Homework Help: Newton-law help

1. Jul 8, 2007

sweet-buds

1. The problem statement, all variables and given/known data
A vertical force F is applied to a block of mass m that lies on a floor. what happens to the magnitude of the normal force N on the block from the floor as magnitude F is increased from zero if force F is a)downward b)upward

2. Relevant equations
Let m be the mass of body. ideally if the body were to be at rest , and considering only vertical motion, N(reaction force on body from surface)=mg
1) if force F is downward, F+mg> N. The N would adjust in a way such by increasing it s value so that it can counter F+mg force else the block will break through the surface

2)the N would decrease or else the block would raise above the surface.

3. The attempt at a solution

a)downward= N increases
b)upward=N decreases

2. Jul 8, 2007

Mindscrape

Yes, you are correct. Intuition plays a good role in this case too because if you push down on a block harder, then the floor is going to have to push back harder for the forces to cancel and the block to stay still.

3. Jul 8, 2007

Dick

Yes, absolutely. But don't even worry about the case where the block has a velocity in the vertical direction. That's not a situation where you would even think much about static normal forces.