# Increasing Mass VS Increasing Force when pushing a Box [closed]

If we assume g=10m/s^2 & negligible friction

The force needed to accelerate that box by 5m/s^2 would be 500N. This is a very basic one.

The confusion came when an force of 250N (the force a 25kg mass/box would experience in a gravitational field) was exerted on the box, I then tought that the 250N would be cancelled out by the Normal force on the surface, thus the only mass would be the 75kg box and the required force to accelerate the box by 5m/s^2 is 375N.

My question is (for the 2nd diagram), should you calculate the force needed to 'push' the box as if there was a 25kg box resting above it or not? My intuition suggests that previous 375N would be the correct answer.

## closed as off-topic by Gert, Jon Custer, stafusa, Cosmas Zachos, PhononMay 30 at 23:30

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Yes, you should calculate it as if there's a $$25kg$$ box on box A.
This is because all that box A experiences is a $$250N$$ force downwards. What actually causes that force is of no concern (as in Newton's laws, $$F=ma$$ tells us the acceleration of a body corresponding to the force experienced by it. What causes that force $$F$$ is of no concern.)