Finding mass given force and acceleration

[MMCS]

a lift ascends with a uniform acceleration of 2m/s^2 whilst transporting a woman. Calculate her mass if the force exerted on her from the floor was 945N.

ANS: 80KG

I have tried F = MA

M = F/A = 945/2 = 472.5

Inertia may have been taken into account but is this possible without being given the mass?

 

[Delphi51]

I hate elevator questions; very tricky!
You have the upward force of 945 N on the lady.
But you forgot gravity also has a grip on her.
Include that, and you will get the 80 kg!

 

[MMCS]

I hate them too! how do i include gravity in there? It would usually be the product of her mass and 9.81 but I am not given her mass

 

[Delphi51]

Write it as mg. You'll have two m's in your F = ma equation but it is not a big problem to collect them as like terms in a linear equation. This is an important idea: write it out even if it strikes you as unsolvable - once it appears in front of you in a mathematical form, a whole different thought process in your mind takes over and often the difficulties disappear.

 

[MMCS]

945 - mg - ma = 0
g + a x m = 945
9.81 + 2 x m = 945
m = 945/11.81 = 80kg
Thanks!