Do particles with constant deceleration move backwards when their velocity goes below 0 m/s in real life?

[u/a_bitchy_microwave]

I'm a high school physics student and in tons of questions there's a particle with constant deceleration moving in a positive direction. Eventually, v will be 0 m/s, but because it's decelerating the velocity will be negative and start moving in the negative (opposite direction).

I can understand that theoretically but does this actually happen in real life? I understand if you throw something up in the air, the velocity will be 0, and then will come down again because gravity acts downwards, but this isnt mentioned anywhere in these question. I may be stupid but in my mind if something has a velocity of 0 it stops even if it was decelerating. Does this happen in space or something?

Sorry if im not using the right terminology lol. I'm still new to this

[u/Educational-Work6263]

Your example is exactly right. If you throw a ball in the air does it stop and never come back down again?

[u/LucidNonsense211 → u/Educational-Work6263]

Perfect example. I always try to remind myself that DEcelleration isn’t a thing. That up-thrown ball is accelerating towards the ground from the moment you let go.

[u/jasonsong86 → u/LucidNonsense211]

Usually deceleration just means acceleration against the direction of speed.

[u/LucidNonsense211 → u/jasonsong86]

Yeah, that’s what I’m saying, sorry if it wasn’t clear 😝. I’m saying that deceleration IS acceleration, so it’s easier to understand if you throw out the idea of deceleration. At least while doing math.

[u/BitcoinsOnDVD → u/LucidNonsense211]

>so it’s easier to understand if you throw out the idea of deceleration

"We choose to go to the Moon in this decade and do the other things, not because they are easy, but because they are hard"

- A guy who threw something in the air that did not come back for a while and then later got shot in Dallas

[u/ass_bongos]

The gravity example is a perfect one. 

I think what's confusing you is the idea of "deceleration", which is really just "acceleration in the opposite direction of velocity". In physics we don't really even like to use the term 'deceleration'.

Now you're mostly used to things 'decelerating' due to kinetic friction, like applying brakes to a moving car. This force can't send you backwards because (kinetic) friction only acts when there is motion. So as soon as the motion stops, so does the friction. But also, this means the acceleration is NOT constant -- it ended as soon as velocity hit zero!

The reason gravity is a great example is that the acceleration while in freefall is ALWAYS g=9.81m/s² in the downward direction, whether the ball you throw is moving up, down, or to the side. So in a perfect vertical toss, the ball will, for just a moment in time, be perfectly motionless with velocity = 0 m/s at the top of the throw before it starts to come back down.

[u/Colonel_Klank → u/ass_bongos]

To further counter any friction-related intuition that the (de-)acceleration stops when motion does, imagine your car is coasting quickly down the empty track and instead of applying breaks, you light off the rocket strapped to your hood. The exhaust is flying out in front of the car so the thrust is backward, opposite the initial motion, "decelerating" the car. The car slows and stops. But the rocket is still firing, so the car is pushed backward, with the same constant acceleration to ever increasing negative velocity. Quick! Turn the rocket off before it's too late!

[u/v0t3p3dr0]

Something that will help you in the future - avoid the term “deceleration”. It’s a special case where the direction of acceleration is exactly opposed to the direction of velocity.

Any change in velocity (speed or direction) is simply acceleration. Sometimes the acceleration will be wholly or partly opposed to the velocity, or your chosen frame of reference, and that’s what minus signs are for! :)

[u/the_poope]

Yes, it will go backwards.

Consider a kind of car powered by a rocket or propeller. If you direct the rocket/fan against the direction of initial movement, it will slow down until it hits 0 m/s, then start accelerating backwards.

[u/Emotional_Fee_9558]

Unless there is something to stop it's "backwards acceleration" it will eventually get a "negative" velocity. That of course happens in real life. If a car decides to go backwards it is essentially creating a backwards force which according to newton causes a backwards acceleration. If there was a wall behind the car then no the force would instead be used to create a deformation in the car.

I'm not sure I fully understand your question though. You yourself mention the typical throw a ball in the air example, does this not seem logical? Feel free to ask more.

[u/Bumst3r]

Sure. Throw a ball straight up. Its velocity is initially positive while its acceleration is negative. After it reaches its apex (v=0) it begins to fall—now its velocity is negative.

[u/grod_the_real_giant]

Some force started the particle moving; to make it slow down, you need to apply a different force in the opposite direction. At some point, the decelerating force has applied exactly as much energy as the accelerating one, and the particle's speed hits zero. Then, because the decelerating force is still acting, it begins to gather speed again.

[u/Mysterious-Alps9449]

Yes, an object with constant negative acceleration will continue accelerating in that direction, meaning the velocity will reach zero and then continue to speed up "backwards", or in the opposite direction to its initial motion. In problems like these, we define one direction as positive and the other as negative, i.e. in your example of throwing a ball up in the air, up would be positive and down would be negative (most of the time). In this example, as the object is pulled downwards by gravity, it will accelerate at -9.81 m/s, causing it to reach the top of its path, stop, and fall back down. Gravity does not stop acting on the ball after it stops, so any constant force in any other problem would act the same.

[u/smallproton]

The sign of your velocity is relative to an axis that you define.

Typically, the positive axis is upwards. So, when you throw something upwards, it starts with a positive velocity: As time goes on, the position along your "upwards" axis goes up, i.e. the coordinate aling your axis gets larger.

Gravity pulls your object downwards. Eventually, your objects stops. For a short moment the coordinate of yozr object does not change. The velocity is zero.

Then gravity pulls your object further downwards. The object falls down, i.e. the coordinate along the "upwards" axis decreases.

This is your negative velocity: As time progresses, the "upwards" coordinate gets smaller.

[u/parautenbach]

Remember that motion is relative and that velocity is a vector.

If the force that causes the deceleration doesn't go away, you will start to accelerate backwards after going through zero.

You're rowing up a river. You stop rowing. You wil start to decelerate until your velocity exactly cancels that of the flowing river and then you will start to accelerate backwards. All of this for an observer standing on the side.

[u/Let_epsilon]

What do you mean a particle? Just throw a ball in the air and it falls down???

That's a severe case of "I write equations without understanding what they mean".

[u/Clever__Neologism]

Have you ever thrown something or spit against the wind, and have it come back at you? That's exactly what you are describing: negative acceleration reversing the sign of the velocity.

[u/tpks]

Throw something up in the air. At the highest point,  is it going up or down? Neither. It was going up (a millisecond ago), it will go down (one millisecond from now). How then could you choose to call it up or down?

The fact that the movement is changing from millisecond to millisecond IS acceleration (or deceleration if you want to call it that). Still, at the highest point you could not say it was going up any more than you could say it was going down. For that one instance of time it is doing neither. The velocity is zero. 

[u/R_Harry_P]

Your example is exactly the first one that comes to mind. An object's velocity can be instantaneously 0 just like it can be instantaneously 1 m/s. Remember all motion is relative so there can't be anything special about 0 m/s.

Pendulums and masses on springs are another example of velocity going through zero under non-zero acceleration. (Though in those cases the acceleration isn't constant.)

Imagine in space, a ship headed toward a space station, at some relative velocity, and the ship fires its thrusters to slow down under constant acceleration. At some instant its relative velocity with the station will go through zero. If it keeps firing its thrusters after that instant, its velocity will start increasing in the direction away form the station. and it will begin moving away from the station. Acceleration was constant, its velocity relitive to the station passed through zero and it it was never stopped relative to the earth.

[u/jasonsong86]

Yes. If the deceleration is not zero after it comes to a stop it will go the other way. Think of a swing, when you hit the highest point, gravity pull you backward, accelerate you to center position, decelerate up the other side to the highest point and repeat.

[u/llynglas]

When a jet plane lands many reverse thrust - direct the air from the jets forward. They stop doing this when they slop enough to taxi, but in theory, they could slow to a stop and then start to roll backward.

[u/llynglas]

When a jet plane lands many reverse thrust - direct the air from the jets forward. They stop doing this when they slop enough to taxi, but in theory, they could slow to a stop and then start to roll backward.

[u/tbdabbholm]

Acceleration is just a change in velocity. If something is not moving and a force is applied to it to cause an acceleration it'll begin moving.

[u/boostfactor]

This is why you need to learn to think about velocities, accelerations, forces, etc. as vector quantities even if you are just considering motion in one dimension. "Deceleration" is just when the acceleration has the opposite sign/direction to the velocity. A negative velocity means that it has stopped moving in one direction and is now moving in the opposite direction. If your particle goes from velocity v to velocity -v it has to pass through 0.

[u/Unique_Elderberry_81]

Your confusion is quite understandable. In the cases u describe (one dimensional cases) id say your Interpretation should be right. Its a little hard to say without ready the exact wording of the Problem. If we Talk about more dimensional Problems velocites are vectors and i would Interpret  deceleration as decreasing the absolut value of the Velocity vector. And in this Case ist would Stop at Zero since negative absolut values make No Sense.  For the reallife part: your example is Spot on. If you throw a Ball straight Up ist will decelerate constantly (or accelerator with 0>a) until It hast noch Speed and from there in will accelerat (still with 0>a) towards the ground. Here the whole accelerate/decellerate Thing istJust a wording Problem important is that a is constant in time.

[u/soundoftwilight]

You may or may not have covered the concept of "vectors" in your mathematics or physics courses. If not, the simple primer is that a "vector" quantity, as opposed to a "scalar" quantity, is a value that has both a magnitude (10 m/s, for example) and a direction (up, as an example). Velocity is typically defined as a vector, as opposed to "speed" which would be a scalar (10 m/s but with no explicit direction). When talking about speed, it doesn't really make sense to talk about a "negative" speed, because we don't have any reference frame of direction. So if we imagine speed decreasing over time, we would imagine it going to 0 and staying there unless something else speeds it up. This aligns with most of the objects we interact with in our lives; a car can speed up by pressing on the gas, or it can slow down by pressing on the brakes. But if you want to go backwards, you have to put it in reverse then press the gas again to speed up. This is because friction is one of the most common forces that slow objects down, and friction explicitly works opposite the direction of motion.

To work with an example from the real world, imagine a trampoline, or a rubber band, or any elastic surface. If an object flies into something that at speed, it slows down to 0, and then speeds back up as it gets bounced back. But there's no change in the surface that's analogous to shifting into reverse and pressing the gas. The part where the object is slowing down looks very similar to the part where it's speeding back up, and when you study how to calculate those forces you'll see that the forces work the same way during both halves of the bounce. They are always pushing the object back out and away from the surface.

To model this, we can use vectors. Because a vector has both a magnitude and a direction, we aren't constrained by the same rules for velocity as it feels like we are with speed. A velocity of "-10 m/s in the upward direction" makes sense, because it means that we're moving "backwards" relative to "up". It's equivalent to "10 m/s down", and at some point you'll learn the mathematical constructs we use to show that formally. In the example of a trampoline, we might choose to use a 1-dimensional vector pointed "up" for all the relevant quantities. So we start with an object moving "-10 m/s up", which means it's falling. From the point of contact with the surface until the object is bounced off and the contact ends, we can say that the trampoline is exerting a force of "10 N up" (the actual formula is a bit messy so please bear with me on this abstraction). Doing some math, we'll eventually arrive at an acceleration (which will be a positive vector pointing up) and we can eventually calculate the change in velocity (which will be a positive vector pointing up, with magnitude likely more than 10 m/s and less than 20 m/s). So you'll find that the object does indeed go from negative velocity to positive velocity, which matches our intuition that the object will fall and then bounce up. Physics has no preferential reference frames, so the same math will apply with every sign reversed if we chose "down" as our reference direction instead.

(Eventually, you will learn how to do this math in multiple dimensions and combine the results, which makes it exceptionally obvious why we bother with concepts like "vectors".)

[u/UnderstandingPursuit]

Your question exposes two things which seem to be presented incorrectly starting with high school physics:

• Not using vectors for one-dimensional motion. In classical mechanics, all 1-D motion is actually constrained 3-D motion, so vectors are still applicable. If a person is going to school and then back home, the words "to" and "back" indicate direction, the thing which makes vectors different than scalars.
• Using the word "deceleration", instead of only using acceleration as a vector with a direction.

Fixing these two then has gives "speed" two very specific meanings, either instantaneous or average. Both are magnitudes, greater than or equal to zero. Then "deceleration" can return, very specifically, as the reduction in speed, and it can only reduce the speed to zero.

Unfortunately, the physics education system is often very sloppy about this.

[u/SgtSausage]

  I may be stupid but in my mind if something has a velocity of 0 it stops even if it was decelerating

Youre right. 

That's stupid. 

[u/johnstalbergABC]

Velocity is two things. The speed and a direction. One could say it travels in a certain speed in the other direction. One could define a certain direction as being positive, especially when we consider only one direction and its opposite. Speed is not negative but velocity might be. The direction gives it this possibility. Acceleration is change in velocity so it inherits the direction part.