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Physics/time calculation

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Question
I have a sack 5 mtrs off the ground. Assuming acceleration of 10mtrs per second per second, how do I calculte when it will land. ie time to fall?

Answer
Virtually all one dimensional kinematics problems can be solved in the same way. Since there are 2 relevant equations:
Df=1/2*a*t^2+Vo*t+Do  and  Vf=a*t+Vo
Which consist of 6 distinct variables:
Do, Df, Vo, Vf, a and t.
Any time you can determine 4 of these variables, you can always solve for the other 2. (with 2 equations you are permitted 2 unknowns!)
In this case the variables are:
Do=5.0m (given), Df=0m (the object hits the ground), Vo=0m/s (the object starts from rest), Vf=? (unknown), a=g=-10m/s^2 (given, the acceleration of gravity) and t=? (unknown, the answer to the problem!)
Using the Df final equation:
Df=1/2*a*t^2+Vo*t+Do
0=1/2*(-10)*t^2+0*t+5.0
Which becomes:
5*t^2=5.0
Therefore, solving for the time:
t=sqrt(1)=1.0s

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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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