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posted by  Plomo on 7/17/2008 12:08:52 AM  |  status: Closed  

Dynamics:

Course Textbook Chapter Problem
Classical Mechanics Vector Mechanics for Engineers: Statics and Dynamics (8th) by Beer, Johnston, Eisenberg 11 91P
Question Details:
Hello,
I'm not sure what I'm doing wrong but I can't get this to solve to match answers in back of book.  Can you help me?  Thank you for looking.
 
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Sage
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posted by ClayL on 7/17/2008 10:36:17 PM  |  status: Live
Asker's Rating: Lifesaver   
Plomo's comment:
"Thank you!"
Response Details:
x = (t+1)2        y = 4(t+2)-2
Vx = x' = 2(t+1)           Vy = y' = -8(t+1)-3
ax = v'x = 2                ay = v'y = 24(t+1)-4

Now solve for (t+1)2 from the the expression for x:
(t+1)2 = x     
Substitute this into the expression for y:
y = 4/x
so xy = 4
This is the equation of a rectangular hyperbola, so

A) at t = 0
Vx = 2 m/s         Vy = - 8m/s
√[22 + (-8)2] = 8.25 m/s
θ = tan-1 (-8/2) = -76o
So v = 8.25 m/s < 76o
ax = 2 m/s2      ay = 24 m/s2
a = √[(22) + (242)] = 24.1 m/s2
θ = tan-1(24/2) = 85.2o
so a = 24.1 m/s2 < 85.2o
 

B) at t = 1/2 s
Vx = 3 m/s       Vy = -2.37 m/s
v = √32 + 2.372 = 3.82 m/s
θ = tan-1(-2.37/3)
 v = 3.82 m/s > 38.3o
ax = 2 m/s2     ay = 4.74 m/s2
a = √22 + 4.742 = 5.14 m/s2
θ = tan-1(4.74/2) = 67.1o
so a = 5.14 m/s2 > 67.1o
 

Note: > and < represent reference angles and not inequalities

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