Question

"An ice skater applies a horizontal force to a
20.-kilogram block on frictionless, level ice,
causing the block to accelerate uniformly at
1.4 meters per second2 to the right. After the
skater stops pushing the block, it slides onto a
region of ice that is covered with a thin layer
of sand. The coefficient of kinetic friction
between the block and the sand-covered ice is
0.28.

Determine the magnitude of the normal force
acting on the block.

Answer 1

     Using the Newton's Secound Law, which comes:

$F_{at}=F_{r} \\ Nu=ma \\ N= \frac{20*1.4}{0.28} \\ \boxed {N=100N}$

If you notice any mistake in my english, please let me know, because i am not native.