Question

While standing in a low tunnel, you raise your arms and push against the ceiling with a force of 200 n . your mass is 80 kg.what force does the floor exert on you?

Answer 1

According to Newton`s third law, when you push against the ceiling with a force of 200 N, ceiling will also apply 200 N  normal force downward on you.

As your weight is also downward force.

Therefore, when you raise your arms and push against the ceiling with a force of 200 N, the total downward force is equal to the sum of these two forces.

That is

$F_{total} =weight \ of\ you + normal\ force\ downward\ on\ you\\\\ F_{total} = 80\kg\times 9.8\ m/s^2 + 200\ N=984\ N$

Thus, from Newton`s third law, 984 N is upward force exert on you by floor.