# Homework Help: Scale in an elevator physics problem

1. Sep 14, 2007

### conniechiwa

A 62-kg girl weighs herself by standing on a scale in an elevator. What does the scale read when the elevator is ascending at 11 m/s but its speed is decreasing by 5 m/s in each second?

I'm not really sure where to begin.

2. Sep 14, 2007

### learningphysics

Hint, you need the force exerted by the girl on the scale... that's the weight the scale reads...

You also know that the force the girl exerts on the scale = the force the scale exerts on the girl.

3. Sep 14, 2007

### conniechiwa

Well I know that normally the scale would read 608.5 N if the elevator was not moving, but I'm not sure what to do here.

4. Sep 14, 2007

### l46kok

First, find what the deceleration of the elevator is.

5. Sep 14, 2007

### learningphysics

draw the freebody diagram of the girl... what are the forces acting on the girl... you know that $$\Sigma\vec{F} = ma$$. follow l46kok's suggestion and find the acceleration of the girl.

6. Sep 15, 2007

### conniechiwa

Would the deceleration be -3.45 m/s squared?

7. Sep 15, 2007

### learningphysics

No, the deceleration is the same given in the question...

8. Sep 15, 2007

### conniechiwa

only the velocity is given?

9. Sep 16, 2007

### learningphysics

They give 5m/s per second... so the acceleration is -5m/s^2

10. Sep 19, 2007

iidesjo!

okay,

Fg=m*g (Fg is gravitational force)
Girl weighs (m) 62 [kg].
Gravitational constant (g) is 9.81 [m/s^2]
Fg= 62*9.81= 608.22 [N]
that is the static value.

With F=m*a you can calculate the difference in force compared to static (v=c)
a= -5 [m/s^2]
F= 62*-5= -310 [N]

so the scale will indicate 608.22-310= 298.22 [N]
to calculate back to mass divide through the graviotational constant.
298.22/9.81= 30.4 [Kg]
you're welcome!
:D