SpeedxWeight=force?

[Deleted User] · 20-08-2021 2:45am #1

I am a dog walker. That's my job. I was rubbish at school so I have a job that doesn't need much maths. Until now. Now this isn't necessary for me to know, it's curiousity.

I have a dog called Frasier. I thought this was so completely unique, so I was not impressed when I got a new client who also had a Frasier!

Now both these boys are young and a bit mental. My Frasier is a Newfoundland so while he runs at me and throws his 50,kg at me, he is half the speed of Other Frasier, who weighs half as much.

People tell me mathematically the two dogs should hit me with the same amount of force, but smaller Frasier is easier to brace myself for (and slower Frasier is easier to avoid completely).

So I ask people who done school good (went to actual classes at leaving cert and got more than a D3 at pass), who would hit me harder? 25kg Frasier at X2 speed, or 50kg Frasier at x1 speed?

Thanks for indulging me.

fritzelly · 20-08-2021 3:23am

Mass and slobbering tongue are a major factor here

[Deleted User] · 20-08-2021 3:31am

https://www.boards.ie/discussion/comment/117778895#Comment_117778895

Slobber is actually just a waste product of the reaction. It takes a while to clean up too, like nuclear waste but less toxic.

This evening actually as we were eating dinner Newfie Frasier decided to shake his head. A big string slobber landed on my face, dunno what landed on my dinner but didn't care, already picked two Newfie hairs and one Labrador hair out of my food.

I do understand why some people keep their dogs outside.

riffmongous · 20-08-2021 7:51am

'acceleration X mass' not 'speed X weight'* is the Newton's 2nd law, and that's about accelerating objects, you need the kinetic energy formula I think, '0.5 X mass X velocity squared'

*Weight is a force, (mass X gravity) that's why your weight changes on different planets

Badly Drunk Boy · 20-08-2021 9:03am

The 'people' are telling you correctly, but only if the dogs are exactly 25kg and 50kg, and their acceleration towards you is double/half of the other dog's.

[Deleted User] · 20-08-2021 9:20am

The laws of classical (bio)mechanics are not for the fainthearted, OP. Plus, there is the aul quantum thing of the observation of your dog causing your dog to change. In lay person's terms this means that, when you observe your dog, it is a Newfoundland, but when you turn away, it instantly turns into a chihuahua.

Then there is the effect of the time-space continuum and general relativity, plus conditions at the moment of impact in a) the solar system and b) our galaxy.

I'd ask one of the neighbours, but I wouldn't hold your breath as the question may be impossible to answer.

magicbastarder · 20-08-2021 9:23am

a dog weighing half as much but going twice as fast has the same momentum (m * v) as the larger dog, but twice the kinetic energy (m * v^2)

Badly Drunk Boy · 20-08-2021 9:24am

I just checked what Newfoundland looks like, and as I thought, it's hairy. Add a bit of rain into the equation and a complete recalculation is needed.

FishOnABike · 20-08-2021 9:54am

You also have to factor in the law of the lever.

A Newfie planting their front paws on your shoulders at full tilt exherts more leverage than a smaller dog hitting you at waist level at a higher speed.

Donald Trump · 20-08-2021 11:25am

https://www.boards.ie/discussion/comment/117779240#Comment_117779240

The hair would increase the drag coefficient and affect the Reynolds number as well

[Deleted User] · 20-08-2021 12:55pm

https://www.boards.ie/discussion/comment/117779240#Comment_117779240

Very hairy but they are practically waterproof. It's very cool. He can get into the sea and swim and be dry twenty minutes later. They evolved to be water dogs.

seamus · 20-08-2021 1:15pm

https://www.boards.ie/discussion/comment/117779362#Comment_117779362

This is one of the reasons I didn't get on so great at physics in the leaving cert. Questions like the OP's seemed to be straightforward enough question about force, like, "A 20kg dog travelling at 20km/h jumps on a human weighing 100kg, what is the force on the human, assuming that they remain static throughout"

But then you look up the answer and it says, "Yeah, you need to consider the length of the lever distance between the two object, also don't forget the inertia coefficient and the gravitational constant".

(╯°□°)╯︵ ┻━┻

Did OK in physics in college though, so it might have been the teacher I had in school.

[Deleted User] · 20-08-2021 1:18pm

Maybe the easiest thing to do is to painstakingly model every scenario in a physics engine, that will take care of the maths. If you do this, remember the ambient temp of the room the PC is running in has to be maintained at between -12C and -13C or the calculations will be off.

magicbastarder · 20-08-2021 1:19pm

https://www.boards.ie/discussion/comment/117780263#Comment_117780263

that only applies to a completely spherical blackbody dog in a vacuum though.

Capt'n Midnight · 20-08-2021 6:52pm

https://www.boards.ie/discussion/comment/117779229#Comment_117779229

The trick here is to sidestep so their Frasier collides with yours. As the momentum is the same they will instantly stop. The excess kinetic energy will be converted into heat and could cause nuclear fusion* if they are travelling fast enough.

*not a good thing if you are nearby.

Bit cynical · 20-08-2021 8:14pm

speed . weight = power

wotzgoingon · 20-08-2021 9:10pm

https://www.boards.ie/discussion/2058204910/speedxweight-force

@budgese

So I ask people who done school good (went to actual classes at leaving cert and got more than a D3 at pass)

Wow look at you with your leaving cert. All I have is my junior cert.

[Deleted User] · 21-08-2021 12:21am

https://www.boards.ie/discussion/comment/117782643#Comment_117782643

No one ever asked for my LC after the age of 25, except for a PLC course I went into at 35 years of age and I pretty much laughed them out of it. I can't even remember my results by now, never mind remember where (or if) I kept them.

SpeedxWeight=force?

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