1. Dec 3, 2012

### Coco12

An elevator weighs 3.0•10^3N. It accelerates up at 1.0m/s^2. What force does the cable exert to give it this acceleration?
I know I have to find the mass first but for the acceleration, would it be gravity plus acceleration given : 9.80+1.0 ?
f=ma
Also for this question:
A 20kg child steps of a skateboard(3kg)with an acceleration of .50m/s^2, what acceleration will the skateboard travel in the opposite direction.
I know u have to find the force delivered by the child first but would u use the mass of only the child or the child and the board(23kg)

2. Dec 3, 2012

### K^2

Yes.

Force applied to child equals to negative of force applied to skateboard. Force applied to child is mass of child times acceleration of child, same as any other system. Skateboard doesn't factor into it at all at this point. Until you are actually looking for acceleration of the skateboard, it doesn't matter if the child pushes off from skateboard, a wagon, or the entire planet.

3. Dec 3, 2012

### Coco12

How do u know when to add gravity onto the equation?