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# Help with mechanics watch

2. (Original post by sienna2266)
iv)
In each case draw a diagram of the forces acting. In the first part on the caravan, and in the second part on the car. And apply F=ma.

What have you been able to do?
3. (Original post by ghostwalker)
iv)
In each case draw a diagram of the forces acting. In the first part on the caravan, and in the second part on the car. And apply F=ma.

What have you been able to do?
Hi, thanks very much for your reply. So I am able to draw diagrams for the caravan and the car. What is confusing me is where it says "force of at least 50N". I don't understand how to apply this knowledge to the first part of the question. So I don't get how to match the information they've provided to answer the first part of iv. It's all too confusing and wordy.

The at least 50N bit makes it sound like an inequality question
4. (Original post by sienna2266)
Hi, thanks very much for your reply. So I am able to draw diagrams for the caravan and the car. What is confusing me is where it says "force of at least 50N". I don't understand how to apply this knowledge to the first part of the question. So I don't get how to match the information they've provided to answer the first part of iv. It's all too confusing and wordy.

The at least 50N bit makes it sound like an inequality question
OK, it's not actually an inequality question here; part iv talks about "just before the brakes come one". I.e. they're on the verge of automatically applying, so the compressive force in the tow bar will be 50N. And the caravan's brakes will not yet be applied.
5. (Original post by ghostwalker)
OK, it's not actually an inequality question here; part iv talks about "just before the brakes come one". I.e. they're on the verge of automatically applying, so the compressive force in the tow bar will be 50N. And the caravan's brakes will not yet be applied.
But the compressive force in the towbar is 50N on the point when the caravan's brakes are applied. And the thrust in the towbar is also changing all the time as the car brakes are applied gradually. So surely we cannot have a thrust of 50N in the towbar before the caravan brakes are even applied?
6. (Original post by sienna2266)
But the compressive force in the towbar is 50N on the point when the caravan's brakes are applied. And the thrust in the towbar is also changing all the time as the car brakes are applied gradually. So surely we cannot have a thrust of 50N in the towbar before the caravan brakes are even applied?
The caravan's brakes are on the point of being applied, i.e. it's just met the criterion for the caravan's brakes to start to be applied, so the force is 50N.

Not sure that addresses your concern. We're looking at one specific instance in time on a situation that is gradually changing.
7. (Original post by ghostwalker)
The caravan's brakes are on the point of being applied, i.e. it's just met the criterion for the caravan's brakes to start to be applied, so the force is 50N.

Not sure that addresses your concern. We're looking at one specific instance in time on a situation that is gradually changing.
Ah ok. So in the question where it says "Show that the acceleration of the caravan just before its brakes come on automatically is -0.65ms^-2" is basically asking you to show that the acceleration of the caravan (and the car) is -0.65ms^-2 when the caravan's brakes are on the point of being applied?

Since the car brakes are applied gradually, do we assume the thrust in the towbar is 50N just before the caravan brakes are applied?

Actually, just had another thought - Since it says that the caravan brakes come on when the thrust is at least 50N, then does that mean the caravan brakes could come on at say e.g. when the thrust in the towbar is 51N or 52N or even 60N etc?
8. (Original post by sienna2266)
Ah ok. So in the question where it says "Show that the acceleration of the caravan just before its brakes come on automatically is -0.65ms^-2" is basically asking you to show that the acceleration of the caravan (and the car) is -0.65ms^-2 when the caravan's brakes are on the point of being applied?
Yes.

Since the car brakes are applied gradually, do we assume the thrust in the towbar is 50N just before the caravan brakes are applied?
Yes.

Actually, just had another thought - Since it says that the caravan brakes come on when the thrust is at least 50N, then does that mean the caravan brakes could come on at say e.g. when the thrust in the towbar is 51N or 52N or even 60N etc?
Although the question talks about the cravan's brakes coming on, it's not just nothing or full braking. It's a gradual process. At 50N the braking due to the cravan's brakes will be 0N. At 51N, it will be a small amount. At 52N, a bit more. etc. A continuous increase (from the 50N mark) as the compression in the tow bar incrases.
9. (Original post by ghostwalker)
Yes.

Yes.

Although the question talks about the cravan's brakes coming on, it's not just nothing or full braking. It's a gradual process. At 50N the braking due to the cravan's brakes will be 0N. At 51N, it will be a small amount. At 52N, a bit more. etc. A continuous increase (from the 50N mark) as the compression in the tow bar incrases.
Thank you very much But what I meant to ask for the last bit is: can the braking due to the caravan's brakes be 0N when the thrust in the towbar is at e.g. 53N and not 50N?
10. (Original post by sienna2266)
Thank you very much But what I meant to ask for the last bit is: can the braking due to the caravan's brakes be 0N when the thrust in the towbar is at e.g. 53N and not 50N?
It doesn't matter as far as the question is concerned. When the thrust in the tow bar is 53N there would be some braking due to the caravan's brakes, so no it's can't be 0N then.
11. (Original post by ghostwalker)
It doesn't matter as far as the question is concerned. When the thrust in the tow bar is 53N there would be some braking due to the caravan's brakes, so no it's can't be 0N then.
I've asked this because just above part iv), It say "The brakes of the caravan come on automatically when the tow bar is subjected to a compression force of at least 50N". Doesn't that mean that the brakes of the caravan could've come on at say 53N?

If not, then what is the significance of the at least 50N?

Many thanks for the help so far btw
12. (Original post by sienna2266)
I've asked this because just above part iv), It say "The brakes of the caravan come on automatically when the tow bar is subjected to a compression force of at least 50N". Doesn't that mean that the brakes of the caravan could've come on at say 53N?

If not, then what is the significance of the at least 50N?

Many thanks for the help so far btw
The car could suddenly brake, in which case the compressive force would leap well above 50N, and the caravan's brakes would come on. So, yes, in that circumstance they could come on at a higher value - depending on how long they take to react. This is really getting down to the real life situation, rather than the idealised mathematical one.

Since the braking is gradual, the caravan's brakes will start to be applied when the compression is 50N, and will initially be 0N.

Perhaps the "at least" is just poor wording. I wouldn't worry about it.
13. (Original post by ghostwalker)
The car could suddenly brake, in which case the compressive force would leap well above 50N, and the caravan's brakes would come on. So, yes, in that circumstance they could come on at a higher value - depending on how long they take to react. This is really getting down to the real life situation, rather than the idealised mathematical one.

Since the braking is gradual, the caravan's brakes will start to be applied when the compression is 50N, and will initially be 0N.

Perhaps the "at least" is just poor wording. I wouldn't worry about it.
Sorry to be so annoying
So the caravan's brakes would not come on(start to be applied) when the thrust in the towbar is 53N? Is that what you mean?

Cause I was thinking that the at least 50N bit was to do with the caravan's brakes starting to be applied when the thrust in the towbar is 50N or 51N etc.. so the caravan's brakes doesn't necessarily have to start being applied when the thrust in the towbar is 50N?

Edit: I get it now
14. (Original post by ghostwalker)
The car could suddenly brake, in which case the compressive force would leap well above 50N, and the caravan's brakes would come on. So, yes, in that circumstance they could come on at a higher value - depending on how long they take to react. This is really getting down to the real life situation, rather than the idealised mathematical one.

Since the braking is gradual, the caravan's brakes will start to be applied when the compression is 50N, and will initially be 0N.

Perhaps the "at least" is just poor wording. I wouldn't worry about it.
Thank you very much! Just re-read this and understand it now! Many thanks once again So just ignore the questions I asked before because you answered them here

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