Is there a time when you wouldn't use -9.8?

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Aletheia

Full Member
It's been way too long since basic phys... In terms of the acceleration due to gravity, would there ever be a situation when the acceleration due to gravity is somehow positive?

Thanks.

ksmi117

GEAUX TIGERS!!!
Moderator Emeritus
10+ Year Member
Well, you shouldn't but sometimes it just makes the calculating easier. Like if you had a question that asked how long will it take for a ball to fall 5 m. You can think of it as +5 m and +10 m/s^2 or -5 m and -10 m/s^2. It all depends on your perspective.

But to make things easier, try and always use -10 m/s^2 and make sure your distances and velocities correlate with up being positive and down being negative.

Also, on the MCAT use 10 instead of 9.8. You can't have a calculator and it makes calculations a whole lot easier if you round some.

PiBond

Call me Bond...PiBond
10+ Year Member
it does depend on your perspective but you can always ask yourself "is the acceleration due to gravity causing my speed to increase or decrease?"

For example, you throw an apple straight up into the air. Acceleration and velocity are pointing in different directions--your velocity is going straight up while the force of gravity is pointing towards the center of the earth. If you chose to make your velocity positive, then by convention your acceleration must be negative.

kentavr

Full Member
10+ Year Member
The acceleration is a vector. So, its sign depend on the coordinate system that you choose for particular problem. Set Z-axes increasing from earth and acceleration will be negative. Set in reverse, then the sign will be positive. Your choice.

DendWrite

Full Member
10+ Year Member
As far as acceleration due to gravity, no, for any MCAT problem it's going to be a constant. Just use 10 and then pick the answer that is smaller but closest to your calculated value.

PiBond

Call me Bond...PiBond
10+ Year Member
it'll only change if we were to change planets that have different masses, radii, etc. this would result in a different acceleration due to gravity constant