What is the kinetic energy of an object with a mass of #1 kg# that has been in freefall for #2 s#?

1 Answer
Mar 9, 2016

Answer:

I found #192J#

Explanation:

We can either:

1) use kinematic to find the final velocity after #2s#:
using: #v_f=v_i+at#
where #v_i# is the initial velocity (assumed as zero);
#a# is the acceleration of gravity (directed downward).
We get:
#v_f=0-9.8*2=19.6m/s#
From this we get:
Kinetic Energy: #K=1/2mv^2=1/2*1*(19.6)^2=192J#

or:

2) use kinematic (and Conservation of Energy) to find the vertical distance described after #2s# and the Potential Energy:
using: #y_f-y_i=v_it+1/2at^2#
Where we set #y_f=0# as the ground (zero Potential Energy).
giving:
#0-y_i=0*2-1/2*9.8*(2)^2#
so that the height (from ground) will be:
#y_i=19.6m#
At this position the object had a Potential Energy of:
#U=mgy_i=1*9.8*19.6=192J# that will be converted entirely into Kinetic Energy during the fall!