Acceleration upwards and the effect of 'g'

[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?

 

[SteamKing]

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.

 

[HallsofIvy]

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?

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!

 

[Xenoned]

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

 

[jtbell]

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?

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.