# Resultant ForceWatch

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Thread starter 3 weeks ago
#1
I don’t understand how having a resultant force of 0 results in an already moving object remaining at the same speed? If the forward force is balanced by a backwards force, how can it be moving at all? And then what does the resultant force have to be for something to stop moving? I don’t get it?
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3 weeks ago
#2
(Original post by EmRep13)
I don’t understand how having a resultant force of 0 results in an already moving object remaining at the same speed? If the forward force is balanced by a backwards force, how can it be moving at all? And then what does the resultant force have to be for something to stop moving? I don’t get it?
If an object is at rest, and there is a resultant force of zero acting on it, then yes, it won't move. But if it is already moving and there is a resultant force of zero on it, then as you say, it will just carry on in a straight line at the same speed. To understand why, you need to switch your way of thinking about it - rather than ask why it will keep moving, you should ask why would it not keep moving? Slowing something down requires a force to act on it, and if there is a resultant force of zero acting, then effectively there is nothing slowing it down.

The problem for most people thinking about this for the first time is that we tend to think that a force is needed to make something keep going. This seems reasonable from common sense - if we are cycling and stop peddling, we come to a halt. If we're driving and we take our foot off the gas, we come to a halt. But in both cases, it is not the fact that we are no longer exerting a forwards force that brings us to a stop - it is the fact that there is a force acting to slow us down (air resistance, friction in the workings of the bike/car, etc.) that has nothing acting to counter it. We don't need a force to make something keep moving forward, but we do need it if we want to keep moving at the same speed, just to overcome resistive forces.

If we imagine a perfect situation with no resistive forces (i.e. no friction and negligable air resistance), then once we have started something moving, there is no need to keep applying a force to continue the motion. If there is no resistive force to overcome, there is no need for a forward force.
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Thread starter 3 weeks ago
#3
(Original post by Pangol)
If an object is at rest, and there is a resultant force of zero acting on it, then yes, it won't move. But if it is already moving and there is a resultant force of zero on it, then as you say, it will just carry on in a straight line at the same speed. To understand why, you need to switch your way of thinking about it - rather than ask why it will keep moving, you should ask why would it not keep moving? Slowing something down requires a force to act on it, and if there is a resultant force of zero acting, then effectively there is nothing slowing it down.

The problem for most people thinking about this for the first time is that we tend to think that a force is needed to make something keep going. This seems reasonable from common sense - if we are cycling and stop peddling, we come to a halt. If we're driving and we take our foot off the gas, we come to a halt. But in both cases, it is not the fact that we are no longer exerting a forwards force that brings us to a stop - it is the fact that there is a force acting to slow us down (air resistance, friction in the workings of the bike/car, etc.) that has nothing acting to counter it. We don't need a force to make something keep moving forward, but we do need it if we want to keep moving at the same speed, just to overcome resistive forces.

If we imagine a perfect situation with no resistive forces (i.e. no friction and negligable air resistance), then once we have started something moving, there is no need to keep applying a force to continue the motion. If there is no resistive force to overcome, there is no need for a forward force.
But in a perfect situation, if we have a forward force of 5N, then if we apply a “backwards” force of 5N, then the resultant force would be 0 right? But then surely, because there’s an equal force combatting the forwards force, the object would have to stop?
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3 weeks ago
#4
(Original post by EmRep13)
But in a perfect situation, if we have a forward force of 5N, then if we apply a “backwards” force of 5N, then the resultant force would be 0 right? But then surely, because there’s an equal force combatting the forwards force, the object would have to stop?
I don't see why it would.

If there was a forward force only, then the object would speed up (you push things to make them move faster than they were going in the first place).

If there was a backwards force only, then the object would slow down.

But if there were equal backwards and forwards forces, there is a total force of nothing. So nothing would happen.
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Thread starter 3 weeks ago
#5
(Original post by Pangol)
I don't see why it would.

If there was a forward force only, then the object would speed up (you push things to make them move faster than they were going in the first place).

If there was a backwards force only, then the object would slow down.

But if there were equal backwards and forwards forces, there is a total force of nothing. So nothing would happen.
But then what do you do to make it stop?
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3 weeks ago
#6
(Original post by EmRep13)
But then what do you do to make it stop?
You apply a larger backwards force than the force pushing it forwards.
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Thread starter 3 weeks ago
#7
(Original post by Pangol)
You apply a larger backwards force than the force pushing it forwards.
But wouldn’t that just accelerate it backwards, not stop it?
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3 weeks ago
#8
(Original post by EmRep13)
But wouldn’t that just accelerate it backwards, not stop it?
Yes, it would accelerate it backwards. You'll need to do that to slow it down! And then once stopped, you'll have to make the backwards force smaller again, to match whatever it pushing it forwards.

This is a bit tricky to disect when we're dealing with an abstract situation. Do you have anything concrete in mind?
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Thread starter 3 weeks ago
#9
(Original post by Pangol)
Yes, it would accelerate it backwards. You'll need to do that to slow it down! And then once stopped, you'll have to make the backwards force smaller again, to match whatever it pushing it forwards.

This is a bit tricky to disect when we're dealing with an abstract situation. Do you have anything concrete in mind?
Not really, do you have a situation that you could best explain with?
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3 weeks ago
#10
(Original post by EmRep13)
Not really, do you have a situation that you could best explain with?
The reason I ask is because your examples seem to imply some sort of forward force that is always acting. I can't think of many common situations where this would happen if you want to stop something moving. But, as I say, if this were what was happening, you would need a larger backwards force to slow the object down, then as it gets closer to stopping, you would have to reduce the backwards force to reduce the decleration, until the backwards and forwards forces are exactly matched at the moment when the object stops moving.

But this all seems a bit artificial.
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Thread starter 3 weeks ago
#11
(Original post by Pangol)
The reason I ask is because your examples seem to imply some sort of forward force that is always acting. I can't think of many common situations where this would happen if you want to stop something moving. But, as I say, if this were what was happening, you would need a larger backwards force to slow the object down, then as it gets closer to stopping, you would have to reduce the backwards force to reduce the decleration, until the backwards and forwards forces are exactly matched at the moment when the object stops moving.

But this all seems a bit artificial.
Yikes okay? I didn’t really think of it as a force that’s constantly moving forward, so I wonder what a situation would look like that didn’t have a constant forward moving force?

What I don’t understand is the last sentence. If you would apply a greater backwards force to reduce the acceleration until the exact point where the forwards and backwards forces are equally balanced then surely at that point, the resultant force is 0?
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3 weeks ago
#12
(Original post by EmRep13)
What I don’t understand is the last sentence. If you would apply a greater backwards force to reduce the acceleration until the exact point where the forwards and backwards forces are equally balanced then surely at that point, the resultant force is 0?
Well, yes. You need to have a non-zero resultant pointing backwards to decelerate the object, but when it has stopped moving, if you want to keep it from moving again, you'll need to have a resultant force of zero.
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Thread starter 3 weeks ago
#13
Kinda going back to posts 4-6 ish...
Here’s what I don’t get. Let’s say there’s a constant forwards force of 5N. Then you immediately add an opposing force of 5N backwards. The resultant force is 0, so the velocity doesn’t change. Yet if you added an opposing force of 2N, the velocity would change because the resultant force isn’t 0, and the object would be travelling at a slower velocity than before, when it had no resistive force acting on it. If you applied an opposing force of 4N, the object would slow down by a greater amount than if you applied 2N. So how is it that if you added an opposing force of 5N, the velocity of the object doesn’t slow down further than if you added an opposing force of 4N, but just maintains the original velocity?
Last edited by EmRep13; 3 weeks ago
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3 weeks ago
#14
Shall I jump in to try to help?
"Let’s say there’s a constant forwards force of 5N. Then you immediately add an opposing force of 5N backwards. The resultant force is 0, so the velocity doesn’t change." that's true.
"if you added an opposing force of 2N, the velocity would change because the resultant force isn’t 0, and the object would be travelling at a slower velocity"
Not sure about this. Is the 5N forward still there, and you're adding 2N backwards? The resultant force would be 3N forward, and the velocity would be increasing, just not as much of an increase if there were only 5N forwards.
Or did you mean there's 5N forwards, 5N backwards and another 2N backwards? This would be a resultant force of 2N backwards, and the object would slow down, decelerate.
Think: 'no resultant force = no change', if it's not moving, no resultant force means it stays not moving, but if it is moving, no resultant force means it carries on moving without getting faster, slower or changing direction.
I hope that helps. 1
Thread starter 3 weeks ago
#15
(Original post by old_teach)
Shall I jump in to try to help?
"Let’s say there’s a constant forwards force of 5N. Then you immediately add an opposing force of 5N backwards. The resultant force is 0, so the velocity doesn’t change." that's true.
"if you added an opposing force of 2N, the velocity would change because the resultant force isn’t 0, and the object would be travelling at a slower velocity"
Not sure about this. Is the 5N forward still there, and you're adding 2N backwards? The resultant force would be 3N forward, and the velocity would be increasing, just not as much of an increase if there were only 5N forwards.
Or did you mean there's 5N forwards, 5N backwards and another 2N backwards? This would be a resultant force of 2N backwards, and the object would slow down, decelerate.
Think: 'no resultant force = no change', if it's not moving, no resultant force means it stays not moving, but if it is moving, no resultant force means it carries on moving without getting faster, slower or changing direction.
I hope that helps. Hi!
Why would the velocity be increasing if there was only 5N forwards?
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3 weeks ago
#16
(Original post by EmRep13)
Hi!
Why would the velocity be increasing if there was only 5N forwards?
The other reply you got was helpful, I thought - but I think I now understand part of your confusion.

If the object is moving forward and there is only a forward force of 5 N, then it will not be moving at a cosntant velocity. It will be accelerating, getting faster and faster. If there is also a backwards force, but this force is less than 5 N, then there will still be some acceleration, the object will still be getting faster and faster - just not as quickly as before. It is only when the backwards force reaches 5 N that the resultant force is zero, the acceleration stops, and the velocity becomes constant. But you should not think of this as being "the original velocity". If there was a consant forwards force before the resistive forces kicked in, then the forwards velocity will be increasing all the time.
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Thread starter 3 weeks ago
#17
(Original post by Pangol)
The other reply you got was helpful, I thought - but I think I now understand part of your confusion.

If the object is moving forward and there is only a forward force of 5 N, then it will not be moving at a cosntant velocity. It will be accelerating, getting faster and faster. If there is also a backwards force, but this force is less than 5 N, then there will still be some acceleration, the object will still be getting faster and faster - just not as quickly as before. It is only when the backwards force reaches 5 N that the resultant force is zero, the acceleration stops, and the velocity becomes constant. But you should not think of this as being "the original velocity". If there was a consant forwards force before the resistive forces kicked in, then the forwards velocity will be increasing all the time.
Oh my god it makes so much sense now?! Wow I feel so stupid for not realising that? Thank you two so so much!!!
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Thread starter 3 weeks ago
#18
Okay so if there wasn't a constant forward force, only the initial one, then would the object be travelling at a constant velocity? And then if a backwards force was applied, would the object move backwards, since there was no initial resultant force?
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3 weeks ago
#19
(Original post by EmRep13)
Okay so if there wasn't a constant forward force, only the initial one, then would the object be travelling at a constant velocity? And then if a backwards force was applied, would the object move backwards, since there was no initial resultant force?
If there was an initial forward force to get things started, but then the force was removed, then yes, the object would move at a constant velocity.

If a backwards force was then applied, there would be a resultant backwards force and therefore a backwards acceleration (or deceleration). This doesn't mena that the object would move backwards straight away - it would first slow down, then stop, and then move backwards. But if the forces stay like this, it would continually accelerate backwards, moving faster and faster in the other direction.
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Thread starter 3 weeks ago
#20
(Original post by Pangol)
If there was an initial forward force to get things started, but then the force was removed, then yes, the object would move at a constant velocity.

If a backwards force was then applied, there would be a resultant backwards force and therefore a backwards acceleration (or deceleration). This doesn't mena that the object would move backwards straight away - it would first slow down, then stop, and then move backwards. But if the forces stay like this, it would continually accelerate backwards, moving faster and faster in the other direction.
So that's partially like how it would work on a car, if you took your foot off the gas like you said before, right? The resistive forces would build up and combat the forwards motion and bring it to a stop? And it wouldn't continue accelerating backwards because those forces (e.g. friction) don't move in a certain direction, but just generally act against the forwards motion, in this case?
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