When is the kinetic energy equal to zero?
2 answers
When it isn't moving
actually, it is more difficult. KE is a movement, but movement is relative to some system. If your sleeping child in a car is not moving relative to the car, we say she has zero movement, but in honesty, she is moving with the car. If you park the car, the car is moving relative so some fixed axis, as the Earth is rotating. So what do we mean by "isn't moving"? Is Kinetic Energy relative.
Take this example.
the baby is holding a pacifier in her mouth and the car is moving a 20m/s. the KE of the pacifier could be written as 1/2 mass*(20^2) or 200m Joules. But in relative velocity, the velocity is zero, so KE could be zero.
Now the baby spits the pacifier forward at .5 m/s.
is KE=1/2 m (.5)^2=.125m J or
is KE=1/2 m (20.5)^2=210.125
in one case the gain of KE was .125m J and the other a gain of 10.125, so what energy did the baby put into it? Well, it may startle you: the baby put .125 m J, and the car put 10J into it .
So Kinetic Energy is a relative quantity, as is motion. KE can be zero if we define motion and KE in a certain system only.
This is a very interesting description of relativity, and how it leads to strange things as one apporaches the speed of light. (At light speed, the baby could not spit out the pacifiers)
http://physics.stackexchange.com/questions/1368/is-kinetic-energy-a-relative-quantity-will-it-make-inconsistent-equations-when
Take this example.
the baby is holding a pacifier in her mouth and the car is moving a 20m/s. the KE of the pacifier could be written as 1/2 mass*(20^2) or 200m Joules. But in relative velocity, the velocity is zero, so KE could be zero.
Now the baby spits the pacifier forward at .5 m/s.
is KE=1/2 m (.5)^2=.125m J or
is KE=1/2 m (20.5)^2=210.125
in one case the gain of KE was .125m J and the other a gain of 10.125, so what energy did the baby put into it? Well, it may startle you: the baby put .125 m J, and the car put 10J into it .
So Kinetic Energy is a relative quantity, as is motion. KE can be zero if we define motion and KE in a certain system only.
This is a very interesting description of relativity, and how it leads to strange things as one apporaches the speed of light. (At light speed, the baby could not spit out the pacifiers)
http://physics.stackexchange.com/questions/1368/is-kinetic-energy-a-relative-quantity-will-it-make-inconsistent-equations-when
1 answer