An object has a mass of 2 kg. The object's kinetic energy uniformly changes from 135 KJ to 36KJ over t in [0, 6 s]. What is the average speed of the object?

1 Answer
Gió
Jun 11, 2016

I found 9.2m/s

Explanation:

We need the speed of the object at each instant so we can use the definition of Kinetic Energy K=1/2mv^2 to find it:
K_i=1/2mv_i^2
and
K_f=1/2mv_f^2

using our data:
135=1/2*2v_i^2
v_i=11.6m/s

and

36=1/2*2v_f^2
v_f=6m/s

we can now use:
v_f=v_i+at
and find a:
6=11.6+6a
a=-0.9m/s^2
we now use:
v_f^2=v_i^2+2as
to find the distance:
36=135-2*0.9*s
s=55m
So:
"average speed"="distance"/"time"=s/t=55/6=9.2m/s

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