1 (50 m/s)(10 s) 1875 m From t 10 s to t 40 s, displacement (area under the curve) is x 1 (50 m/s + 33 m/s)(5 s) + (50 m/s)(5 s) 1457 m a (m/s ) (c) 0 t 15 s: a 1 v (50 0) m/s 15 s m/s 5 15 s < t < 40 s: a t (s) 40 s t 50 s: a 3 v (0 50) m/s 50 s 40 s 5.0 m/s 5 (d) (i) x a 1 t 1 (3.3 m/s ) t, or x 1 (1.67 m/s )t (ii) x 1 (15 s) [50 m/s 0] + (50 m/s)(t 15 s), or x (50 m/s)t 375 m (iii) For 40 s t 50 s, x 3 area under v vs t from t 0 to 40 s + 1 a 3 (t 40 s) + (50 m/s)(t 40 s) or x m m + 1 ( 5.0 m/s )(t 40 s) + (50 m/s)(t 40 s) which reduces to x 3 (50 m/s)t (.5 m/s )t 4375 m (e) v total displacement total elapsed time 1875 m 50 s 37.5 m/s *.7 (a) Compare the position equation x t 4.00t to the general form x x i + v i t + 1 at to recognize that: x i.00 m, v i 3.00 m/s, and a 8.00 m/s The velocity equation, v v i + at, is then v 3.00 m/s (8.00 m/s )t. The particle changes direction when v 0, which occurs at t 3 8 s. The position at this time is: x.00 m + (3.00 m/s)error! s) (4.00 m/s )Error! s) Error!

12 1 Chapter Solutions From x x i + v i t + 1 at, observe that when x x i, the time is given by t v i a.8 v i 5.0 m/s. Thus, when the particle returns to its initial position, the time is (3.00 m/s) t 8.00 m/s 3 4 v 3.00 m/s (8.00 m/s ) s and the velocity is 3 4 s 3.00 m/s (a) v(t.50 s) v i + at 5.0 m/s + (3.00 m/s )(.50 s) 1.7 m/s v(t.50 s) v i + at 5.0 m/s + ( 3.00 m/s )(.50 s).30 m/s.9 (a) x 1 at (Eq.11) 400 m 1 (10.0 m/s ) t t 8.94 s v at (Eq.8) v (10.0 m/s )(8.94 s) 89.4 m/s.30 (a) Take t i 0 at the bottom of the hill where x i 0, v i 30.0 m/s, and a.00 m/s. Use these values in the general equation x x i + v i t + 1 at to find x t m/s + 1 (.00 m/s ) t when t is in seconds x (30.0t t )m To find an equation for the velocity, use v v i + at 30.0 m/s + (.00 m/s )t v ( t) m/s

13 Chapter Solutions 13 The distance of travel x becomes a maximum, xmax, when v 0 (turning point in the motion). Use the expressions found in part (a) for v to find the value of t when x has its maximum value: From v ( t) m/s, v 0 when t 15.0 s Then x max (30.0t t ) m (30.0)(15.0) (15.0) 5 m.31 (a) v i 100 m/s, a 5.00 m/s v v i + ax x 1000 m and t 0.0 s 0 (100) (5.00)x No, at this acceleration the plane would overshoot the runway. *.3 In the simultaneous equations we have v x v xi + a x t x x i 1 (v xi + v x) t v x v xi (5.60 m/s )(4.0 s) 6.4 m 1 (v xi + v x )4.0 s So substituting for v xi gives 6.4 m 1 [v x + (5.60 m/s )(4.0 s) + v x ]4.0 s 14.9 m/s v x + 1 (5.60 m/s )(4.0 s) v x 3.10 m/s

14 14 Chapter Solutions *.33 Take any two of the standard four equations, such as v x v xi + a x t x x i 1 (v xi + v x )t solve one for v xi, and substitute into the other: v xi v x a x t x x i 1 (v x a x t + v x ) t Thus x x i v x t 1 a xt Back in problem 3, 6.4 m v x (4.0 s) 1 ( 5.60 m/s )(4.0 s) v x 6.4 m 49.4 m 4.0 s 3.10 m/s.34 We assume the bullet is a cylinder. It slows down just as its front end pushes apart wood fibers. (a) a v v i x (80 m/s) (40 m/s) (0.100 m) m/s t s (c) v i 40 m/s, v 0; a m/s ; v v i + ax x v v i a v i a (40 m/s) ( m/s ) x m *.35 (a) The time it takes the truck to reach 0.0 m/s is found from v v i + at, solving for t yields t v v i a 0.0 m/s 0 m/s.00 m/s 10.0 s The total time is thus 10.0 s s 35.0 s

15 Chapter Solutions 15 The average velocity is the total distance traveled divided by the total time taken. The distance traveled during the first 10.0 s is x 1 v t (10.0) 100 m The distance traveled during the next 0.0 s is x v i t + 1 at (0.0)(0.0) m, a being 0 for this interval. The distance traveled in the last 5.00 s is x 3 v t (5.00) 50.0 m The total distance x x 1 + x + x m, and the average velocity is given by v x t m/s *.36 Using the equation x v i t + 1 at yields x 0.0(40.0) 1.00(40.0) / 0, which is obviously wrong. The error occurs because the equation used is for uniformly accelerated motion, which this is not. The acceleration is 1.00 m/s for the first 0.0 s and 0 for the last 0.0 s. The distance traveled in the first 0.0 s is: x v i t + 1 at (0.0)(0.0) 1.00(0.0 )/ 00 m During the last 0.0 s, the train is at rest. Thus, the total distance traveled in the 40.0 s interval is 00 m..37 (a) a v v i t 63(580/3600) ft/s 0 m/s x v i t + 1 at (63)(580/3600)(1.40) 1 66(1.40) 649 ft 198 m.38 We have v i m/s, v m/s, x x i m (a) x x i 1 (v i + v) t t (x x i) v i + v ( m) m/s m/s s

16 16 Chapter Solutions v v i + a(x x i ) a v v i (x x i ) ( m/s) ( m/s) ( m) m/s.39 (a) Take initial and final points at top and bottom of the incline. If the ball starts from rest, v i 0, a m/s, x x i 9.00 m Then v v i + a(x x i ) 0 + ( m/s ) 9.00 m v 3.00 m/s x x i v i t + 1 at 9.00 m (0.500 m/s ) t t 6.00 s (c) Take initial and final points at the bottom of the planes and the top of the second plane, respectively. v i 3.00 m/s v 0 x x i m v v i + a(x x i ) gives a (v v i) (x x i ) [0 (3.00 m/s) ] (15.0 m) m/s (d) Take initial point at the bottom of the planes and final point 8.00 m along the second: v i 3.00 m/s x x i 8.00 m a m/s v v i + a(x x i ) (3.00 m/s) + ( m/s )(8.00 m) 4.0 m /s v.05 m/s

17 Chapter Solutions Take the original point to be when Sue notices the van. Choose the origin of the x-axis at Sue's car. For her we have x is 0 v is 30.0 m/s a s.00 m/s so her position is given by x s (t) x is + v is t + 1 a st (30.0 m/s)t + 1 (.00 m/s ) t For the van, x iv 155 m v iv 5.00 m/s a v 0 and x v (t) x iv + v iv t + 1 a v t 155 m + (5.00 m/s)t + 0 To test for a collision, we look for an instant t c when both are at the same place: 30.0t c t c t c 0 t c 5.0t c From the quadratic formula t c 5.0 ± (5.0) 4(155) 13.6 s or 11.4 s The smaller value is the collision time. (The larger value tells when the van would pull ahead again if the vehicles could move through each other). The wreck happens at position 155 m + (5.00 m/s)(11.4 s) 1 m..41 Choose the origin (y 0, t 0) at the starting point of the ball and take upward as positive. Then, y i 0, v i 0, and a g 9.80 m/s. The position and the velocity at time t become: y y i v i t + 1 at y 1 gt 1 (9.80 m/s ) t and v v i + at v gt (9.80 m/s )t (a) at t 1.00 s: y 1 (9.80 m/s )(1.00 s) 4.90 m at t.00 s: y 1 (9.80 m/s )(.00 s) 19.6 m at t 3.00 s: y 1 (9.80 m/s )(3.00 s) 44.1 m

18 18 Chapter Solutions at t 1.00 s: v (9.80 m/s )(1.00 s) 9.80 m/s at t.00 s: v (9.80 m/s )(.00 s) 19.6 m/s at t 3.00 s: v (9.80 m/s )(3.00 s) 9.4 m/s *.4 Assume that air resistance may be neglected. Then, the acceleration at all times during the flight is that due to gravity, a g 9.80 m/s. During the flight, Goff went 1 mile (1609 m) up and then 1 mile back down. Determine his speed just after launch by considering his upward flight: v v i + a(y y i ) 0 v i (9.80 m/s )(1609 m) v i 178 m/s His time in the air may be found by considering his motion from just after launch to just before impact: y y i v i t + 1 at 0 (178 m/s)t 1 ( 9.80 m/s ) t The root t 0 describes launch; the other root, t 36. s, describes his flight time. His rate of pay may then be found from pay rate $ s $ 3600 s s 1 h $99.4/h.43 (a) y v i t + 1 at 4.00 (1.50)v i (4.90)(1.50) and v i 10.0 m/s upward v v i + at 10.0 (9.80)(1.50) 4.68 m/s v 4.68 m/s downward.44 We have y 1 gt + v i t + y i 0 (4.90 m/s )t (8.00 m/s)t m 8.00 ± Solving for t, t 9.80 Using only the positive value for t, we find t 1.79 s

19 Chapter Solutions 19 *.45 The bill starts from rest v i 0 and falls with a downward acceleration of 9.80 m/s (due to gravity). Thus, in 0.0 s it will fall a distance of y v i t 1 gt 0 (4.90 m/s )(0.0 s) 0.0 m This distance is about twice the distance between the center of the bill and its top edge ( 8 cm). Thus, David will be unsuccessful. Goal Solution Emily challenges her friend David to catch a dollar bill as follows. She holds the bill vertically, as in Figure P.45, with the center of the bill between David s index finger and thumb. David must catch the bill after Emily releases it without moving his hand downward. If his reaction time is 0. s, will he succeed? Explain your reasoning. G: David will be successful if his reaction time is short enough that he can catch the bill before it falls half of its length (about 8 cm). Anyone who has tried this challenge knows that this is a difficult task unless the catcher cheats by anticipating the moment the bill is released. Since David s reaction time of 0. s is typical of most people, we should suspect that he will not succeed in meeting Emily s challenge. O: Since the bill is released from rest and experiences free fall, we can use the equation y 1 gt to find the distance y the bill falls in t 0. s A: y 1 (9.80 m/s )(0. s) m > 0.08 m Since the bill falls below David s fingers before he reacts, he will not catch it. L: It appears that even if David held his fingers at the bottom of the bill (about 16 cm below the top edge), he still would not catch the bill unless he reduced his reaction time by tensing his arm muscles or anticipating the drop. *.46 At any time t, the position of the ball released from rest is given by y 1 h 1 gt. At time t, the position of the ball thrown vertically upward is described by y v i t 1 gt. The time at which the first ball has a position of y 1 h/ is found from the first equation as h/ h 1 gt, which yields t h/g. To require that the second ball have a position of y h/ at this time, use the second equation to obtain h/ v i h/g 1 g(h/g). This gives the required initial upward velocity of the second ball as v i gh.

20 0 Chapter Solutions.47 (a) v v i gt (Eq..8) v 0 when t 3.00 s, g 9.80 m/s, v i gt (9.80 m/s )(3.00 s) 9.4 m/s y 1 (v + v i) t 1 (9.4 m/s)(3.00 s) 44.1 m Goal Solution A baseball is hit such that it travels straight upward after being struck by the bat. A fan observes that it requires 3.00 s for the ball to reach its maximum height. Find (a) its initial velocity and the maximum height it reaches. G: We can expect the initial speed of the ball to be somewhat greater than the speed of the pitch, which might be about 60 mph (~30 m/s), so an initial upward velocity off the bat of somewhere between 0 and 100 m/s would be reasonable. We also know that the length of a ball field is about 300 ft. (~100m), and a pop-fly usually does not go higher than this distance, so a maximum height of 10 to 100 m would be reasonable for the situation described in this problem. O: Since the ball s motion is entirely vertical, we can use the equation for free fall to find the initial velocity and maximum height from the elapsed time. A: Choose the initial point when the ball has just left contact with the bat. Choose the final point at the top of its flight. In between, the ball is in free fall for t 3.00 s and has constant acceleration, a -g m/s. Solve the equation v yf v yi gt for v yi when v yf 0 (when the ball reaches its maximum height). (a) v yi v yf + gt 0 + (9.80 m/s )(3.00 s) 9.4 m/s (upward) The maximum height in the vertical direction is y f v yi t + 1 at (9.4 m/s)(3.00 s) + 1 ( 9.80 m/s )(3.00 s) 44.1 m L: The calculated answers seem reasonable since they lie within our expected ranges, and they have the correct units and direction. We must remember that it is possible to solve a problem like this correctly, yet the answers may not seem reasonable simply because the conditions stated in the problem may not be physically possible (e.g. a time of 10 seconds for a pop fly would not be realistic).

21 Chapter Solutions 1.48 Take downward as the positive y direction. (a) While the woman was in free fall, y 144 ft, v i 0, and a g 3.0 ft/s Thus, y v i t + 1 at 144 ft 0 + (16.0 ft/s )t giving t fall 3.00 s. Her velocity just before impact is: v v i + gt 0 + (3.0 ft/s )(3.00 s) 96.0 ft/s. While crushing the box, v i 96.0 ft/s, v 0, and y 18.0 in 1.50 ft. Therefore, a v v i ( y) 0 (96.0 ft/s) (1.50 ft) ft/s, or a ft/s upward (c) Time to crush box: y y (v + v v i )/ (1.50 ft) ft/s or s.49 Time to fall 3.00 m is found from Eq..11 with v i 0, 3.00 m 1 (9.80 m/s ) t ; t 0.78 s (a) With the horse galloping at 10.0 m/s, the horizontal distance is vt 7.8 m t 0.78 s

22 Chapter Solutions.50 Time to top 10.0 s. v v i gt (a) At the top, v 0. Then, t v i g 10.0 s v i 98.0 m/s h v i t 1 gt At t 10.0 s, h (98.0)(10.0) 1 (9.80)(10.0) 490 m.51 v i 15.0 m/s (a) v v i gt 0 t v i g 15.0 m/s 9.80 m/s 1.53 s (c) h v i t 1 gt v i g m 11.5 m At t.00 s v v i gt m/s a g 9.80 m/s.5 y 3.00t 3 At t.00 s, y 3.00(.00) m, and v y dy 9.00t 36.0 m/s If the helicopter releases a small mailbag at this time, the equation of motion of the mailbag is y b y bi + v i t 1 gt t 1 (9.80) t Setting y b 0, t 4.90t Solving for t, (only positive values of t count), t 7.96 s

23 Chapter Solutions 3.53 (a) J da constant da J a J Jt + c but a a i when t 0 so c 1 a i, Therefore, a Jt + a i a dv ; dv a v a (Jt + a i ) 1 Jt + a i t + c but v v i when t 0, so c v i and v 1 Jt + a i t + v i v dx ; dx v x v 1 Jt + a i t + v i x 1 6 Jt3 + 1 a it + v i t + c 3 x x i when t 0, so c 3 x i Therefore, x 1 6 Jt3 + 1 a it + v i t + x i a (Jt + a i ) J t + a i + Ja i t a a i + (J t + Ja i t) a a i + J 1 Jt + a i t Recall the expression for v: v 1 Jt + a i t + v i So (v v i ) 1 Jt + a i t Therefore, a a i + J(v v i )

24 4 Chapter Solutions.54 (a) a dv d [ t t] Error! t m/s ) Take x i 0 at t 0. Then v dx t x 0 0 t v 0 ( t t) x t t x ( m/s 3 )t 3 + ( m/s )t The bullet escapes when a 0, at ( m/s 3 )t m/s 0 t s s (c) New v ( )( ) + ( )( ) v 450 m/s m/s 450 m/s (d) x ( )( ) 3 + ( )( ) x m m m.55 a dv 3.00v, v i 1.50 m/s Solving for v, dv 3.00v v 0 v dv 3.00 v v i t 0 1 v + 1 v i 3.00t or 3.00t 1 v 1 v i When v v i, t v i 0. s

25 Chapter Solutions 5.56 (a) The minimum distance required for the motorist to stop, from an initial speed of 18.0 m/s, is x v v i a 0 (18.0 m/s) ( 4.50 m/s 36.0 m ) Thus, the motorist can travel at most (38.0 m 36.0 m).00 m before putting on the brakes if he is to avoid hitting the deer. The maximum acceptable reaction time is then t max.00 m v i.00 m 18.0 m/s s In s, the distance traveled at 18.0 m/s is x v i t 1 (18.0 m/s)(0.300) 5.40 m The displacement for an acceleration 4.50 m/s is m. v v i + ax (18.0 m/s) (4.50 m/s )(3.6 m) 30.6 m /s v m/s.57 The total time to reach the ground is given by y y i v i t + 1 at m ( 9.80 m/s ) t t (5.0 m) 9.80 m/s.6 s The time to fall the first fifteen meters is found similarly: 15.0 m 0 1 (9.80 m/s ) t 1 t s So t t 1.6 s 1.75 s s suffices for the last ten meters.

26 6 Chapter Solutions *.58 The rate of hair growth is a velocity and the rate of its increase is an acceleration. Then v i 1.04 mm/d and a 0.13 mm/d. The increase in the length of the hair (i.e., w displacement) during a time of t 5.00 w 35.0 d is x v i t + 1 at x (1.04 mm/d)(35.0 d) + 1 (0.13 mm/d w)(35.0 d)(5.00 w) or x 48.0 mm.59 Let path (#1) correspond to the motion of the rocket accelerating under its own power. Path (#) is the motion of the rocket under the influence of gravity with the rocket still rising. Path (#3) is the motion of the rocket under the influence of gravity, but with the rocket falling. The data in the table is found for each phase of the rocket's motion. (#1): v (80.0) (4.00)(1000); therefore v 10 m/s (4.00)t giving t 10.0 s (#): 0 (10) ( 9.80) x giving x 735 m t giving t 1. s This is the time of maximum height of the rocket. (#3): v 0 ( 9.80)( 1735) v 184 ( 9.80)t giving t 18.8 s (a) t total s x total 1.73 km (c) v final 184 m/s t x v a 0 Launch #1 End Thrust # Rise Upwards #3 Fall to Earth

27 Chapter Solutions 7.60 Distance traveled by motorist (15.0 m/s)t Distance traveled by policeman 1 (.00 m/s ) t ( a ) intercept occurs when 15.0t t t 15.0 s v (officer) (.00 m/s )t 30.0 m/s (c) x (officer) 1 (.00 m/s ) t 5 m *.61 Area A 1 is a rectangle. Thus, A 1 hw v i t. Area A is triangular. Therefore A 1 bh 1 t(v v i). The total area under the curve is v A A 1 + A v i t + (v v i )t/ and since v v i at v v i A A v i t + 1 at A 1 The displacement given by the equation is: 0 t t x v i t + 1 at, the same result as above for the total area..6 a m/s, a m/s x 1000 m 1 a 1t 1 + v 1 t + 1 a t t t 1 + t and v 1 a 1 t 1 a t a 1t 1 + a 1 t 1 a 1t 1 a + 1 a a 1 t 1 a

28 8 Chapter Solutions a 1 1 a 1 t 1 a t 1 0, s t a 1t 1 a s Total time t 155 s.63 (a) Let x be the distance traveled at acceleration a until maximum speed v is reached. If this achieved in time t 1 we can use the following three equations: x (v + v i) t 1, 100 x v(10. t 1 ) and v v i + at 1 The first two give t 1 v t 1 at 1 a For Maggie a 00 (0.4 t 1 )t (18.4)(.00) 5.43 m/s 00 For Judy a (17.4)(3.00) 3.83 m/s v at 1 Maggie: v (5.43)(.00) 10.9 m/s Judy: v (3.83)(3.00) 11.5 m/s (c) At the six-second mark x 1 at 1 + v(6.00 t 1) Maggie: x 1 (5.43)(.00) + (10.9)(4.00) 54.3 m Judy: x 1 (3.83)(3.00) + (11.5)(3.00) 51.7 m Maggie is ahead by.6 m.

29 Chapter Solutions 9 *.64 Let the ball fall 1.50 m. It strikes at speed given by: v x v xi + a(x x i ) v x 0 + ( 9.80 m/s )( 1.50 m) v x 5.4 m/s and its stopping is described by v x v xi + a x (x x i ) 0 ( 5.4 m/s) + a x ( 10 m) a x 9.4 m /s m m/s Its maximum acceleration will be larger than the average acceleration we estimate by imagining constant acceleration, but will still be of order of magnitude ~ 10 3 m/s..65 Acceleration a 3.00 m/s Deceleration a' 4.50 m/s (a) Keeping track of speed and time for each phase of motion, v 0 0, v m/s s v m/s t s v m/s, v s v 0 m/s, v m/s s v m/s t s v m/s, v m/s s v m/s t s v m/s, v s Σt 45.7 s x Σ v i t i 6.00(4.00) + 1.0(5.00) (.67) (6.00) (0.0) + 1.0(.67) (4.00) (1.33) 574 m (c) v x 574 m 45.7 s 1.6 m/s

30 30 Chapter Solutions (d) t WALK x (574 m) v WALK (1.50 m/s) 765 s (about 13 minutes, and better exercise!).66 (a) d 1 (9.80) t 1 d 336t t 1 + t t 4.90(.40 t ) 4.90t 359.5t t ± (359.5) 4(4.90)(8.) 9.80 t ± s d 336t 6.4 m Ignoring the sound travel time, d 1 (9.80)(.40) 8. m, an error of 6.8%..67 (a) y v i1 t + 1 at t + 1 (9.80) t t.99 s after the first stone is thrown. y v i t + 1 at and t s substitute 50.0 v i (1.99) + 1 (9.80)(1.99) v i 15.4 m/s downward (c) v 1 v i1 + at.00 + ( 9.80)(.99) 31.3 m/s v v i + at ( 9.80)(1.99) 34.9 m/s.68 The time required for the car to come to rest and the time required to regain its original speed of 5.0 m/s are both given by v 5.0 m/s a.50 m/s. The total distance the car travels in these two intervals is x car x 1 + x (5.0 m/s + 0) (10.0 s) + ( m/s) (10.0 s) 50 m

31 Chapter Solutions 31 The total elapsed time when the car regains its original speed is total 10.0 s s s 65.0 s The distance the train has traveled in this time is x train (5.0 m/s)(65.0 s) m Thus, the train is m 50 m m ahead of the car..69 (a) We require x s x k when t s t k x s 1 (3.50 m/s )(t k ) 1 (4.90 m/s )(t k ) x k t k t k t k 5.46 s x k 1 (4.90 m/s )(5.46 s) 73.0 m (c) v k (4.90 m/s )(5.46 s) 6.7 m/s v s (3.50 m/s )(6.46 s).6 m/s.70 (a) In walking a distance x, in a time, the length of rope l is only increased by x sin θ. The pack lifts at a rate x sin θ. h v a l v x sin θ v boy x l v boy x x + h m x v boy a dv v boy l dx + v boy x d 1 l a v boy v boy l v boyx l dl, but dl v a v boy l 1 x l v boy l h l h v boy (x + h ) 3/ (c) v boy h, 0 (d) v boy, 0

32 3 Chapter Solutions.71 h 6.00 m, v boy.00 m/s v x sin θ v x boy l v boy x (x + h ) 1/ However x v boy t (a) v boyt v (v boyt + h ) 4t 1/ (4t + 36) 1/ t(s) v(m/s) v (m/s) t (s) From problem.70 above, h v boy a (x + h ) 3/ h v boy (v boyt + h ) 144 3/ (4t + 36) 3/ t(s) a(m/s ) a (m/s ) t (s)

33 Chapter Solutions 33.7 Time t(s) Height h(m) h (m) (s) v (m/s) midpt time t (s) v (m/s) t (s) acceleration slope of line is constant. a 1.63 m/s 1.63 m/s downward

34 34 Chapter Solutions.73 The distance x and y are always related by x + y L. Differentiating this equation with respect to time, we have x dx + y dy 0 y Now dy is v B, the unknown velocity of B; and dx v. B x From the equation resulting from differentiation, we have dy x dx y x y ( v) y O L α x A v But y x tan α so v B 1 tan α v When α 60.0, v B v tan 60.0 v v Goal Solution Two objects, A and B, are connected by a rigid rod that has a length L. The objects slide along perpendicular guide rails, as shown in Figure P.73. If A slides to the left with a constant speed v, find the velocity of B when α G: The solution to this problem may not seem obvious, but if we consider the range of motion of the two objects, we realize that B will have the same speed as A when α 45, and when α 90, then v B 0. Therefore when α 60, we should expect v B to be between 0 and v. O: Since we know a distance relationship and we are looking for a velocity, we might try differentiating with respect to time to go from what we know to what we want. We can express the fact that the distance between A and B is always L, with the relation: x + y L. By differentiating this equation with respect to time, we can find v B dy/ in terms of dx/ v A v. A: Differentiating x + y L gives us x dx + y dy 0 Substituting and solving for the speed of B: v B dy x dx y x y ( v) Now from the geometry of the figure, we notice that y x tan α, so v v B tan α v When α 60.0, v B tan 60 v 0.577v (B is moving up) 3 L: Our answer seems reasonable since we have specified both a magnitude and direction for the velocity of B, and the speed is between 0 and v in agreement with our earlier prediction. In this and many other physics problems, we can find it helpful to examine the limiting cases that define boundaries for the answer.

35 Chapter Solutions 35