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Exercicios resolvidos James Stewart vol. 2 7ª ed - ingles

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168 D CHAPTER 13 VECTOR FUNCTIONS<br />

13.4 Motion in Space: Velocity and Acceleration<br />

1: (a) Ifr(t) = x(t) i + y (t) j + z(t) k is the position vector of the particle at timet, then the average velocity over the time<br />

.interval [0, 1] is<br />

_ r(l)- r (O) _ (4.5 i + 6.0j + 3.0k)- (2.7 i + 9.8j + 3.7k) _ . 1 8 3 8 . 0 ? k s· . - . 1 I h th<br />

1 - . J - . . 1m1 ar y, over t e o er<br />

1-0 1<br />

•<br />

v ave - -<br />

intervals we have<br />

[0.5, 1] :<br />

[1, 2] :<br />

[1, 1.5]:<br />

_ r(1)- r (0.5) _ (4.5 i + 6.0j + 3.0 k)- (3.5 i + i2j + 3.3 k) _ 2 0<br />

. _ 2 4<br />

. _ 0 6<br />

k<br />

Vave- 1 - 0.5 - 0.5 - . 1 . J .<br />

V ave -<br />

_ r (2)- r (1) _ (7.3i+7.8j+2.7k) - (4.5i+6.0j +3.0k) _ . 28<br />

2 _ 1<br />

- .<br />

1<br />

1 8 ._ 03 k .<br />

- · 1 + · J ·<br />

_ r(1.5) -r(1) _ (5.9i+6.4j+2.8k)-(4.5 i +6.0j+3.0 k) _ 28<br />

. · 08<br />

._ 04<br />

k<br />

Yavo- - - · 1 + · J •<br />

1.5 -1 0.5 . .<br />

(b) We can estimate the velocity at t = 1 by averaging the average velocities over the time intervals [0.5, 1] and [1, 1.5]:<br />

v(1) ~ ~{(2 i - 2.4j- 0.6k) + (2.8 i +0.8j- 0.4k)] = 2.4i - 0.8j - o·.5k. Then the spe<strong>ed</strong> is<br />

Jv (1)\ ~ )(2.4) 2 + ( - 0.8)2 + ( -0.5)2 ~ 2.58.<br />

. 1 2 )<br />

3. r(t) = (- 2 t ,t =><br />

v{t) = r'(t) = (-t, i)<br />

a(t) = r"(t) = (-:- 1, 0)<br />

Att = 2:<br />

v{2) = ( -2, 1)<br />

a(2) .= (-1,0)<br />

jv(t)\ = Jt 2 + 1<br />

5.r(t)=3costi+2sintj =><br />

v (t) = -3sinti+2costj<br />

a(t) = -3costi- 2sintj<br />

Att = 71'/3:<br />

v(~) =-¥i+j<br />

a(i) = - ~ i - J3j<br />

(3,0)<br />

/<br />

X<br />

Jv (t)\ = )9sin 2 t + 4cos 2 t;:::: .) 4 + 5sin 2 t<br />

Notice that x 2 /9 + y 2 j4 = sin 2 t + cos 2 t = 1, so the path is an ellipse.<br />

Att = 1:<br />

z<br />

v(t) = i + 2tj<br />

a(t) = 2j<br />

v(1) = i + 2j<br />

a(1) = 2j<br />

Jv(t)l=~<br />

Here x = t, y = t2 => y = ~ 2 and z = 2, so the path of the particle is a<br />

parabola in the plane z = 2.<br />

X<br />

Y<br />

© 2012 Ccngugc Learning, AU Rights Rescr.·cd. Mny not be scann<strong>ed</strong>, copi<strong>ed</strong>, or duplicat<strong>ed</strong>, or post<strong>ed</strong> to a publicly accessible website, in whole or in pru1.

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