Projectile Motion Ay (1 Viewer)

[Currybear]

Hey,

so im having trouble with understanding verticle acceleration, question say to take g=10 or some other positive number. What i dont get is how can g be a positive number, if it was the object would be moving up and never fall.

Why dont they tell us g is negative instead of having us remember Ay=-g

It makes no sense!

 

[namelyanonymous]

g is just a constant representing the acceleration due to gravity. It is *not* a negative value because there's no direction specified, so a negative value would not be appropriate.


When we're talking about a body affected by gravity, however, the gravity accelerates it downwards at a constant rate "g". Because it's accelerating downwards (hence, a direction that is negative), Ay = -g.

I tried using it as a positive value once, and it just took me into the complex field, so always stick to it being negative.

 

[omgiloverice]

give us the question

 

[Carrotsticks]

Hey,

so im having trouble with understanding verticle acceleration, question say to take g=10 or some other positive number. What i dont get is how can g be a positive number, if it was the object would be moving up and never fall.

Why dont they tell us g is negative instead of having us remember Ay=-g

It makes no sense!

Say they make g be a negative quantity.

Your Cartesian, assuming the particle is projected from the origin, would be



Although we know that g is a negative quantity, this equation does not portray that the equation is a negative parabola as well as if we were to take g to be positive, where our equation would then be



Regarding your statement about the object moving up but never falling. The 9.8 is a QUANTITY, but we have not taken into consideration its direction. By taking its direction to be downwards, we therefore have our gravity actually being g=-9.8 (since it is natural to take upwards to be a positive direction in the definition of projectiles).

You wish to take g = -9.8, but this has 'inbuilt' the fact that g goes downwards. To allow students to better understand the concept of directions and signs (positive vs negative), we take g to be positive, THEN we add the negative symbol in front so it is clearly seen that gravity goes downwards, whereas just having g=-9.8 does not make it as clear.

 

[Currybear]

That makes sense, but gravity by definition is a force that brings 2 or more objects closer to each other. When would this quantity be >0?

 

[braintic]

Despite the attempts at rationalisation below, the answer is much more simple: an arbitrary convention. But probably the most sensible choice.

 

[Fus Ro Dah]

Despite the attempts at rationalisation below, the answer is much more simple: an arbitrary convention. But probably the most sensible choice.

For what reason(s) is it the most sensible choice?

 

[Currybear]

In the HSC they never specify what g is, in my school exam they did and i freaked the f out and lost 6 marks

 

[braintic]

For what reason(s) is it the most sensible choice?

Ease of translation into English. If someone asks you 'what is the acceleration due to gravity near the earth's surface', you don't really want to answer 'minus 9.8 m/s/s'. So if we are going to define g as the acceleration due to gravity, then we are using the best option. It is better to keep the -ve out of the English and save it for the maths.

But as I said, its only an arbitrary choice, and this is merely my opinion/rationalisation.