# Homework Help: Newton's law-need quick help.

1. Oct 8, 2009

### stanton

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

A 3kg object hangs at one end of a rope that is attached to a the ceiling of a railroad car. When the car accelerates to the right, the rope makes an angle of 4 degrees with the vertical. Find the acceleration of the car.
the answer is 0.69m/s^2

2. Relevant equations

sigma F=ma, tension*cos(theta) = mg, Fnet - ma = 0
yup. looks good to me. but I don't what values to plug in.

3. The attempt at a solution

First I draw free body diagram. gravity acts on the string. And I thought I could make some equation like Fsin(delta) like sort of thing by drawing triangle with 4 degree and...oh, I got all messed up.
I tried to use an equation :tension*cos(theta) = mg
plug value: tension*cos(4)=3.0x9.81 I think I can figure out what the tension is. However,
I do not know how to get the acceleration.
I would be very appreciated if you give me specific procedure how the answer came out by giving information about what equation was used and so forth. Thanks!
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Oct 8, 2009

### Donaldos

The first equation you found is correct:

$$T \cos \theta =mg$$

Using Newton's second law you should also be able to show that:

$$ma=T sin \theta$$

Combine the two equations to find $$a$$.

3. Oct 9, 2009

### stanton

My, I didn't know this can be solved so easily. I remember my teacher said about it, but I lost my note. When I followed your advice and used the equation i am given, I got my result 0.685! Thank you so much. Have a nice day =)