The heavier car would because it has more KE. but what if you sped the lighter car up to match the KE of the heavier car , say to 20 mph ( I'm guessing) then which is harder to stop?
In the bullet situation, because velocities are different but the energy is the same I'm starting to think they would penetrate to equal distance.
The one harder to stop it the one with the highest momentum.
Kinetic energy is one half mass times velocity squared .5(m*V^2)
Momentum is mass times velocity m*v
I will convert and use SI units...
5000lb=2267.96kg
10mph=4.47m/s
Kinetic energy is 22.65794kJ (I feel the extra significant digits are important due to the conversion of joules to kilojoules)
So we need to match that with the lighter car.
2000lb=907.18kg
22.65794kJ=22657.94J
Lets work backwards...
22657.94*2=45315.88
45315.88/907.18=49.95
square root of 49.95=7.07m/s
7.07m/s=15.81mph
5000lb car at 10mph has the same energy as a 2000lb car moving 15.81mph
Now lets look at the Momentum (we will ignore the vector quantity aspect, as it is unimportant to our purposes at the moment, we only need the magnitude, not the vector.)
momentum of the 5000lb car
2267.96kg*4.47m/s= 10137.78 kg per m/s
momentum of the 2000lb car
907.18kg*7.07m/s=6413.76 kg per m/s
The 5000lb car has a much larger momentum, and therefore a much larger force will be needed to stop it.
A you can see from the calculations... while energy is the same, momentum is not... To equal momentum you will need much more velocity. Of course energy will also be higher...
The 2000lb car would need to be moving 11.18m/s or 25mph to equal the momentum of the 5000lb car. More than double the speed...
Since force is quantified as acceleration or deceleration... it is has a function of time. Example- 5m/s per second
And since Momentum is a vector quantity, time means movement in the direction of the vector. So given a constant decelerating force, the object with the highest momentum will travel farther before stopping.
This is why heavy for caliber bullets penetrate farther than lighter ones, even ones with higher kinetic energy.
It is also why in hollowpoints that they are more consistent. As every shot fired is slightly different, and forces can be different... more momentum helps ensure better more consistent penetration.
Sectional density comes into play when comparing different calibers. As well as frontal surface area and drag characteristics... Fontal area is larger on higher calibers, and that affects the drag and therefore the deceleration force acting on it.
Any friction between a longer heavier bullet and a shorter lighter one in a given caliber will be minor overall. Given solid non expanding projectiles that is.
Sectional density, frontal surface area and drag, also comes into play in expanding bullets because they change as the round expands.
As a round expands, the decelerating force acting upon it also increases.
This all combines into a very complex set of variables that determine final penetration.