How come if a car traveling at constant velocity hits me I feel

>>8389978
>How come if a car traveling at constant velocity hits me I feel a force?
How do you know? Have you ever been hit by a car before?

You actually don't feel a force. This is a fact, provable by the laws of physics.

>>8389978
>How come if a car traveling at constant velocity hits me I feel a force?

You don't feel a force, though. But don't take my word for it, go see for yourself!

>>8389978

normal force.

>>8389978
Force = mass x acceleration
When you're standing still and a car hits you. You have the mass of the car multiplied by the change in velocity over time of you going from still to however fast the car was going. Faster velocity and higher mass means more force.

>>8389989
except that a car doesn't slow down when it hits you, so a=0 and F=m*0=0

>>8389993
Physicists BTFO

>>8389993
You're absolutely right, make your own physics book please.

All bait aside, these are concepts that students actually struggle with.

For instance, if a car accelerates to some speed and then applies the breaks, they think the car has a forward force that is being decreased by the breaking force, and that the car will come to a constant velocity or rest when the forces are equal. The reasoning being, that if that car hits you, you still feel a force so there must have been a force in that direction.

Awful misconceptions like that. Or they fail to understand why when you throw a ball in the air, it continues going up despite gravity acting on it. And then at what point does it begin to come back down? They run through the calculations but they don't get it.

I think it's because physics classes jump right into forces while mostly avoiding inertia. It also doesn't help that students were trained from high school to think in terms of F = ma, which can be dangerously misleading if you don't know how to use it correctly. But since they're unable to introduce the far superior F = dp/dt (students haven't had calculus), you get a lot of misconceptions.

A lot of students don't even think to relate momentum and force or momentum and energy, and that's even after a college level mechanics course. The standard textbooks don't hit these relations hard enough before jumping into energy, which of course is used to solve most of the problems from there on.

>>8390022
>the far superior F = dp/dt (students haven't had calculus), you get a lot of misconceptions.
This. I didn't understand forces until I learnt this

>>8389978
if a car is truly traveling at constant velocity, then you can't prove that it hit anything.

>>8390022
This is why I'm extremely glad I had a literal poet for a highschool physics teacher.

Nearly his entire class just focused on physical intuition, and he shoehorned in a intuitive version of integrals and derivatives at the very beginning to help students move up and down through kinematics.

I haven't needed to actually learn any more physics until my junior year.

>>8390022

>F = dp/dt

Is that the derivative of momentum with respect to time? That's fucking sick.

>>8389978
Because the car is not actually traveling at a constant velocity. There is some tiny variation.

>>8390269

Makes that whole "Velocities don't change unless acted upon by a force" seem a lot less mystical, because you've DEFINED a force as a rate of change of velocity

>>8389993
Energy is still being put into the system. If the driver shifted into neutral immediately before hitting you, the car would slow down.

>>8390269
well it's actually not hard at all and you technically don't even need calculus
f=m*a but a=v/t
so f=mv/t=p/t

>>8389993
Define "hitting you." It's not instantaneous. There's an impulse, a force exerted on you over time changing your momentum.

>>8389978
Momentum

>>8390290
you're the man now dog

>>8390290

I don't think you understand what technically means. What you calculated are averages that are far less fundamental. The differential formulation really is the key here, so I'm not being pedantic either.

Keeping in mind that these are vector quantities,

F = dp/dt, v = dr/dt, a = d^2r/dt^2, dW = F dot dr

with a few related concepts, those will get you through 90% of basic mechanics. With what you have, you won't even get close to a quarter of the way because you've already lost a considerable amount of information.

In fact, you'll derive lots of erroneous results that way if you're not careful (which you won't be).

[I am wasting time and so there's pretty much a separate rant beyond here]

In college, physics 1 (mechanics) classes tend to be watered down junk because it's focused on reteaching all the nonsense and picked up in high school physics or "college physics" that relied only on algebra and trig, rather than exploring more advanced material.

Of course you can keep on increasing the mathematical complexity, and I'm not saying that's necessary, but calculus 1 really truly is fundamental and I'd even say the bare minimum. You either abandon most of the math and take a primarily qualitative approach or your introduce some basic calculus. It is absolutely irritating to see 50 page formula sheets thrown at students, knowing they have no idea where any of these equations came from. Then they shrug their shoulders when students come to hate physics and math and say they don't know what to do.

>>8389978
acceleration and mass...

literally the most basic equation in all of physics

>>8389993
Nice

>>8389993
Einstein reborn

>>8390423
>tfw when your physics 1 course was calculus based an mostly theoretical
It is a good feel.
And mind you Analysis was a course we had to do at the same time, no way to take it beforehand.
They pretty much tell everyone that's not willing to learn the necessary calc on his own to fuck right off.
I mean it's not optimal but certainly efficient.

>>8389978
>How come if a car traveling at constant velocity hits me I feel a force?
Because humans are deformable, not rigid bodies.

>>8390022
>F = dp/dt
This is cool, I get it, and it is new to me. I did A Level physics a long time ago and know of F=ma.

So does this imply that when talking about Force we aren't talking about the force 'of' the hurtling vehicle, but the change in momentum of the person it hits? That is, the vehicle doesn't 'have' a Force, but the person being hit *experiences* a Force as their p alters. Is this correct?

>>8389993
But the car is slowed down by hitting you.

>>8389993
10/10

Since there are physics students in the thread, when does a physics student generally learn the Lagrangian formalism? First year, second year?

>>8391308
High school

>>8389993
> Being this dumb and then samefagging hard to make yourself seem smart

The sum of all forces is 0, however its momentum is not. If the car travels 100 km/h = 27,78 m/s and weighs 1000 kg, then its momentum becomes p = m*v = 1000*27,78 = 27780 kg*m/s.
Now the car hits you. You as a person weigh 200 kilos cuz youre a fat neckbeard. The car hits you with its momentum and transfers a part to you.
v' = ma*v/(ma+mb),
this v' = 1000*27,79/1200 = 23,15 m/s.
With p_begin = p_end
so p_car = P_car + P_neckbeard
27,780 = 23,15*1000 + P_neckbeard
P_neckbeard = 4,630

So V_neckbeard = 4630/200 = 23,15 m/s
lets say the car hits you for 0,5 seconds,
F = dP/dt = m*a = M *dv/dt = 200*23,15/0,5 = 9260 Newtons.

That concludes your fucking mongloid physics lesson for this week newfag.

> Lel he took the bait
Doesnt matter I have to revise this shit for an upcoming exam anyways, thanks for the exercise

>>8391317
Epic. But it's related to multivariable calculus, so wouldn't it be maybe second semester of first year (or first for a very rigorous school)?

>>8391308

You generally take the upper division mechanics course in your third year. This is mainly because if you start at a calculus 1 as a freshman, you won't finish differential equations until Spring of sophomore year. Then you start all your upper division courses that next semester (Junior year).

>>8391188

Do realize that that the forces are equal and opposite (third law) so the dp/dt is the same for both the car and the person. The person has small mass and and a large change in velocity, but the car has a small change in velocity but it's compensated by having a large mass and so the magnitude of dp/dt is the same

>>8391333
Just finished multivariable calculus (I study chemistry). Is that enough to start teaching myself Lagrangian mechanics?

>>8391344
Yes, the other math tools you need will be developed as you go through the material (e.g. calculus of variations and some basic differential equations.)

>>8391501
Thanks man I'm gonna start right now

>>8391343
Ah, of course, thanks.

>>8391960

I realize I didn't really answer your question, but yes you have the right idea.

No it doesn't make sense to associate any kind of force with car. The net force acting on the car is zero. What you do associate with the car is momentum. By Newton's first law, it has a tendency to want to keep at the same velocity v and to keep in that same direction, which is why it's continuing to come toward you. It doesn't make sense to introduce forces at this stage. A force got it going that way but when it has a constant velocity, the only thing keeping it going that way is inertia.

Now when the person gets hit by the car, the instantaneous rate of change of their momentum is the force. That is dp/dt = F(t) (explicitly noting that force is a function of time depending on the collision).

If we rearrange this,

dp = F(t)dt

So yes they experience a force as their momentum changes but you must realize that the change in momentum associated with the collision is done as soon as the collision is done. Once that interaction time is over there is no more change in momentum associated with the collision. In real life, you would continue on at your new velocity if it weren't for other forces like air resistance and friction bringing you to rest.

To find dp, we need to integrate

P(t) - P(0) = integral [ F(t)dt ] from 0 to t

So unless we actually know how the force changes with time during the collision (the force isn't constant during the collision), there isn't too much we can do to find it explicitly. But the total change in momentum over the interaction time (0 to t) is still useful this expression is called the impulse.

J = P(t) - P(0) = mv(t) - mv(0)

Well by Newton's third law we know that the car experiences an equal and opposite force during the collision and so

J' = P(t)' - P(0)' = Mv'(t) - Mv'(0)

J = - J'

P(t) - P(0) = - P'(t) + P'(0)

Rearranging gives you

P(0) + P'(0) = P(t) + P'(t)

which is the conservation of momentum

>>8392200
Okay, I understand all that. I just have one question. What exactly is causing the instantaneous change in momentum? Does that force have a name? I know we can solve this problem using Lagrangian mechanics but can't we solve it with Newtonian mechanics too?

>force is a function of time depending on the collision
What form do these forces usually take? Like I know we can model billiard balls as a spring but I don't know why.

>>8389984
Underrated

>>8389993
The car actually *speeds up* when it hits (((you))).

>>8391321
>I have to revise this shit for an upcoming exam anyways

underage ban coming right up

File: 1444591201683.jpg (18KB, 313x286px) Image search: [Google]

18KB, 313x286px

>>8389993
Holy shit how did he do it.

>>8389993
>a=0 and F=m*0=0
Le force on person and not a force acting on the car maymay

>>8389993
*Hard Drivin' crash replay theme plays*

>>8392267

I'm not entirely sure what you're asking but

all contact forces are electromagnetic in origin. The momentum changes because of the electron charge repulsion at the collision point.

> know we can solve this problem using Lagrangian mechanics but can't we solve it with Newtonian mechanics too?

I completely lost you here.

>What form do these forces usually take?

In terms of a real collisions it's unknown because of the complexity of the interactions, but it's not very useful anyway. There are an endless number of factors that are for more important that need to be determined.

>Like I know we can model billiard balls as a spring but I don't know why.

It gets complicated but it's mainly because you can. A lot of things can be modeled as springs, like the bonds between atoms.

>>8392267
>I know we can solve this problem using Lagrangian mechanics but can't we solve it with Newtonian mechanics too?
You can use either, they're just different mathematical models of physics. Not to be rude, but you don't seem to know physics so how do you know Lagrangian mechanics? It's relatively advanced.

>>8389978
I'm fascinated and dissapoint about how a shitty easy first year problem troll /sci/.

>>8392391
He just know how to de the exercises, but nothing about the physics beyond.

>>8392411
Once again, I hate to be rude but how the fuck? Is he like a pure math guy?

>>8392391
>>8392435

Based on his question and he how he phrased it, he clearly doesn't "know" Lagrangian mechanics. He out of an interest in physics is probably just familiar with it on a Wikipedia level and knows you can use it to simplify to certain problems.