# Two objects acceleration problem

1. Oct 7, 2015

### MightyDogg

1. The problem statement, all variables and given/known data
A baby bird falls from a nest 18m high. After the bird falls 2 meters its mother realizes and catches it 1.5m off the ground. How fast must its mother accelerated down in order to catch its baby?

2. Relevant equations
$$x= \frac{1}{2} a t^2$$

3. The attempt at a solution
I calculated the time it took the baby bird to fall 2m. It was .638 s. Then I calculated the total it takes for the baby bird to fall 16.5m (1.5m off the ground) and it was 1.835 s. Then I subtracted the two to get the time the mother has to reach its the baby bird. That time was 1.19s. Then I plug it back into the equation with 'x' as 16.5m. I calculate acceleration to equal -23m/s^2.

Is that correct? I think it is, I just want to double check. Any help would be appreciated. Thanks.

2. Oct 7, 2015

### TSny

Your answer looks correct. I guess the negative sign indicates that the acceleration is downward. It could be that the problem is asking only for the magnitude of the acceleration in which case you could drop the sign. Hard to tell.

3. Oct 8, 2015

### MightyDogg

I forgot to mention when I put 16.5m into the last equation I made it negative because it was going downward, thus my acceleration turned out negative.

As long as the magnitude is correct then I'm ok with the problem. Thank you for double checking my work.

4. Oct 8, 2015

### Kilgour22

As you get into more complicated questions, make sure to draw your coordinate system indicating which directions are positive! This can save you lots of heartache in the future, especially with problems relating to force.