Confused about projectile motion regarding acceleration due to gravity

Question

the acceleration due to garvity is 9.8m/s^2. but im confused in its signs when computing for horizontal and vertical motion especially when an obejct is thrown upward and downward.... when will i use (-9.8m/s^2) and ( 9.8m/s^2)... or shall i use constant (-9.8m/s^2)?... help

Answer 1

If you're considering the upward direction as positive and downward direction negative, then it's -9.8 m/s^(2). I remember doing this recently and I always used the negative value of gravitational acceleration when using the equations for projectile motion.

Answer 2

Generally any direction down or left is considered negative.  When you're dealing with up or down motion, it's best to use down as negative, so you can certainly write gravity as -9.8 m/s^2.

Answer 3

It's up to you to define which directions to use as positive and negative. Typically, down is taken to be negative, so we use -9.8 m/s^2. Just make sure that you are consistent with your signs.

Answer 4

That depends on which direction on the vertical axis you want to be positive direction. The acceleration a is always downward in direction and has as magnitude |a| = g = +9.8 m/s^2

If you choose the upward direction to be positive, then acceleration a will be negative, a = -g = -9.8 m/s^2 (g = 9.8 m/s^2). The vertical component of velocity will be + for upward movement and - for downward movement.

If you choose the downward direction to be positive, then acceleration a will be positive, a = g = 9.8 m/s^2 (g = 9.8 m/s^2). The vertical component of velocity will be - for upward movement and + for downward movement.

In most cases we choose the positive direction upward and let a = -9.8, but we can do the other way round, especially when there is no up-moving phase.