A'ight, so, in basic physics, let's say you've got a person who threw a ball in the air and you have to calculate, I dunno, time or distance or something. Anyway, that's irrelevant. Someone threw a ball vertically in the air, and you use acceleration due to gravity even for its upward acceleration. This doesn't make sense to me, because isn't gravity pulling the ball down? My physics teacher hasn't given me a satisfying answer (what he says kinda just boils down to “It is what it is”) and my college physics textbook was either very vague in why or didn't give a reason, either. I can't remember. I just know it didn't help.

Yeah I can do the calculations and stuff and get the questions right, but I really want to know WHY.

If anyone could explain, I'd be very grateful. Thanks!

EDIT: By the way, I'm not asking on StackOverflow because I'm afraid I'll get ripped apart, ha ha…

Gravity is indeed pushing the ball down, but the person threw the ball up with an acceleration in an upward direction.

What might help is the actual value of the acceleration due to gravity: 9.81 m/sec^2

Now you might be going, seconds squared!? But here's how I think about it. It just means that the acceleration is changing 9.81 metres per second per second. So for every second that the ball is under the influence of gravity, its speed increases 9.81 metres per second in a downward direction.

Perhaps a project explaining this would be better? Would you find this helpful?

(And this is technically the wrong forum to post this in, but I'm not entirely sure which one it should be in. I'm still more than happy to help, I personally love physics)

I think you might be confusing acceleration and velocity. Velocity is the speed and direction of an object. You can be going 5 meters per second to the left, 2 miles an hour up, or 15 feet per second at a 45° angle from the ground. Those are all velocities. The direction of the velocity is always the direction which the object is moving.

Acceleration is different. It's defined as the change in velocity over a period of time. Let's imagine a car slowing down for a red light. The car is moving to the right at 30mph towards the red light. When the light turns red, the driver begins to slow down. 10 seconds later the car is moving at 5mph to the right. Our velocity changed from 30mph right to 5mph right in 10 seconds. Notice that the car was only ever traveling to the right, never left (it isn't going backwards!), and so the velocity was only ever to the right as well. Acceleration, however, is a different story. Remember: acceleration is the change in velocity over time. We can calculate the change in velocity by doing (final velocity - initial velocity). This is 5mph right - 30mph right = -25mph right. If the acceleration is negative in the rightward direction, then we can also say it is positive in the leftward direction (25mph left). But wait! The car was never travelling left! What gives?

Well, the key to remember is that acceleration is not about the way that the position of the car is changing. It's about the way that the velocity of the car is changing. And when the driver puts on the breaks, it's essentially equivalent to the car being gently pulled backwards. It isn't moving backwards, but it's moving towards going backwards. Hence, the negative acceleration.

The same goes for gravity. If an object is moving up, gravity will still be pulling it down (which is why acceleration due to gravity on earth = -9.8m/s^2). This negative acceleration doesn't necessarily mean that the object is moving downwards (only a negative velocity could tell us that), but it does tell us that the velocity of the object is changing towards being downards. It might not be going down yet, but the velocity is changing so that it will be soon (which is why the object will eventually fall).

Sorry for a wordy post. I can't claim it's an especially thrilling read, but I'm hoping it can help.

Basically, think of it like this:
The O represents the ball.
At the beginning, the force of the ball being thrown upwards overpowers gravitational force:
However, there's a difference: As soon as you let go of the ball, there is no more force applied to it. On the other hand, gravitational force is still there, even after you let go of the ball.
Now, the ball will inevitably succumb to gravitational force (DUM DUM DUM) and go down, after it undoes the acceleration the throwing force (upwards acceleration) has inflicted on the ball (downwards acceleration).

The_Grits wrote:

Gravity is indeed pushing the ball down, but the person threw the ball up with an acceleration in an upward direction.

What might help is the actual value of the acceleration due to gravity: 9.81 m/sec^2

Now you might be going, seconds squared!? But here's how I think about it. It just means that the acceleration is changing 9.81 metres per second per second. So for every second that the ball is under the influence of gravity, its speed increases 9.81 metres per second in a downward direction.

Perhaps a project explaining this would be better? Would you find this helpful?

(And this is technically the wrong forum to post this in, but I'm not entirely sure which one it should be in. I'm still more than happy to help, I personally love physics)

Mm, I do get that the acceleration is 9.81 m/s^2, and I do understand what that means, but I don't quite understand where in the calculations it would indicate the speed is slowing down. For instance, one of my formulas is vf = vi + at. Is there something here I'm missing that helps indicate that the ball is slowing down? And if you did do a project, I would be very grateful. You don't have to do that, though.

PullJosh wrote:

The same goes for gravity. If an object is moving up, gravity will still be pulling it down (which is why acceleration due to gravity on earth = -9.8m/s^2). This negative acceleration doesn't necessarily mean that the object is moving downwards (only a negative velocity could tell us that), but it does tell us that the velocity of the object is changing towards being downards. It might not be going down yet, but the velocity is changing so that it will be soon (which is why the object will eventually fall).

Here's where I get confused; our physics teacher tells us to plug in the acceleration as +9.81 m/s^2, not -9.81 m/s^2, and I've gotten into a couple disccusions/borderline arguments over this because he can't quite explain why he's doing it that way. Should I be using a negative acceleration then?

iamunknown2 wrote:

Now, the ball will inevitably succumb to gravitational force (DUM DUM DUM) and go down, after it undoes the acceleration the throwing force (upwards acceleration) has inflicted on the ball (downwards acceleration).

And that acceleration due to gravity is acting against your initial velocity, correct? Correct me if I'm wrong, but your final velocity is 0, right?

Thank you so much everybody! I guess my main concern here is that the teacher says use a positive acceleration, which doesn't make sense to me when the ball is being pulled downwards (PullJosh touched on this). Is the teacher wrong in this case? I apologize that I wasn't quite clear in what I did know and didn't ask my real question.

Pturretdactyl wrote:

Thank you so much everybody! I guess my main concern here is that the teacher says use a positive acceleration, which doesn't make sense to me when the ball is being pulled downwards (PullJosh touched on this). Is the teacher wrong in this case? I apologize that I wasn't quite clear in what I did know and didn't ask my real question.

The whole positive/negative acceleration thing is completely up to you. (sort of) What's important is that you keep it in reference. So if you decide that gravity will be a negative acceleration (which makes more sense to me personally) then you need to remember that once you're finished your calculation.

May I suggest you watching some Khan Academy videos on this subject? https://www.khanacademy.org/science/physics/one-dimensional-motion

Ah, I see. One of the things to keep in mind with motion is that everything is relative. As I'm typing this, I'm sitting in place without moving. You could say that my velocity is 0. But that's just relative to the earth. When compared to the sun, I'm moving at 30km/s. If we're doing motion math, we get to decide the perspective from which we view our subjects. Generally we think of positive motion as going up, and negative motion as going down. However, what if the “camera” we use to view the situation is turned upside down? Now, when the object moves up, it appears as down on our camera, so up become a negative direction. Likewise, down can become positive. This is entirely up to the person doing the math. The only requirement is that you have to be consistent throughout the entire problem.

My guess, then, is that your teacher has decided that up is negative and down is positive. In physics, you can do that sort of thing.

If the ball starts out moving away from the ground, gravity should have the opposite sign of initial velocity.

If the ball is launched downwards, gravity should have the same sign as initial velocity.

If the ball is dropped, pick a sign for gravity and stick to it - initial velocity is zero.

Which sign you pick for gravity and velocity is really up to you - but I would try to answer using whatever signs the teacher had specified.

Also note that in free body diagrams, gravity can have two signs at once, as all signs in each diagram have to match the direction of the signs in other diagrams.

Your physics teacher either has to define the velocity of the ball when thrown upwards as a negative velocity or has to define gravity pulling downwards as a negative acceleration - he can't have it both ways. If he tries to, then get him to write the equations(*) out in full and then substitute some actual numbers. If he insists that both downward gravity and upward motion are positive values, he'll have some difficulty getting the answers to work out as claimed. :-)

(*: for the height of the ball ‘y’ against time ‘t’ after being released with an upward velocity ‘v’ and an acceleration due to gravity ‘g’.)

Even better, do the calculation yourself in Scratch and plot the graph of y against t for a given v and g. Once for positive v negative g and once for positive v positive g. (You can cut the graph off before the ball reaches the moon, to save paper…)

gtoal wrote:

Your physics teacher either has to define the velocity of the ball when thrown upwards as a negative velocity or has to define gravity pulling downwards as a negative acceleration - he can't have it both ways. If he tries to, then get him to write the equations(*) out in full and then substitute some actual numbers. If he insists that both downward gravity and upward motion are positive values, he'll have some difficulty getting the answers to work out as claimed. :-)

(*: for the height of the ball ‘y’ against time ‘t’ after being released with an upward velocity ‘v’ and an acceleration due to gravity ‘g’.)

Even better, do the calculation yourself in Scratch and plot the graph of y against t for a given v and g. Once for positive v negative g and once for positive v positive g. (You can cut the graph off before the ball reaches the moon, to save paper…)

Ahhhhhhhhh that makes sense now. Alright, I'll give some practice problems a shot. Thank you!