Negative 9.8 or not !? (1 Viewer)

[exa_boi87]

Practicing projectile motion questions something chronic thismorning, but stuck on a trend ive noticed but cannot decipher. In some questions, "-9.8" is used as the value of "g" where as others "+9.8" is used, regardless of what polarity of 9.8 is specified in the question. My question is, is there a specific time I should be using positive and/or negative?

From what I gather, if the motion is from a horizontal downward, the value is positive, if it is before the midpoint of a parabola, it is negative. Is this true or am i missing something? .. thanks

 

[Antwan23q]

To my knowledge it is always negative 9.8, but can you post up some of these questions where its positive?

 

[serge]

yep, negative all the time

 

[Jago]

it depends on what direction you take as positive. If you take "up" as positive, then g = -9.8 but if you take "down" as positive, then g = 9.8

 

[exa_boi87]

Ok, heres an example of what I mean (thx for replies by the way) ..

The excel book uses two different polarities for 9.8 in two questions:

"A rock is thrown horizontally out to sea from the top of a vertical cliff face with an initial velocity of 20 m.s. It is seen to reach the water after 3.0s. Find: (a) The height of the cliff"

They substitute the value "9.8" for a[g] in their formula:

(change in)y = u[y]t + 0.5a[g]t^2

Now in the infamous michael jordan question, they use a negative value for g in the first section, and a positive in the second! (HSC 2004 Q27) .. where the formula

v = u + at substitutes -9.8 as "a" when determining the initial velocity, and for the second part subsititutes positive 9.8 when determining the maximum height!! (solution taken from the markers comments, Phys HSC 04)

 

[acmilan]

Ok, heres an example of what I mean (thx for replies by the way) ..

The excel book uses two different polarities for 9.8 in two questions:

"A rock is thrown horizontally out to sea from the top of a vertical cliff face with an initial velocity of 20 m.s. It is seen to reach the water after 3.0s. Find: (a) The height of the cliff"

They substitute the value "9.8" for a[g] in their formula:

(change in)y = u[y]t + 0.5a[g]t^2

Now in the infamous michael jordan question, they use a negative value for g in the first section, and a positive in the second! (HSC 2004 Q27) .. where the formula

v = u + at substitutes -9.8 as "a" when determining the initial velocity, and for the second part subsititutes positive 9.8 when determining the maximum height!!

They are right in both cases. It all comes down to interpretation. In the first example, the rock never goes upwards, it only ever falls down, thus they take down to be the positive direction. Thus g is also positive, since g goes downwards. However itd work out even if you take g as negative, this would give the height of the cliff as negative, but its still right.

In the second direction, the ball goes upwards first, thus they take up as positive, and thus g as negative.

Its all relative, doesnt really matter which direction you take as positive or negative as long as you are consistent throughout the entire question.

 

[Ghost1788]

couldnt have put it better myself....

 

[exa_boi87]

Ok thanks for that .. so just to confirm the last question (Jordan), as the upward motion was taken as positive, the value of g was negative, as the projectile begain its decent (from the midpoint) g becomes positive as the motion is now downward? ..

Thanks again, its finally all fitting together

 

[Jago]

no, if you initially take upward motion as positive, you have to keep it as positive throughout the whole question.