A head on collision between two cars of equal mass each going 80 m/s or a collision between the same two cars, one stationary and the other going at 160 m/s? My hunch is the collision between the car going 80 m/s and the stationary car. I am curious to know because the total kinetic energy of the system in the head on collision seems to be m*80^2 and the total kinetic energy in the other case seems to be m*160^2
2007-03-03 23:50:09 · 3 answers · asked by z_o_r_r_o 6 in Science & Mathematics ➔ Physics
Oops I mean 0.5 m* 160^2
2007-03-03 23:53:32 · update #1
I guess my question is more about how frame of reference plays a role. In the case where 2 cars approach each other at 80 m/s, relative to one car, the other car is going 160 m/s
2007-03-04 00:10:33 · update #2
Your intuition tells you that these two situations *must* be exactly the same, because the speed at which the two cars are approaching is the same regardless of who measures it - it is always 160 m/s.
So how can your first answerer have got it *so* wrong.
The problem is that while laws like conservation of energy must apply between frames, you cannot assume that all frames will get the same result for, say, energy.
This is easiest to see if you are moving with respect to someone stationary on the ground. In their frame of reference your kinetic energy is 1/2mv^2. In yours (in which you are stationary) it is zero. Both results are perfectly correct, but you clearly cannot compare results directly from different reference frames.
So in this case, take the perspective of the person in car A.
If both cars are going at 80 m/s towards each other relative to the ground (a second frame), they will conclude that car B is moving towards them at 160 m/s. They will also conclude this if they are moving towards car B at 160 m/s relative to the ground or if car B is moving towards them at 160 m/s relative to the ground.
2007-03-04 01:22:36 · answer #1 · answered by Anonymous · 0⤊ 0⤋
the head on would be better because the kenetic energy would be converted into potential energy.
in the other case, with the statoinary car, the energy of the car going 160 m/s would fling the other car away and probably hit something.
2007-03-04 18:31:49 · answer #2 · answered by James 1 · 0⤊ 0⤋
The KE is 6400m joules in the 1st case and 12 800m joules (double)in the 2nd case...therefore the conclusion is easy!
2007-03-04 08:05:30 · answer #3 · answered by physicist 4 · 0⤊ 0⤋