Newton's second law with vectors

[Shadow236]

Homework Statement

A 9 kg object is subjected to two forces, [PLAIN]http://www.webassign.net/images/Farrowbolditalic.gif1 = 7 N i - 4 N j and [PLAIN]http://www.webassign.net/images/Farrowbolditalic.gif2 = 7 N i - 9 N j. The object is at rest at the origin at time t = 0.

(a) What is the object's acceleration?

(b) What is its velocity at time t = 14 s?

(c) Where is the object at time t = 14 s?

(All answers in unit vector notation)

Homework Equations

(a) F_net=ma

(b) V = V₀ + at

(c) X = X₀ + V₀t + 1/2at²

The Attempt at a Solution

(a) F_net = F1 + F2 = 14i - 13j

F_net = ma -> a = (14i-13j)/9
a = 1.56i - 1.44j (MARKED INCORRECT)

Why is this incorrect? If a = F/m then all I should have to do is add up the forces to get F_net.

(b and c) I've used the equations above and got the answers wrong. Overall, I won't be able to get these answers correct unless I get the correct acceleration.

 

[LCKurtz]

Homework Statement

A 9 kg object is subjected to two forces, [PLAIN]http://www.webassign.net/images/Farrowbolditalic.gif1 = 7 N i - 4 N j and [PLAIN]http://www.webassign.net/images/Farrowbolditalic.gif2 = 7 N i - 9 N j. The object is at rest at the origin at time t = 0.

(a) What is the object's acceleration?

(b) What is its velocity at time t = 14 s?

(c) Where is the object at time t = 14 s?

(All answers in unit vector notation)

Homework Equations

(a) F_net=ma

(b) V = V₀ + at

(c) X = X₀ + V₀t + 1/2at²

The Attempt at a Solution

(a) F_net = F1 + F2 = 14i - 13j

F_net = ma -> a = (14i-13j)/9
a = 1.56i - 1.44j (MARKED INCORRECT)

Why is this incorrect?

Probably because ##1.56\ne14/9## and ##-1.44\ne -13/9##. They are just decimal approximations to the fractions, which are the correct answers.

 

[Shadow236]

So you think that I was correct to add F1 and F2? If that is true I should just leave F_net as 14/9i - 13/9j?

 

[LCKurtz]

Yes. You could always ask your teacher, or if it's an online problem, try typing in the fractions. In that case be sure to use parentheses as in (14/9)i

 

[Shadow236]

Alright, I will try that. Thank you very much. Are my equations for (b) and (c) okay? I just want to check because I've already submitted this question a few times.

EDIT: Hey, I got it right! =D Fractions for life, haha. Thanks a lot!

 

[vela]

Those equations (b and c) are valid only if you have constant acceleration. Do you have constant acceleration? If so, you can use them.

 

[Shadow236]

Yes, I had constant acceleration. Thank you.