Sample Solution from
Vector Mechanics for Engineers: Dynamics
11th Edition
ISBN: 9780077687342
Chapter 11.1
Problem 11.8P
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Textbook Problem

The motion of a particle is defined by the relation x = t 2 ( t 2 ) 3 , where x and t are expressed in feet and seconds, respectively. Determine (a) the two positions at which the velocity is zero, (b) the total distance traveled by the particle from t = 0 to t = 4 s.

Expert Solution
To determine

(a)

The positions at which velocity is zero.

Answer

Time x1=8.88ftandx2=2.03ft

Explanation of Solution

Given information:

The motion of the particle is given by the equation:

x=t2(t2)3

x=t2(t3(2)33t×2(t2))x=t2(t386t2+12t)x=t2t3+8+6t212t

x=t3+7t212t+8 __________ (1)

Where (x) in feet and (t) is in seconds.

We can obtain the velocity (v) at any time (t) by differentiating (x) with respect to (t),

Since, v=dxdt

v=ddt(t3+7t212t+8)v=3t2+14t12 __________ (2)

The acceleration (a) can be obtained by differentiating again the above equation with respect to (t)

a=dvdta=ddt(3t2+14t12)

a=6t+14 _________ (3)

When velocity is zero, v=0

From equation (2),

v=3t2+14t12or,0=3t2+14t12or,3t2+14t12=0t1=3.53st2=1.131s

Now, the position when t1=3.53s.

x1=t3+7t212t+8x1=(3.53)3+7(3.53)212(3.53)+8x1=43.98+87.2242.36+8x1=95.2286.34x1=8.88ft

Now, the position when t1=1.131s:

x2=t3+7t212t+8x2=(1.31)3+7(1.31)212(1.31)+8x2=2.25+1215.72+8x2=2017.97x2=2.03ft

Conclusion:

The two positions when velocity is zero x1=8.88 ft and x2=2.03ft.

Expert Solution
To determine

(b)

The total distance traveled by the particle when, t = 0s to t=4s.

Answer

Total distance travel by the particle is 8 ft.

Explanation of Solution

Given information:

The motion of the particle is given by the equation:

x=t2(t2)3

x=t2(t3(2)33t×2(t2))x=t2(t386t2+12t)x=t2t3+8+6t212t

x=t3+7t212t+8 __________ (1)

Where (x) in feet and (t) is in seconds.

We can obtain the velocity (v) at any time (t) by differentiating (x) with respect to (t),

Since, v=dxdt

v=ddt(t3+7t212t+8)v=3t2+14t12 __________ (2)

The acceleration (a) can be obtained by differentiating again the above equation with respect to (t):

a=dvdta=ddt(3t2+14t12)

a=6t+14 _________ (3)

From equation (1), when t = 0s.

x1=t3+7t212t+8x1=(0)3+7(0)212(0)+8x1=8ft

Again, from equation (1), when t=4s

x2=t3+7t212t+8x2=(4)3+7(4)212(4)+8x2=64+11248+8x2=120112x2=8ft

Conclusion:

The total distance travel by the particle is 8 ft.

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