How Do You Calculate the Tension in a String Lifting an Accelerating Mass?

[iamtrojan3]

Homework Statement

A 1.65 kg mass is suspended from a string which is pulled upward. The mass accelerates upwards with an acceleration of 3.10 m/s^2. What is the tension in the string?

Homework Equations

f = ma

The Attempt at a Solution

seems really easy but i can't get the right answer

the regular tension would be 1.65kg * 9.8m/s^2 = 16.17N

if its pulled up 3.1 m/s^2, the forces in that would be 1.65kg * 3.1m/s^2 = 11.05N

then i subtracted the forces to get 5.1, but its wrong.

that wasn't right so i added the forces, still wrong.

then i did the acceleration as a whole, as in 9.8 + 3.1 = 12.9
then found the forces which is 21.3N... still wrong

anyideas? thanks

 

[womfalcs3]

Draw a free body diagram, and solve the problem as you see the forces there (gravity, upward acceleration included). The acceleration would be modified.

 

[rock.freak667]

seems really easy but i can't get the right answer

the regular tension would be 1.65kg * 9.8m/s^2 = 16.17N

yes regular if it was in equilibrium, so just know that is the weight of the mass.

if its pulled up 3.1 m/s^2, the forces in that would be 1.65kg * 3.1m/s^2 = 11.05N

3.1 m/s² is the resultant acceleration, multiplying it by the 1.65kg gives the resultant force.



In your free body diagram, the tension acts upwards while the weight acts downwards. Can you make a relationship between the tension,weight and resultant force? If you can do that, then the question become quite easy.