What is the magnitude of the average frictional force on the block?

Question

A $2.0 \space kg$ block, initially moving at $10.0 \space m/s$, slides $50.0 \space m$ across a sheet of ice before coming to rest. What is the magnitude of the average frictional force on the block?

I know the answer is 2.0 N.

Answer 1

$F_f^2 = F_0^2 + 2a(\Delta x)$

$F_f = 0 m/s$

$F_0 = 10 m/s$

$\Delta x = 50 m$

$a = -1 m/s$

$F = ma = 2 * -1 = -2N$ (negative because opposite to direction of motion)