Density is the best property to use, as while multiple different metals could create cubes with the same color, mass, or volume, no different metal could create a cube with the same mass and volume. Density is based on mass and volume, and as a result no two different metals will have the same density.
Explanation:
Joule (J) is the MKS unit of energy, equal to the force of one Newton acting through one meter.
She knows the speed limit in the area, and also saw the speed you were going on the speedometer. The speed you were going was faster than the limit allowed, so that's how she knew you were going too fast.
