Quick question on projectile motion (1 Viewer)

[kooltrainer]

for those equations tht has "a" (for acceleration due to gravity)
should i put -9.8 or +9.8 ?? For a projectile that is parabolic, do we change the signs of 9.8 when we go up and when we go down? or do we keep the sign of 9.8 constant ..

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[decypher]

Sign convention really doesn't matter so long as you are consistant. For example if you assume a falling object to be travelling in a negative direction in one problem you should do the same in other problems. Also remember to state your assumption.

Generally though, motion to the left is positive (and negative to the right) and positive upwards and negative down. Like on a cartesian plane.

 

[davidbarnes]

I always use -9.8 m/s. As that value always acts on a projectile whether it is moving up or down.

 

[Forbidden.]

I always use -9.8 m/s. As that value always acts on a projectile whether it is moving up or down.

- 9.8 m/s/s or - 9.8 m.s^-2 for acceleration acting downwards.

 

[kooltrainer]

I always use -9.8 m/s. As that value always acts on a projectile whether it is moving up or down.

if u always take -9.8, wouldnt that contradict wot decypher said?
he said that if the ball moves up, u take +9.8 (assuming that up is positive) BUT,
-9.8 when the ball goes down..

so u cant "always" take -9.8 right?

 

[S1M0]

- 9.8 m/s/s or - 9.8 m.s^-2 for acceleration acting downwards.

Why exactly is it m/s/s anyway?

 

[Mark576]

Because the acceleration is increasing at a rate of 9.8 m/s every second. Hence, we define it as 9.8 m/s/s. In reference to the OP's question, acceleration should be taken always as negative, as long as you're consistent with signs attributed to any values that indicate motion upwards to be positive, where negative would indicate downward motion. Acceleration due to gravity always acts downwards, I think that's where you're slightly confused.

EDIT: kooltrainer, the acceleration due to gravity is constant, and hence doesn't change direction mid-flight, which is what you're supposing. That is the simplest explanation why the acceleration due to gravity (g) will always be negative (if that's what you choose it to be) throughout a trajectory.

 

[Steth0scope]

Because the acceleration is increasing at a rate of 9.8 m/s every second. Hence, we define it as 9.8 m/s/s. In reference to the OP's question, acceleration should be taken always as negative, as long as you're consistent with signs attributed to any values that indicate motion upwards to be positive, where negative would indicate downward motion. Acceleration due to gravity always acts downwards, I think that's where you're slightly confused.

EDIT: kooltrainer, the acceleration due to gravity is constant, and hence doesn't change direction mid-flight, which is what you're supposing. That is the simplest explanation why the acceleration due to gravity (g) will always be negative (if that's what you choose it to be) throughout a trajectory.

Spot on.

You guys are thinking of velocity - that changes direction (i.e. from up to down). But the only acceleration acting on any projectile (not considering air resistance) is due to the weight force of gravity which always acts down.

Acceleration is the change of velocity over time. That's why it is metres per second (velocity) per second i.e. m/s/s or ms^-2.

 

[Danger]

Why exactly is it m/s/s anyway?

metres per second per second, which is basically metres per second squared.

 

[decypher]

if u always take -9.8, wouldnt that contradict wot decypher said?
he said that if the ball moves up, u take +9.8 (assuming that up is positive) BUT,
-9.8 when the ball goes down..

so u cant "always" take -9.8 right?

To further clarify what I said earlier. (I read it again and I could've been clearer).

I always define gravity as -9.81 ms^-2, because I allocate my sign convention based on the cartesian plane (on this case the ground would be y=0). Therefore anything above the ground would have positive displacement. If however the source of the projectile (e.g. cannon etc) was above the ground by say 10m then I would set y=0 to be 10m above the ground, so the final displacement of the projectile would be -10m in the y direction when it hits the ground.

EDIT: Make sure you state your assumption that g=9.81 ms^-2 because gravity varies according to height and geographical location.

Why exactly is it m/s/s anyway?

A unit of acceleration. Essentially it means that an object is getting faster (or slower)/changing direction or doing a combination of the two (remember acceleration is a vector and hence has magnitude and direction) by X m/s each second.

Note:

v=ds/dt (change in displacement/change in time, hence m/s)

a=dv/dt or a=d^2s/dt^2 (2nd derivative, and hence ms^-2)

 

[YannY]

Can you like not introduce maths into physics... jeez dy/dx? we dont need that stuff. These people hardly understand physics let alone maths.

 

[Steth0scope]

Best way to think of it is like this.

Velocity is the change of position over time. That equates to metres per second (since metres = distance and seconds = time).

Now, since acceleration is the change of velocity over time, the units are now velocity (if u like) per second i.e. metres per second (velocity) per second i.e. m/s/s or ms^-2.

Ok maybe thats not the best way to think of it lol but it might help