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Question
I was wondering if you can help me out and tell me how fast a object is falling from 78 feet it weighs 12 pounds, also what can I figure out if the kinetic energy is 266.71229 of force hitting the object? what does this mean in terms ofthe damage it will do to a human body?

Answer
If an object falls from 78 feet the velocity when the object reaches the ground can be determined from kinematics where the known variables are:
Do=78ft (given), Df=0ft (object hits ground), Vo=0ft/s (object dropped from rest), Vf=? (answer to problem), a=-32.2ft/s^2 (known acceleration of gravity near the Earth's surface) and t=? (unknown).
The displacement of an accelerating body is given by:
Df=1/2*a*t^2+Vo*t+Do  which becomes  0=1/2*(-32.2)*t^2+0+78
Solving for the time:
16.1*t^2=78  therefore  t=sqrt(78/16.1)=2.20s
This time can now be used to determine the final velocity using:
Vf=a*t+Vo  which becomes  Vf=(-32.2)*2.20+0=-70.8ft/s
The weight does not come into the calculation unless air friction is a significant factor!
As for the 2nd part of your question "the kinetic energy of force hitting the object" makes no sense. Can you clarify?

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James J. Kovalcin

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I am teaching or have taught AP physics B and C [calculus based mechanics & electricity and magnetism] as well as Lab Physics for college bound students. I have a BS in Physics from the University of Pittsburgh and a Master of Arts in Teaching from same. I have been teaching physics for 34 years. I am constantly updating my skills and have a particular interest in modern physics topics.

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