Homework Help: Newton's Law Problems

1. Jul 8, 2009

Smile101

1. The problem statement, all variables and given/known data
A block of mass m1=2.0 kg on a frictionless inclined plane of angle 20 degrees is connected by a rope over a pulley to another block of mass m2=1.0kg. What are the magnitude and direction of the acceleration of the second block?

2. Relevant equations
a=m2g-m1gsin(angle)/ma1+m2

3. The attempt at a solution

how would i calculate m1g and m2g in the first place?
other then that...

a=m2g-m1gsin20/2+1

Your help will beeee very appreciated seeing as I have a test on it tom!!!

2. Jul 8, 2009

rock.freak667

g is acceleration due to gravity = 9.81 m/s2

3. Jul 8, 2009

Smile101

so basically i would multiply mass with 9.81
m2 = 1(9.81) = 9.81
m1= 2(9.81) = 19.62

a= -19.62-(-9.81)sin20/3
a= 3.3m/s^2 (down)

but the answer is supposed to be -1.03 m/s^2 (down)

4. Jul 8, 2009

rock.freak667

Well for one, it seems that you switched around m1g and m2g. It should be

$$a=\frac{9.81-(19.62)sin20}{3}$$

Also, do you know how to get that formula?

$$a=\frac{m_2g-m_1gsin\theta}{m_1+m_2}$$

5. Jul 8, 2009

Smile101

No i don't! But the modified version you gave me is right! I got 1.03! :) thank you soo much

6. Jul 8, 2009

rock.freak667

If the resultant force of m2 is downwards, then for the system vertically. (T=Tension)

m2a=m2g-T

therefore on the incline, m1 moves up, so that

m1a=T-m1gsin$\theta$

two equations where you can eliminate T by adding them. That is how to derive that formula. It's better to know how to do these kinds of of problems from first principles than to memorize a formula