A point-like mass moves horizontally between two walls on a frictionless surface with initial kinetic energy $E$. With every collision with the walls, the mass loses $1/2$ its kinetic energy to thermal energy. How many collisions with the walls are necessary before the speed of the mass is reduced by a factor of $8$?

$ \textbf{(A)}\ 3\qquad\textbf{(B)}\ 4\qquad\textbf{(C)}\ 6\qquad\textbf{(D)}\ 8\qquad\textbf{(E)}\ 16 $
1. Let's denote the initial kinetic energy of the mass as \( E_0 \). The kinetic energy after the first collision will be \( \frac{1}{2} E_0 \), after the second collision it will be \( \left(\frac{1}{2}\right)^2 E_0 = \frac{1}{4} E_0 \), and so on. After \( n \) collisions, the kinetic energy will be \( \left(\frac{1}{2}\right)^n E_0 \).

2. The kinetic energy is given by \( \text{KE} = \frac{1}{2} mv^2 \). Since the kinetic energy is directly proportional to the square of the speed, if the speed is reduced by a factor of 8, the kinetic energy is reduced by a factor of \( 8^2 = 64 \).

3. We need to find the number of collisions \( n \) such that the kinetic energy is reduced by a factor of 64. This means:
   \[
   \left(\frac{1}{2}\right)^n E_0 = \frac{1}{64} E_0
   \]

4. Simplifying the equation:
   \[
   \left(\frac{1}{2}\right)^n = \frac{1}{64}
   \]

5. Recognizing that \( 64 = 2^6 \), we can rewrite the equation as:
   \[
   \left(\frac{1}{2}\right)^n = \left(\frac{1}{2^6}\right)
   \]

6. Therefore, \( n = 6 \).

Conclusion:
\[
\boxed{6}
\]