Calculate Tension in String for 1.49 kg Mass with 3.1 m/s2 Acceleration

[Amel]

Homework Statement

A 1.49 kg mass is suspended from a string which is pulled upward. The mass accelerates upwards with an acceleration of 3.10 m/s². What is the tension in the string?

Homework Equations

F_net= T + F_g =ma

so

T = m(a + g)

The Attempt at a Solution

Ok so I pluged in the numbers

T = (1.49kg)(3.1m/s²+(9.89m/s²)

and I got 19.221 kN but its wrong.

The title for this is wrong I was going to ask another question originally.

 

[CFDFEAGURU]

3.1 m/s^2 is accelerating the mass upwards. Ask yourself, what direction is the 9.81 m/s^2 acting in? Draw a free body diagram of the forces acting on the mass.

Thanks
Matt

 

[Amel]

I know gravity is downwards but there is a problem in the book just like this and this is how they get to that equation.

T + F_g = ma

so since F_g is negative

T - mg =ma

T = ma+mg

T = m(a+g)

I know that gravity was negative but that's why they added mg over to the other side.

 

[CFDFEAGURU]

Yes, sorry I looked right past that. Not sure where you went wrong then. Are you close to the correct answer?

Thanks
Matt

 

[Amel]

It just says 19.221 kN is wrong it doesen't tell me anything else. Are my units right? I don't get why I am getting this wrong.

 

[CFDFEAGURU]

The units are correct. N = kg*m/s^2.

Not sure why it isn't right. Maybe someone can help us both out. LOL

Thanks
Matt

 

[Amel]

I was getting it wrong because I was putting kN instead of N. I missed that the book converted their answer to kN, Thanks I needed to see N = kg*m/s^2 to catch my mistake.

 

[CFDFEAGURU]

LOL geeze I missed it too. Whoops.

Thanks
Matt