Meg N.

asked • 01/20/15

Find the airplane's acceleration and time?

An airplane flies eastward and always accelerates at a constant rate. At one position along its path it has a velocity of 33.5 m/s, it then flies a further distance of 43300 m and afterwards its velocity is 46.7 m/s. Find the airplane\'s acceleration and calculate how much time elapses while the airplane covers those 43300 m.

Christopher R.

There's a formula for this in any physics text book being:
 
v2^2-v1^2 = 2*a*s
 
Where v2 = the final velocity
           v1 = the initial velocity
             a = acceleration
             s = the change in distance
 
I thought I would simply help you out in how the formula was derived.
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01/20/15

Logan D.

I'm not sure about the votes, but there are several mistakes in your derivations and usage.
 
First, s = displacement, not distance. Distance is not the same thing as displacement. Second, PE = mgh, not mad. h is the height above a uniform mass with gravitational pull, not distance or displacement. Therefore, you can't use it in the airplane problem, where there is no height. Also, a = g for PE, which is 9.8m/s2, therefore you also can't use PE to determine a, because it's already assigned a value. Third, vf2 = vi2 + 2as is not derived by equating PE to KEs. (On top of the reasons already stated, you left out the other PE for the conversation of energy. The full equation relating PE to KE is KEi + PEi = KEf + PEf.)
 
The derivation of vf2 = vi2 + 2as is found by substituting the the equation of acceleration into the equation for average velocity.
 
a = (vf - vi) / t
⇒ t = (vf - vi) / a
 
vavg = (vf + vi) / 2 = s / t
⇒ s = (vf + vi)t / 2
Substitute:
⇒ s = (vf + vi)((vf - vi) / a) / 2
 
FOIL and rearrange and you'll get the solution.
 
Finally, we can tell from the problem and how early it in the semester that the student isn't using conservation of energy, but is still learning how to solve these equations with basic kinematic equations.
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01/21/15

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Logan D. answered • 01/20/15

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Christopher R.

Thanks for your input. Yeah, your right. PE and KE are related to the potential energy and kinetic energy due to gravity. However, I remember a professor deriving this formula using calculus where a=dv/dt=dv/ds*ds/dt=v*dv/ds in which implies a*ds=v*dv. Then, he worked out the math and came up with a formula that you and I agree upon. v2^2=v1^2+2as. Again, thanks for your input.
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01/21/15

Christopher R. answered • 01/20/15

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