Acceleration due to gravity in Projectile motions (1 Viewer)

[ThreeOne]

My physics teacher said to us that the acceleration due to gravity in projectile motions questions is always -9.8ms^-2. But a friend of mine who has done the HSC says to me that it can be negative or positive 9.8 depending on which way the projectile is moving.

Who's right?

 

[airie]

If you assign the upwards direction as the vertical positive direction, then the acceleration due to gravity would be negative. If, however, you take the positive direction as the direction towards earth, which would be downwards for a projectile motion over a flat ground, then the acceleration due to gravity would be positive.

So in the end it really depends on what direction do you assign to be notionally positive. Usually we work in right-handed coordinates ie. horizontal positive direction is "to the right", while vertical positive direction is "upwards"

 

[ThreeOne]

Okay.

Let's say I throw a ball from the ground to the air at 30 degrees to the horizontal and it lands some metres away. Up is positive, down is negative.

So the acceleration will be -9.8 for the first half of the motion (until it reaches maximum height) and then it is +9.8 for the second half (from maximum height back to the ground). Correct?

 

[airie]

Erm, no. The positive direction hasn't changed when the ball is falling down, therefore the acceleration is still negative.

Think about it this way: On its way up, the ball is gradually slowed (in the vertical direction) under the effect of gravity, which applies towards the centre of the earth (assumed downwards as curvature of ground can be disregarded here). It therefore eventually slows, and stops (in vertical direction), then comes back down again. The ball is still under the effect of gravity, which is acting in the same direction as before.

Basically, there is no need to reverse the whole direction of the coordinate axes Take the whole motion as one entity, all the equations apply to it for the whole course. The laws of physics don't change when the ball starts to fall

 

[ThreeOne]

So let's say I assign up as positive. If I were to stand at the edge of the cliff xm high and I simply drop a ball directly downwards, then the acceleration will still be negative since I assigned "upwards direction as the vertical positive direction"?

 

[airie]

That's right

See, regardles if the ball is moving upwards or downwards, the gravity it experiences is always AGAINST the positive direction, therefore the acceleration is always negative

 

[twilight1412]

That's right

See, regardles if the ball is moving upwards or downwards, the gravity it experiences is always AGAINST the positive direction, therefore the acceleration is always negative

no your wrong its not always against the positive direction
its just conventional to take up as the positive direction
but if you took down as the positive direction then acceleration would be positive as well
first think about a car moving in a straight line if it was moving to the right then you would assume the right as positive right? and if it was accelerating then it would be a positive acceleration
but if the it was slowing down then the acceleration would be negative right? because it is accelerating in the opposite direction

READ HERE IF YOU DONT WANT EXPLANATION

basically positive and negative is only direction and so if you are accelerating in the same direction as the motion then the acceleration is positive

 

[ronaldinho]

what the.. i dont get it? after it pasts the maximum is the gravity postivte or negative.. i thought positive because its acting WITH the ball...

 

[falcon07]

Think of velocity as being a number line, with right being positive and left being negative velocity. Initial velocity is to the right. Gravity moves velocity to the left at a constant speed during flight. Hence at one point velocity will be 0, then following that there will be a negative velocity becoming more negative.

 

[ronaldinho]

aha
i get it..
that was an excellent analogy man.. thanks

 

[twilight1412]

geez >< i need to work on better explanations if i wanna do good in hsc next year ......

 

[airie]

no your wrong its not always against the positive direction
its just conventional to take up as the positive direction

Actually...

So let's say I assign up as positive...then the acceleration will still be negative since I assigned "upwards direction as the vertical positive direction"?

He DID assign upwards as the positive direction, so gravity IS always against the positive direction

 

[twilight1412]

Actually...


He DID assign upwards as the positive direction, so gravity IS always against the positive direction

if you didnt get it from my explanation then i'll explain it again ><

IF AND ONLY IF he assigns down as the positive direction then gravity would also be positive

ask your teacher if you dont believe me ... its true ^^

 

[airie]

IF AND ONLY IF he assigns down as the positive direction then gravity would also be positive

Exactly. Since he assigned the upwards direction positive, gravity is ALWAYS negative as it is therefore against the positive direction

 

[Dr_Doom]

wow I'm glad I didn't do physics xD

 

[Forbidden.]

That's right

See, regardles if the ball is moving upwards or downwards, the gravity it experiences is always AGAINST the positive direction, therefore the acceleration is always negative

Otherwise everything would fly up faster and faster into the sky, into the atmosphere and then into space, if g was positive 9.8ms^-2

 

[airie]

Otherwise everything would fly up faster and faster into the sky, into the atmosphere and then into space, if g was positive 9.8ms^-2

Assuming your positive direction is upwards, that is :uhhuh: XP

 

[IceOnFire]

My teacher says the hardest type of questions within the HSC would be something like this:

A cannon ball is shot from a cliff 20 meters high at 30 degrees to the horizontal.
Then the questions could be:
1) Determine the time taken for the cannon ball to reach impact.
2) Determine how far the ball has travel horizontally.

Is that the hardest questions you can get? Is my teacher correct?

Because otherwise I will have to work more on my projectile motions then.

 

[airie]

My teacher says the hardest type of questions within the HSC would be something like this:

A cannon ball is shot from a cliff 20 meters high at 30 degrees to the horizontal.
Then the questions could be:
1) Determine the time taken for the cannon ball to reach impact.
2) Determine how far the ball has travel horizontally.

Is that the hardest questions you can get? Is my teacher correct?

Because otherwise I will have to work more on my projectile motions then.

I don't know, what else is there? Find max height of the object? Find the time passed at max height, when the point of launch is above ground level? Find the velocity of impact of the object? Find the position of the object and its instantaneous velocity at a given time?

Meh, I'm just blabbing. Not actually sure if they'd appear in an HSC paper

 

[IceOnFire]

I don't know, what else is there? Find max height of the object? Find the time passed at max height, when the point of launch is above ground level? Find the velocity of impact of the object? Find the position of the object and its instantaneous velocity at a given time?

Meh, I'm just blabbing. Not actually sure if they'd appear in an HSC paper

Hahahaha!!

What's "when the point of launch is above ground level?" and "Find the position of the object and its instantaneous velocity at a given time?" ? How do you find them? I haven't seen any questions related to them... yet.