Reason for different type of energy transfer for two kinds of collisions According to my physics book, if an electron were accelerated with 15 MeV of (kinetic?) energy and collided into a 100g thermally insulated copper block (not sure if the fact it is thermally insulated is relevant) the energy would transfer into thermal energy on the copper block and the copper block would heat up by a few degrees.
However, if I were to collide into that same copper block with a bowling ball travelling with 15 MeV of kinetic energy then that energy would be transferred into kinetic energy on the copper block and the copper block would move.
It seems to me, that in both cases, there is a thing smashing into another thing with 15 MeV of energy.  When the first thing is small (e.g. electron) the energy is transferred into thermal energy.  When the first thing is large the energy is transferred into kinetic energy.
Why doesn't the electron's energy get transferred into kinetic energy as well?
 A: Momentum is transferred in both cases (conservation of momentum is absolute), but how much?
Figure the momentum of the two projectiles.


*

*The electron is fully relativistic, so you get
$$p^2 = E^2 - m^2 = \left(15.0^2 - 0.5^2\right)\,\mathrm{MeV}^2$$
or $p \approx 1.5\times 10^{7}\,\mathrm{eV}$ (in $c = 1$ units, of course).

*The bowling ball is fully classical, so you get
$$p^2 = 2mE$$
with a mass in the $10^{36}\,\mathrm{eV}$ range. So $p \approx 6 \times 10^{21}\,\mathrm{eV}$.
The bowling ball transfers roughly 14 orders of magnitude more momentum than the electron!
The energy from the electron ending in thermal modes because there aren't any other places for it to go.

Figuring it again in mks units is left as an exercise for the interested student.
A: So I will take a stab from a more "big picture" point of view. 
When an electron hits the block of copper, it's mass (and physical size) is considerably less than the size of the copper block. So at impact, it's really hitting what could be thought of as a single atom in the block. 
Now, temperature is the measure of kinetic energy of molecules/atoms. So when all of the kinetic energy from the electron gets converted (let's say no losses) into kinetic/vibrational energy of the atom it hit, that atom then vibrates in the lattice and this vibration distributes the energy throughout the block. All of the atoms begin to vibrate more and this is measured as a temperature increase in the block. 
An example of what this looks like (this is a copper atom hitting a copper block)[from Wikipedia]:

As you can see, the motion of the atoms is not coherent but is random as they bounce around. 
When a bowling ball hits the block, it's of the same/bigger scale than the block so it's not transferring energy to the kinetic modes of the atoms. Rather, it's transferring momentum to the entire block and not just to the random vibrations/translations of the atoms itself. So this will cause the kinetic energy of the block to increase, rather than internal energy. 
TL;DR: Electron is tiny and hits the atom, converting kinetic energy into random motion/vibration of the atoms in the lattice. Temperature is defined as the average kinetic energy of the atoms. Bowling ball is huge and provides a coherent translation, rather than a random one, to the block. This is kinetic energy of the block rather than internal.
