Calculating Force in an Accelerating Elevator - Solving for Normal Force

[contlee]

A 70kg person is in an elevator the elevator has an acceleration of 3m/s/s DOWNWARD. What force will the floor exert on the person.

I am confused on the the equation. So far I think (mass)(acceleration)=(mass)(gravity)-(normal force). I know the mass is 70kg or 700N. I also know the acceleration is 3m/s/s. so i started to fill in my equation an i got -210=700-(Normal Force). But after solving I don't see that method resulting in the correct answer. HELP

 

[willem2]

You put a minus sign in front of the acceleration, so your positive direction is up and the
negative direction is down. Gravity is also downwards, so it should get a minus sign as well.
The normal force is upwards, so it should not get a minus sign.

 

[contlee]

i am still confused, So i worked it out the way you told me, but since i am looking for the amount of force the floor is exerting on the person should i find the normal force. or should it just be Mass x Gravity

Here is my work
(Mass)(Acceleration)=Normal Force-(Mass)(Gravity)
-210=Normal Force -700
Normal Force= 490

 

[willem2]

The amount of force the floor is exerting IS the normal force, so your answer is
correct.

 

[contlee]

thanks for the help. really appreciate it