Stopping of A Steel Rod

Unanswerable. Insufficient (and even contradictory) information.

from responses below, we all are assuming the rod is travelling in an axial direction, because we choose to stop it with a radial force, which we assume to be a clamp?

I think the rod in either case would turn into a pretzel unless it is closely guided/restrained along much of its length

I was tempted to say instant spaghetti sculpture.

The point of contact surface smoothness needs to be known, too. Smooth steel wire and textured rebar will have very different coefficients of friction.

To be fair, I think "dead stopped" is the OP's way of saying "stopped dead", i.e. "brought to a halt". As for the 0.4 sec, that has to be interpreted as the maximum time allowed, so the calculation is of the minimum force required

Determine the total kinetic energy of the rod.This is the total amount of energy required to stop it.Parcel it into any time units you desire.

Er, ah, actually that is the total energy to be dissipated when it stops.

Yes, opposing radial forces, like opposing disc brake pads, are required.

So, the answers aren't in the back of the text book?

Why is that a surprise (rhetorical question - NNTR)?

This sort of thing is unlikely to be solved during a telephone call to the original equipment manufacturer's Technical Helpline either.

The rod should weigh around 4.54kg...

The force (F) required to move an object of mass (m) with an acceleration (a) is given by the formula F = m x a. So, force = mass multiplied by acceleration.^[1]

F(N) = m(kg) times a(m/s² )

https://www.wikihow.com/Calculate-Force

Or it weighs about 10 pounds

Interesting that it is a nice round number in imperial units.

Perhaps propelled by sling, this arrow of outrageous fortune is but a dream ...an American dream...

An object moving in a straight line that reverses direction in the same line must come to a stop before the direction changes.

A fly traveling East hits a train traveling West.

Ignoring wind, and any other common sense factors, when the fly and train meet,the fly comes to a brief total stop before it changes direction and moves West.

If the fly comes to a total and complete stop,does the train also stop?

I'm (hic) puttin' the cork back in the bottle now (hic)!

If the fly comes to a total and complete stop, does the train also stop?

No, not until it gets to the station. When it hits the fly, it just slows down a tiny amount.

This should do it. Just add a simple mechanical release mechanism

Del

Adding unnecessary complexity...
Your geometry is of too
Del

LOL
Del

Ah!

Good old Pneumatics still lives!

Ye gods! There it is, all designed, built and everything!

One wonders what damage this missile, being <...steel rod...8mm dia and 12 metre long travelling at 30 metre per second...>, will do until the correct amount of weight to be added has been determined!

It would make a rather spectacular arrow beforehand, perhaps?

Rev.3:

Homework?

You try it first then run the answer by us! And show your working!!

If we do it for you how will you learn?

ps: in a steel mill, they wouldn't bother with such stuff. There would be solid stop at the end of the line to halt the rod dead!
As for it being hot and maybe still slightly elastic/bendy.... it's rod, it will come good after the other processes. I know, I working in a steel mill..

It would take 0.4 seconds after which you stop applying the force if I am not mistaken.

Your working will be to calculate an area where you want to apply the force, the force that is required to stop and to calculate how much heat you will produce in the process.

Some side calculations will be required which you will find out if you try to answer the above questions.

Have fun!

NOT off-topic!!

Friction force is (at least largely) independent of area, so area is irrelevant, hence OT.

Not off topic. The amount of frictional force is proportional to the coefficient_of_friction x area x pressure, but pressure is force/area so area cancels out, leaving coefficient_of_friction x force. This can be demonstrated by sliding a block down an inclined plane that can be adjusted so that the block just slides without accelerating. If the characteristics of the surfaces of the block are the same, it makes no difference if the block is oriented with a small surface down or a larger surface.

https://sciencenotes.org/friction-example-problem-sliding-inclined-plane/

Agree entirely with your point that friction is independent of area.

However I did mark the post back on topic because of the first sentence, which does go a little way to address the completely nonsensical "question" in the original post:-

"how much time would it take to stop after applying the force for 0.4 sec."

To the OP: if it hasn't stopped by the time you remove the force, then, its not going to stop at all.

The rod weighs 4.8 kg.

The kinetic energy of the rod at 30M/s =??? J

The power required to dissipate this much energy is proportional to the kinetic energy of the mass.

The length of the rod required to dissipate this energy is approx ??? meters.

Hint: the rod will get warm (extra credit: calculate the KWH required)

Now you have all of the information needed to figure this out, all of which is available online.

If you find that I have made an error, please show me. Then we both can learn.

Making a presumption that the rod is smooth, what about an eddy current brake? Run the rod thru a poly bushing to hold concentricity near the stopping point and place three coils at 120° running axially. This will induce the appropriate stopping force.

The OP asked for force and time,which is Power required.
No response from the OP after giving him the required variables,so here they are:
Rod weight=4.8kg
Velocity of rod:30 M/sec
Kinetic energy of rod=2160 Joules
2160 J=7200Watts.(7.2KW)
This is the amount of power required to stop the rod.
The rod is moving at 30 M/sec.
The stopping time requested is .4 sec.
The rod will travel 12 meters during this time.
This power will have to be dissipated in a length of 12 meters.
This will raise the temperature by .965665C ( 33.7382F) above the starting temperature of the rod.

No, assuming that one succeeds in stopping the bar the average velocity during braking will be 15M/sec. Stopping in 0.4 seconds the rod will travel 6 meters.

How do you get the average velocity?

If the bar decelerates at a constant rate,however,the braking force could be applied in pulses,then you would have to average the speed at many different points within that .4 second interval.The length required could vary in that case.

However,it will still require the same amount of power to stop the rod.

Wrong on both counts:

Average velocity = total distance ÷ total time. The rest is irrelevant.

It takes the same energy to stop the rod, but not the same power.

IMHO,The amount of Power and Energy will be unchanged since the total time to stop is fixed at .400 seconds,the kinetic energy of the rod is fixed.

Slice it anyway you please,these factors are unchanged.

Perhaps I dozed off in physics class 50 years ago,or perhaps my memory has some gaps.

If I am wrong,I will appreciate being corrected.

Huh?

2160 Joules in 0.4 seconds is 5.4 KW

A difference of 0.97 ^oC is a difference of 1.75 ^oF (you've done a conversion of absolute temperature rather than difference).

You are right on the Joules,but not the temperature.

I stated"above the starting temperature of the rod."

Did you forget to add 32 degrees when converting to F?

After further consideration of my reply to you it finally dawned on me (at sunrise),that you are correct on both counts.

I was wrong.

I referred to a conversion table for the C to F conversion instead of a manual calculation.

On the Joules calculation,I don't know what I was thinking,must have mixed up terms.

As the carpenter said"Measure twice,cut once."

To paraphrase,"Calculate twice,post once."

Lesson learned.

I stand corrected.

Thanks for pointing out my errors.

...plus this is the heat that needs to be dissipated to stop it, not the power required to stop it. Were it to impact on some solid object that were not plugged into the mains, there would be no power applied to stop it and yet it would still stop.

The only thing to be experienced is the heat to be dissipated from the stopping apparatus.

Maybe I'm all wet by taking an opposite viewpoint! I take the rod motion to be axial. And the force is radial; that is, perpendicular to the direction of motion, thus it will provide no retarding force at all. Unless there is a coefficient of friction to convert the applied radial force to an axial force.