Why Does the Elevator's Acceleration Calculation Keep Giving the Wrong Answer?

[um_alim]

I've been trying this question over and over, getting the same exact answer (0.270 m/s^2), however, it doesn't seem to be the right answer.

Can someone please help?

A man stands on a scale in an elevator that is accelerating upward. The scale reads 795.5 N. When he picks up a 27.0 kg box, the scale reads 1067.4 N. The man's mass is 79.0 kg. What is the acceleration of the elevator?

 

[deltabourne]

Originally posted by um_alim
I've been trying this question over and over, getting the same exact answer (0.270 m/s^2), however, it doesn't seem to be the right answer.

Can someone please help?

A man stands on a scale in an elevator that is accelerating upward. The scale reads 795.5 N. When he picks up a 27.0 kg box, the scale reads 1067.4 N. The man's mass is 79.0 kg. What is the acceleration of the elevator?

Net_F=Fn-m*g=m*a
we know the scale reads 795.5, so that is the normal force.. and his weight is 79.0*9.81 = 774.99 kg*m/s^2

so we get 795.5-774.99 = 79.0kg*a
or a = 0.2596202531645569620253164556962 = 0.260 m/s^2

if you do it with the box, you end up with the same answer.. maybe you had a math error?

 

[um_alim]

Oh, also, here's my work for the problem:

We know that the man, man and the box, situations are all accelerating at the same acceleration. So, we only need to find the acceleration of one situation. (I did the one with the box, but both give relatively the same answer +/- 0.001 m/s^2)

Apparent weight = mass x gravity + mass x acceleration

- solve for acceleration

795.5 N = 79.0 kg x 9.8 m/s^2 + 79.0 kg x (acceleration)
795.5 N = 774.2 N + 79.0 kg x (acceleration)
795.5 N - 774.2 N = 79.0 kg x (acceleration)
21.3 N = 79.0 kg x (acceleration)

[divide by 79.0 kg]

acceleration = 0.2696202532

Which should be the answer, right? But the computer doesn't accept that answer.

 

[um_alim]

That's right! 0.260! Thanks!

 

[deltabourne]

edit: :)

it seems as if it wants you to use 9.81, not 9.8

 

[um_alim]

Yeah, I can't believe it! The textbook itself uses 9.8 m/s^2, yet it won't except answers using those numbers.