- #1

potatogirl

- 8

- 1

- Homework Statement
- I do know that it depends on the axis/reference frame chosen. In my class, we always use up as positive and down as negative. However, I am still confused. I know an object in freefall will accelerate at -9.8 m/s^2 with this axis reference...but sometimes in our homework problems, the only way to get a correct answer is for g to be positive.

See the following homework problem:

A rocket of mass 4.50 × 10^5 kg is in flight. Its thrust is directed at an angle of 55.0° above the horizontal and has a magnitude of 7.50 X 10^6 N. Find the magnitude and direction of the rocket's acceleration. Give the direction as an angle above the horizontal.

- Relevant Equations
- F=ma

I know to break it down into its x and y components and then use Pythagorean:

Acceleration in the x direction is Fx/m ---> (7.50 x 10^6*cos55) / (4.50 x 10^5 kg) = 9.56 m/s^2

Acceleration in the y direction is: (Fy - mg)/m ---> ((7.50 x 10^6*sin55) - (4.5 x 10^5* 9.8 m/s^2)) / (4.5 x 10^5 kg)

Then I can take it from there...but this problem (like many others) hinges on g being positive 9.8m/s^2 and not negative. In the above equation we have it as positive (I saw a solution key online that shows it like that). When I use g as negative I get the wrong answer. So I am just trying to get the concept down so I can keep my neg/positive signs correct for future problems.

I feel like this is a stupid question but I am just not getting it unfortunately. Any help in understanding is appreciated! Is it because the rocket is moving upwards and not downwards? Do we only use -9.8 m/s^2 in freefall??

Thank you!

Acceleration in the x direction is Fx/m ---> (7.50 x 10^6*cos55) / (4.50 x 10^5 kg) = 9.56 m/s^2

Acceleration in the y direction is: (Fy - mg)/m ---> ((7.50 x 10^6*sin55) - (4.5 x 10^5* 9.8 m/s^2)) / (4.5 x 10^5 kg)

Then I can take it from there...but this problem (like many others) hinges on g being positive 9.8m/s^2 and not negative. In the above equation we have it as positive (I saw a solution key online that shows it like that). When I use g as negative I get the wrong answer. So I am just trying to get the concept down so I can keep my neg/positive signs correct for future problems.

I feel like this is a stupid question but I am just not getting it unfortunately. Any help in understanding is appreciated! Is it because the rocket is moving upwards and not downwards? Do we only use -9.8 m/s^2 in freefall??

Thank you!