# Laws of Motion, vertical acceleration question.

"In a lift accelerated upwards at a certain rate, a spring balance indicates a weight to have a mass of 10kg. When the lift is accelerated downwards at twice the upward rate, the mass appears to be 7kg. Find the actual mass and the upward acceleration of the lift."

In short, im stuck. I think this question involves simultaneus equations, but i cant get a set of equations that make sense. Ive looked at the textbooks I have but they all concern themselves with the resistance forces or tensions in the strings.

try to put in pseudo forces on the block

OlderDan
Homework Helper
Bucky said:
"In a lift accelerated upwards at a certain rate, a spring balance indicates a weight to have a mass of 10kg. When the lift is accelerated downwards at twice the upward rate, the mass appears to be 7kg. Find the actual mass and the upward acceleration of the lift."

In short, im stuck. I think this question involves simultaneus equations, but i cant get a set of equations that make sense. Ive looked at the textbooks I have but they all concern themselves with the resistance forces or tensions in the strings.

A spring balance really measures force, not mass. It is marked with mass units assuming you will be using it in an inertial (non-accelerating) frame of reference. When it reads 10kg that really means the force it is measuring is 10kg(g) and when it reads 7kg, that means the force is 7kg(g)

Call the upward acceleration a and the downward acceleration -2a, taking positive upward in all cases.

F = ma = Force of spring balance (up) acting on m - weight
F = -2ma = Force of spring balance (down) acting on m - weight

Replace the words with the correct quantities and you are on your way.

thanks for the help but im still not sure what im doing here...

f=ma
f=(10)a
f=10a (1)

f=ma
f=(7)(-2a)
f=-14a (2)

that seems wrong.

HallsofIvy