Finding mass from normal forces at top and bottom of Ferris wheel

[takelight2]

Homework Statement

A woman rides on a Ferris wheel of radius 16m that maintains the same speed throughout its motion. To better understand physics, she takes along a digital bathroom scale (with memory) and sits on it. When she gets off the ride, she uploads the scale readings to a computer and creates a graph of scale reading versus time. (Figure 1) Note that the graph has a minimum value of 510N and a maximum value of 666N. The acceleration due to gravity, g=9.80m/s2.

Relevant Equations

What is the woman's mass?

Express your answer in kilograms.

So this is what I've attempted:

666 = m*a1
510 = m*a2

a1= ac + 9.8
a2= ac-9.8

666 = m(ac+9.8)
510 = m(ac-9.8)

666 = m*ac + m*9.8
510 = m*ac - m*9.8

156 = 2m(9.8)

m = 7.9 kg (which seems very wrong haha)

any ideas?? I thought my reasoning was okay, since I considered that at the top of the wheel (where weight is max), both the centripetal acceleration and the gravitational acceleration point downward, and at the bottom (weight is min) they point in opposite directions so they must subtract.. What am I doing wrong here?

 

[BvU]

What am I doing wrong here?

You are making a sign error. The 510 N and 666 N are in the same direction as g

 

[takelight2]

You are making a sign error. The 510 N and 666 N are in the same direction as g

Oh ok. So then i can just change a2= -ac +9.8, making downward positive.

Ok thanks a lot.