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/sci/ - Science & Math

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1057788 No.1057788 [Reply] [Original]

A brick of mass <span class="math">m[/spoiler] is falling through the air with downwards acceleration <span class="math">g[/spoiler] when it impacts a tabletop.

What is the magnitude of the force the brick exerts on the table, and from which of Newton's laws can this be deduced?

and no, it's not homework.

>> No.1057811

Hello 3rd grade

>> No.1057831

Thats easy. Force=Mass*Gravity
F=mg or F=ma a=Acceleration
This is deduced from Newtons 2nd Law of Gravity

>> No.1057839

this is like first day of retard physics class.

>> No.1057833

F=uck you, nigger.

>> No.1057850

Why would you present us a third grade physics problem if it weren't your homework?

>> No.1057853

Newton's second law. Also you forgot to provide the mass m.

Average red building brick is 1.2 kg. Falling at gravity, 9.8ms**2.

F = 1.2kg * 9.8ms**2
F = 11.76 kg**2

>> No.1057863

Is it not an integral with g geing the upper limit and h being the lower limit?

>> No.1057865

lol at everyone who thinks the velocity of the brick is not relevant.

>> No.1057869

Not enough information to solve the problem. The easiest way to solve it would be to measure the impact time, given the velocity of the brick. If you wished to solve analytically, you'd need to know the height from which it was dropped, the elasticity and geometry of the brick, the elasticity and geometry of the table, and you'd be doing one hell of a complex analysis - probably finite element.

At the essence of the problem is that some parts of the brick (and table) are accelerating faster than others.

>> No.1057876

>>1057831
Doesn't F=ma mean that an object of mass <span class="math">m[/spoiler] accelerating at rate <span class="math">a[/spoiler] must be subject to a net force F?

I didn't think it covered how much force an object "imparts" when it impacts another object.

>> No.1057900

>>1057876
It still covers the force during impacts, but if you're going to approximate the body as a point object you're going to need the duration of the impact - and then you'd only be able to find the average force.

>> No.1057910

>>1057876
Perhaps we could speak of Surface area of contact,

>> No.1057924

>>1057869
You're aware you can solve an equation with just the variables, yes? like F=ma is applicable to everything within the frame of newtonian mechanics.

>> No.1057936

>>1057853
Alice, you're retarded. Tripfags should make a point to not be wrong, being the only people who can make a reputation here.

>> No.1057938

This is why I hate physics...

>> No.1057941

>>1057924
You are aware that you need boundary conditions to solve a differential equation, right?

>> No.1057942

>>1057924
wat

>> No.1057944

>>1057900
No, it doesn't.

What matters is the momentum of the object - the acceleration is irrelevant except to calculate the velocity at the time of impact.

All you idiots upthread who said the problem was 'simple' when infact it is impossible. GTFO.

Here's a hint - what if the brick is dropped from zero height - why dosn't it break the table? Because it exerts less force.

>> No.1057965

>>1057944
wrong again

>> No.1057967

>>1057900
Well according to Wiki,

> Second Law: A body of mass m subject to a force F undergoes an acceleration a that has the same direction as the force and a magnitude that is directly proportional to the force and inversely proportional to the mass, i.e., F = ma.

It doesn't say anything about impacts.

>> No.1057980

>>1057965
He's right. This is an impulse problem; ie: you need to know the distance and/or time of deceleration of the brick.

>> No.1057984

>>1057944
>>1057967
Does an object (with some mass) have a velocity before an impact? Does that object have a velocity after the impact (zero or otherwise)? Are these velocities equal? If not, must the object not have undergone an acceleration? Thus, the second law is still valid (although of little use).

>> No.1057989

WHY THE FUCK DO YOU IDIOTS RESPOND TO SHITTY THREADS LIKE THIS

ARE YOU SUCH FUCKING IDIOTS THAT THIS IS THE ONLY CALIBER OF THREAD YOUR PUNY BRAINS CAN UNDERSTAND ?

GET THE FUCK OFF MY /SCI/

>> No.1057995

>>1057942
>>1057941
I don't know.
but you can solve an equation without values by assigning letters for unknowns

>> No.1058003

>>1057980
Your post is contradictory; you say you need to know the "time of deceleration" to solve the problem. However, the first sentence of >>1057944 (which you apparently agree with) was "No, it doesn't" in response to >>1057900 stating it was still valid to use Newton's Second Law if you had the "duration of impact".

Unless "time of deceleration" and "duration of impact" are appreciably different in your opinion.

>> No.1058014

>>1057965
Gah - idiots everywhere.

Take two cars both accelerating at (say) 1m/s^2 - now let them to accelerate for one second before colliding head on. Alternatively, allow them to accelerate for several minutes before colliding.

Now, put yourself between the two cars. Yo will discover that the force applied to squish your puny body will be dramatically different, due to the difference in kinetic energy per momentum.

>> No.1058010

>>1057995
wat

>> No.1058024

>>1058014
Yeah, but that's not what you said.

>> No.1058034

>>1058014
*per as in 'as in', not per as in 'divided by'

>> No.1058035

>>1058010
stop sayin wat fag. answer my question.

>> No.1058037

>>1058014
The post you replied to as wrong was not wrong in stating you were wrong about the third nested quoted post being wrong.

GET IT!?

>> No.1058042

>>1058035
wat

>> No.1058055

>>1058042
wat

>> No.1058076

OP here. What the fuck happened in this thread?

>>1057984
That only implies that a force must have acted on the brick, it doesn't say anything about what kind of force the brick exerted on the table.

>> No.1058106

You can't really use the 3rd law. It's not F=mg.

The opposite force to the dropping brick is the earth moving upwards.

The brick hitting the table is a different matter and the change in velocity must be taken into account.

>> No.1058190

Jesus H. Christ guys. It's sad how many of you are wrong.

The brick is traveling at some velocity v when it impacts the table. If we knew what height the brick was dropped from we could calculate its velocity. The brick decelerates from v to 0 over some short distance/time. If you want to throw in an approximation lets say 1/10th of a second. Now you can find the deceleration "a". NOW you can apply F = ma.

>>1057910
Surface of contact only matters if you want to calculate the momentary pressure exerted as pressure is expressed in force per unit area.

>> No.1058223

OP again.

>>1058190
Ok, so you know what it's deceleration is. So if you knew for sure that the table was what had caused that deceleration, you could use the third law to work out what force the brick applies to the table. So how do you work out what caused the brick to decelerate?

>> No.1058274

>>1058223
>So how do you work out what caused the brick to decelerate?
It hit the fucking table. That's why it decelerated. You said so yourself in the previous sentence.

It doesn't even matter why the brick decelerated so long as you know in what distance/time it did so.

>> No.1058355

>>1058274
Sure, the table must have exerted a force on it and there for was at least PARTLY responsible for its deceleration. But there could have been some other unknown force acting on it as well, so we can't use that deceleration to deduce the force that the table exerted on it.

>> No.1058367
File: 17 KB, 320x352, TROLL_01.jpg [View same] [iqdb] [saucenao] [google]
1058367

>>1058355
Are you being intentionally dense?

>> No.1058375

>>1057785

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>> No.1058395

>>1058367
Why, because I'm asking questions? Surely a system of physical laws as renowned as Newton's isn't dependent on baseless assumptions like that a table is wholly responsible for a brick's deceleration.

>> No.1058400

>>1057786

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>> No.1058684

arrrrhh, damnit /sci/. Fine, I'll just go read the Principia.

>> No.1058718

>>1058395
How is that a baseless assumption?

>> No.1058726

To get an estimate, find the elastic modulus of the brick and table and do unit analysis. You will be off by a factor of two or 1/2 or 1/3 or 2/pi or something, but computing that factor will required a detailed computation of how the brick vibrates which will be a lot harder.

>> No.1058760

Why aren't here any anons who EXACTLY know what they're talking about.

Also: bricks, how do they work?

>> No.1058792

It depends on the table.

A weak table can only withstand so much force before it collapses and no more forces are exerted. A strong stiff table will decelerate the brick quickly, which requires a strong force, and by Newton's third law the brick exerts a strong force on the table.

>> No.1058800

>>1057787

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