# G=9.8 or -9.8

1. Jun 25, 2015

### Falley

So,while learning mechanics,I found that most questions use g=-9.8 ,but when calculating the velocity the instant a rocket touches the ground,the solution instead used g=9.8,which is very confusing.Other questions of similar workings used g=-9.8,anybody care to help?

2. Jun 25, 2015

### e.bar.goum

It depends on which direction you define as "up". As long as you are consistent, it doesn't matter which you use.

3. Jun 25, 2015

### Falley

But for the same question,part a),b)c) a n b used -9.8 while c used 9.8

4. Jun 25, 2015

### Staff: Mentor

In every problem we get to choose whether we consider forces, velocities, and accelerations to be positive when they're in the upwards direction and negative when they're in the downward direction, or the other way around. Gravity pulls downward, so it will be negative if you use the "positive up" convention, positive if you use the "positive down" convention.

Either way works just fine - you just have to be consistent throughout the problem.

5. Jun 25, 2015

### Staff: Mentor

Most likely your textbook author was most comfortable thinking about parts a and b one way and part c the other way. Once you get used to it, you'll find yourself picking whichever one feels most natural for a given problem or subproblem without even thinking about it.

Or maybe he did it in purpose, just to keep you on your toes... Recognizing which sign convention is being used and being able to handle either one is an important skill and exercising it might have been the point of this problem.

Or maybe he was just careless. It happens

6. Jun 25, 2015

### CWatters

Post the exact question otherwise we are guessing.

7. Jun 25, 2015

### Falley

Example 3,the rocket

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8. Jun 25, 2015

### Erik Cox

Both are meaningless without units... :)

9. Jun 25, 2015

### Staff: Mentor

That is just a typo. The equation, as written, doesn't have the solution 25.9 s. The solutions for +9.8 m/s^2 are both negative. Only -9.8 m/s^2 gives a positive solution and that soluiton is 25.9 s.

10. Jun 25, 2015

### CWatters

In case it's not obvious... Their approach is far more complicated than necessary. They are working out the impact velocity starting from the point where the rocket motor shuts off on the way up. It's possible to do that but it's much easier to assume it just falls from the max height which you worked out in part (b).

11. Jun 26, 2015

### Falley

Thanks a lot,cleared up a lot of things for me