# Homework Help: Laws of motion/vectors question?

1. Sep 4, 2010

### ooohlala

1. The problem statement, all variables and given/known data

A 10 kg block is subjected to two forces (10i + 20j) N and (6i - 8j) N. The magnitude of the resultant acceleration is

(a) 2 (b) 3 (c) 4 (c) 5

2. Relevant equations

F=ma

3. The attempt at a solution

I was thinking maybe I should add the horizontal components to get the force acting horizontally, and then substitute that and the mass into F=ma to get 'a', but that way I get 1.6 m/s^2 which isn't even an option. Someone help, please?

2. Sep 4, 2010

ehild

3. Sep 4, 2010

### ooohlala

Yes, I don't know how I'm supposed to take them into account. I thought there's no need to. :S Tell me how to start?

4. Sep 4, 2010

### ehild

You have learnt vector addition. The resultant of two vectors added is a new vector. All vectors have components. How do you get the components of the resultant vector?

ehild

5. Sep 4, 2010

### ooohlala

By adding 10 to 6 and 20 to -8 to get (16i + 12j)?

6. Sep 4, 2010

### ehild

Yes. Now you have F=ma. The acceleration is vector, and it points at the same direction as F. Therefore its magnitude is proportional to the magnitude of the force. You have to find the magnitude or the force first. How do you get the length (magnitude) of a vector?

ehild

7. Sep 4, 2010

### ooohlala

Haha thank you, I got the answer. :)

8. Sep 4, 2010

### ehild

Congratulation!

ehild