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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Physics => Topic started by: kenhung123 on November 25, 2009, 11:54:37 pm

Title: Gravity acceleration
Post by: kenhung123 on November 25, 2009, 11:54:37 pm: Since gravity acts downwards and an object is falling downwards its acceleration is 9.8ms-2? Why do some people say its -9.8ms-2 because its going down?
Like I am thinking since the gravity acts down and the object complements that direction (goes down) it means its positive and if it goes up (counter acting gravity) its -9.8ms-2?
Title: Re: Gravity acceleration
Post by: TrueTears on November 25, 2009, 11:55:48 pm: The negative and positive is simply there to dictate direction.

If you say let up be positive and down be negative then all vectors that's pointing down must be negative.

Since $g$ is a vector and if you denote down as negative, then an object would accelerate downwards at $-9.8 ms^{-2}$
Title: Re: Gravity acceleration
Post by: kenhung123 on November 26, 2009, 12:06:18 am: Oh but what if its just the use of formulas? Its convention down is negative and up is positive??
Title: Re: Gravity acceleration
Post by: TrueTears on November 26, 2009, 12:08:10 am: In the formulas you must also specify positive/negative as direction. Then all the vectors in your formula must follow the same rule.
Title: Re: Gravity acceleration
Post by: Gloamglozer on November 26, 2009, 12:08:26 am: Quote from: kenhung123 on November 26, 2009, 12:06:18 am
Oh but what if its just the use of formulas? Its convention down is negative and up is positive??

Yes. When using those formulae remember to define your vectors. And yes, it is convention to do what you just said but you can do it the other way around and still get the same answer.

Just remember to mark down on the paper what you defined up or down as.

EDIT: Who here on VN can beat the goddess?
Title: Re: Gravity acceleration
Post by: QuantumJG on November 26, 2009, 12:49:39 am: Quote from: kenhung123 on November 25, 2009, 11:54:37 pm
Since gravity acts downwards and an object is falling downwards its acceleration is 9.8ms-2? Why do some people say its -9.8ms-2 because its going down?
Like I am thinking since the gravity acts down and the object complements that direction (goes down) it means its positive and if it goes up (counter acting gravity) its -9.8ms-2?

The negative represents the direction in which it acts. The direction is towards the centre of the Earth, but since in high school physics you are working on problems that allows for the Earth to be approximated to a flat surface, you can solve your problem by drawing a Cartesian coordinate system. Since the gravity force points down it would be normal to say it's negative, because it's downwards.

This negative should not confuse you, it does not mean gravity will always go against motion, bit if say I throw a ball up in the air it has positive velocity which is slowed down by it's downwards acceleration (by g) to the point it will stop and start gaining negative velocity, since, it's accelerating downwards (by g).

Hope this helps.