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I think it's about materials. A pebble, with the same mass as the bug, would crack my windshield. In fact, I have cracks in my windshield from probably pebbles from the truck in front of me.
At some speed, certainly. Sandblasting clear glass will produce frosted glass.jaketodd said:Ok, imagine driving at 50 mph through a sandstorm. Wouldn't that mess up your windshield?
You're car would generally have momentum, not force. As does the bug. When the bug hits the windshield, it decelerates very rapidly (this is basically an impulse force, or more thoroughly an elastic collision). The question you should ask is: how much does your car (or windshield) decelerate? It is negligible because the car is massive compared to the bug.jaketodd said:F=ma
So my car has an incredibly high amount of force.
If the bug gave that back, it would surely crack my windshield.
That idea was debunked just a few posts ago!valenumr said:You're car would generally have momentum, not force. As does the bug. When the bug hits the windshield, it decelerates very rapidly (this is basically an impulse force, or more thoroughly an elastic collision). The question you should ask is: how much does your car (or windshield) decelerate? It is negligible because the car is massive compared to the bug.
Not everyone is an expert and everyone makes mistakes. You still need to exercise judgement over what is posted on here.jaketodd said:You guys are the experts.
Force is equal to mass times acceleration (F = m*a). Momentum is mass times velocity (p =m*v) If you consider acceleration is change of velocity with respect to time (a = dv/dt), you can work out that force is also equivalent to change in momentum with respect to time. So I was just trying to clarify that your car doesn't necessarily have a pre-existing "force" if it is moving at a constant velocity, but it can still impart a force (change of momentum) on the bug, because the car has momentum with respect to the bug.jaketodd said:If it's momentum, not force, then why does Newton say equal and opposite force? You guys are the experts. And I believe you. Just a confusing way of wording it I think - equal and opposite force. It conjures ideas of the bug hitting your windshield with as much force as the car hitting it - like a head on collision with a car of equal mass, except it's a bug doing it.
When we are analyzing the details of energy transfer in inelastic collisions (ones in which some of the kinetic energy is spent cracking, breaking, squashing, heating, deforming, spattering things like bugs and windshields) then yes, the characteristics of the materials involved are important.jaketodd said:Elastic. So it does come down to materials science?
It seems like you're still not getting Newton's 3rd law. The bug does hit the windshield with exactly the same force (opposite direction, of course) as the windshield exerts on the bug. Really!jaketodd said:If it's momentum, not force, then why does Newton say equal and opposite force? You guys are the experts. And I believe you. Just a confusing way of wording it I think - equal and opposite force. It conjures ideas of the bug hitting your windshield with as much force as the car hitting it - like a head on collision with a car of equal mass, except it's a bug doing it.
Which is exactly what happens if you do the calculation.jaketodd said:It conjures ideas of the bug hitting your windshield with as much force as the car hitting it
I don't think you actually have to ignore anything in your computation since you are computing conservation of momentum, not conservation of energy. In other words, you are not assuming an elastic collision, since momentum is conserved whether the collision is elastic or inelastic (which this collision is).Nugatory said:I have simplified the calculation by ignoring the tiny amount of kinetic energy that was spent turning the bug into bug pulp.
Because it's not momentum, it's the rate of change in momentum. There's a big difference. Newton just used the common name for the rate of change of momentum. Reread this:jaketodd said:If it's momentum, not force, then why does Newton say equal and opposite force? You guys are the experts. And I believe you. Just a confusing way of wording it I think - equal and opposite force. It conjures ideas of the bug hitting your windshield with as much force as the car hitting it - like a head on collision with a car of equal mass, except it's a bug doing it.
jbriggs444 said:There are different ways to state Newton's third law.
A "force" can be understood as one important aspect of an interaction between two objects. It tells you how fast momentum is being transferred from one object to the other.
Large force: Large rate of change of momentum.
Small force: Small rate of change of momentum.
That is pretty much the second law. F=ma and ma is the rate of change of momentum (as long as mass is not changing).
The third law asserts that the rate at which momentum is increasing in the one object (the force of A on B) matches the rate at which momentum is decreasing in the other (the additive inverse of the force of B on A). It is essentially a statement that all interactions conserve momentum.
That’s fair, but I still have to ask: if you don’t like the wording, what might be a better way of stating Newton’s third law?jaketodd said:"The wording is fine and cannot be blamed for your misguided ideas."
Ya that's really constructive.
But a bug doing it is not the same as a car of equal mass doing it at the same relative speed. That's the whole point. Force is not mass alone, it's mass times acceleration. If a bug hits your windshield, the bug decelerates a lot, but its mass is tiny; the car has a large mass, but a tiny deceleration. The result is a tiny force--the same tiny force both ways.jaketodd said:It conjures ideas of the bug hitting your windshield with as much force as the car hitting it - like a head on collision with a car of equal mass, except it's a bug doing it.
Oh gosh, I can see how this might be confusing the way it is written. I wasn't referring to the collision. I was referring to the properties of the car before the collision.PeroK said:That idea was debunked just a few posts ago!
A pebble can shatter the windscreen, but it's not going to slow the car down significantly.valenumr said:Oh gosh, I can see how this might be confusing the way it is written. I wasn't referring to the collision. I was referring to the properties of the car before the collision.
Spark plug fragments can do a number on the tempered glass in side windows.PeroK said:A pebble can shatter the windscreen, but it's not going to slow the car down significantly.
I really don't know. You guys are the experts, and it seems a conclusion hasn't been reached yet. Maybe Newtons 3rd Law should be introduced or worded with caveats like the ones we see in this thread.Nugatory said:That’s fair, but I still have to ask: if you don’t like the wording, what might be a better way of stating Newton’s third law?
What caveats? The force of the bug on the windshield and the force of the windshield on the bug are exactly equal and opposite with no ifs, ands, buts or other hedging. That has been stated explicitly and repeatedly, starting with the very first reply you received and most recently in posts #47, #48, and following.jaketodd said:I really don't know. You guys are the experts, and it seems a conclusion hasn't been reached yet. Maybe Newtons 3rd Law should be introduced or worded with caveats like the ones we see in this thread.