# An application of Dynamics, very difficult, really need help

## The Attempt at a Solution

please help with this question.

## Answers and Replies

NascentOxygen
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The template has yet to be filled in ...

Relevant equations??

Your attempt??

The template has yet to be filled in ...

Relevant equations??

Your attempt??
actually, i can't even understand it. so, please try to help with this question

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CWatters
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Which bit don't you understand?
Try describing the motion of the print head in your own words.

PS: Forum rules don't allow us to solve homework for you. We can only help you solve it by giving hints or correcting errors in your own attempt.

I'm sure you understand at least some of the problem statement. ..You know what a print head is? You know it has mass? You know masses accelerate when forces are applied to them? You may have heard of Newton's laws or the equations of motion?

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Which bit don't you understand?
Try describing the motion of the print head in your own words.

PS: Forum rules don't allow us to solve homework for you. We can only help you solve it by giving hints or correcting errors in your own attempt.

I'm sure you understand at least some of the problem statement. ..You know what a print head is? You know it has mass? You know masses accelerate when forces are applied to them? You may have heard of Newton's laws or the equations of motion?
ok. sorry. when i tried again, i could do the calculations part

but can you explain to me the part iv. I got a lot of problems with the applications part. the only info I know about it is what is mentioned in the question

and also, for part (a), i needed to explain whether acceleration and velocity can be in different directions. for part (b), I had to explain whether resultant force and acceleration can be in different directions.
i think they should be related.

+ that part is 6 points. so, I don't think it's a simple reason.

so, please help me with that part

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NascentOxygen
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i needed to explain whether acceleration and velocity can be in different directions. f
When a ball is thrown up in the air, its velocity for a while is directed upwards, yet all the time its acceleration is g, and that's directed downwards. A moving object can be brought to rest if you give it an acceleration opposite in direction to its velocity',

When a ball is thrown up in the air, its velocity for a while is directed upwards, yet all the time its acceleration is g, and that's directed downwards. A moving object can be brought to rest if you give it an acceleration opposite in direction to its velocity',
well, these parts about directions of acceleration and force - i already explained.
i was thinking may be they are related to the last part
there's only the last part that i can't answer right now.
thanks anyway

NascentOxygen
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I don't know what answer they are looking for, but maybe noise, vibration, or heat buildup?

Perhaps something to do with the time required to start/stop/measure the ink flow to each dot?

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I don't know what answer they are looking for, but maybe noise, vibration, or heat buildup?
would that suffice for 6 points?
can you come up with a suitable reason that could be linked with parts i, ii and iii and the parts a and b that i mentioned (relations between directions of accelration and velocity and force and acceleration)

I don't know what answer they are looking for, but maybe noise, vibration, or heat buildup?

Perhaps something to do with the time required to start/stop/measure the ink flow to each dot?
during my calculations, i found times (for the forces) to be 8.66ms and 2.88ms
does this help???

ii was 11.5 millisec
and iii was 51.2 per sec

anyone please?

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CWatters
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for iv) the most obvious thing is the actual time to print the character once the head has stopped moving.

As for what would you change to make printing faster... What factors effect how fast the print head accelerates?

for iv) the most obvious thing is the actual time to print the character once the head has stopped moving.

As for what would you change to make printing faster... What factors effect how fast the print head accelerates?
can you be a bit more clear. i can't get what you are trying to say here?

anyone please. can you suggest your ideas.
and how can changes be made to increase the speed?

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haruspex
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there's only the last part that i can't answer right now.
It's an ugly question. You've calculated the print speed based on the data given. Why might it print more slowly? Because the data provided are unreliable?
If it is, e.g., a matter of heat build up, then the data should have been qualified, something like "in tests of printing a few characters..."

For speeding it up, based on your equations, which data modified in which directions, would increase the print speed?

Well, they've given you the mass, the forces and the distance the head needs to cover. You've got to know that everytime the printer gets ready to print a character, it's velocity is zero. But, force acts on it AFTER printing the character, so then you've got to calculate the velocity it attains while covering the distance to the next character (2.5 mm) just before the retarding force stops it again to zero velocity. So all you've got to worry about is one cycle of 'printing, moving and stopping', the graph will repeat itself. Use Newton's laws of motion with the proper signs for acceleration and you will get all the skeleton data required. The last part of the question is a real world scenario given to you, they've asked you for physical conditions, so perhaps it could be friction, heating, mechanical advantage loss due to improper parts and all that kind of stuff. So then you know what changes you have to make.

haruspex
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Well, they've given you the mass, the forces and the distance the head needs to cover. You've got to know that everytime the printer gets ready to print a character, it's velocity is zero. But, force acts on it AFTER printing the character, so then you've got to calculate the velocity it attains while covering the distance to the next character (2.5 mm) just before the retarding force stops it again to zero velocity. So all you've got to worry about is one cycle of 'printing, moving and stopping', the graph will repeat itself. Use Newton's laws of motion with the proper signs for acceleration and you will get all the skeleton data required.
I believe PhysicStud01 did all that successfully to arrive at the answer in post #11.

I believe PhysicStud01 did all that successfully to arrive at the answer in post #11.

Oh that's good.....I failed to notice. Well then I guess PhysicStud01 has solved his probem :)

Oh that's good.....I failed to notice. Well then I guess PhysicStud01 has solved his probem :)
i've solved the problem. only the part o=about physical problems, ... i can't figure
as for the calcualtions, it's ok

The last part of the question is a real world scenario given to you, they've asked you for physical conditions, so perhaps it could be friction, heating, mechanical advantage loss due to improper parts and all that kind of stuff. So then you know what changes you have to make.
I don't think there are any calculations involved....the numerical answer you computed only works in ideal situations (like zero friction, drag, etc). The answer to the last part must be qualitative not quantitative because they haven't given you any data to compute losses in speed due to the so called 'physical conditions'. So you've got to think of the real world and figure out what makes the printer work slower and suggest necessary changes.

For speeding it up, based on your equations, which data modified in which directions, would increase the print speed?
haruspex has put it in a good way......just have a closer look at the quote
Strictly speaking from the questions perspective perhaps the force acting to accelerate the printer could be increased, the actual head could be made lighter so it accelerates faster....etc (gosh am I breaking any guidelines!?)

CWatters
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for iv) the most obvious thing is the actual time to print the character once the head has stopped moving.

As for what would you change to make printing faster... What factors effect how fast the print head accelerates?

can you be a bit more clear. i can't get what you are trying to say here?

Well I would try and make the print head lighter.

F=ma
so
a=F/m

A lighter head would accelerate faster so it would take less time to move between characters and give a faster print rate.

You could also increase the forces applied to the print head.

or both.

any other suggestion. because of the fact that the answer is 6 points. So, I'm thinking maybe there's some more complicated reason, but a reason that can be reached from the simple concepts that need to be known at A-level??

CWatters
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How about overshoot? I suppose if the drive system wasn't very "stiff" the head might carry on moving after the drive has stopped due to the inertia of the head. That might allow the head to overshoot the required position, or perhaps oscillate about the required position before coming to rest. So after commanding the head decelerate and stop you might have to allow a "settling time" for oscillation to die away before printing the character? I don't know if is an issue for real printers but it's plausible.

haruspex
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you might have to allow a "settling time" for oscillation to die away
I thought of that, but shouldn't that be part of the 8ms already specified for how long it takes to print the character?

I knew a fellow who used to work in a printer test lab for a large computer company. He got thrown out of the lab for sending a printer a document in which each line consisted of a pair of characters separated by various gaps. He found the natural frequency and shook the printer apart.