# Acceleration upwards and the effect of 'g'

1. Aug 24, 2013

### Xenoned

Hi,

If a statement something like this is given:
"A body is going up with a constant acceleration of 2 m/s^2"

Does it mean that acceleration due to gravity acts on it and we have to subtract 9.8 from 25?

We subtract forces by newton's laws of motion right?

There is a question like this:
"A lamp hangs vertically from a chord in a descending lift. The lift has a deceleration of 5.2 m/s2 before coming to a halt. If the tension in cord is 30 N, find the mass of the lamp."

We answer it by T=m(g+a) ; a= acceleration/deceleration and get the answer.
but here we use g+a .

why not just acceleration?
Why shouldn't we subtract or add g in case of 1D motion?

Last edited: Aug 24, 2013
2. Aug 25, 2013

### SteamKing

Staff Emeritus
Where do you get 25 from?

If a body is accelerating upward at 2 m/s^2, it cannot at the same time be accelerating at 9.8 m/s^2 in the opposite direction.

In the elevator question, you should draw a free body diagram of the lamp and the cord to determine all of the forces acting on the lamp while the elevator is coming to a stop.

3. Aug 25, 2013

### HallsofIvy

Those are two contradictory questions! You ask why we use g at all in the first question then ask why we shouldn't use it in the second question!

4. Sep 1, 2013

### Xenoned

I am sorry. It's not 25 but 2.

5. Sep 1, 2013

### Staff: Mentor

The force of gravity acts on it. The net force on the body produces the given acceleration of 2 m/s2 upwards. The net force is comprised of the gravitational force and the force exerted by whatever mechanism is propelling the body upward.