Math Is Fun Forum

paulb203

Member

Registered: 2023-02-24

Posts: 313

Momentum

Momentum is instantaneous, yeah?

I’m 90kg, walking east at 3m/s, my momentum (p) is;

p=mass x instantaneous velocity

= 90kg x 3m/s

=270kg m/s

Yeah?

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What’s the significance of momentum?

I’m guessing Force is part of it.

If I bump into you while I’m walking at 3m/s, with my mass of 90kg, I’ll hit you (push you?) with quite a force

But I’d hit you with a greater force if I was travelling at 6m/s (p=540kg m/s), or I had a mass of 180kg (also p=540kg m/s)

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Also, how do I work out what that force is?

F=ma

F=90kg x a

What is a?

a=v-u/(t)

Answer; we don’t have enough information? We only have instantanous v, we don’t have final v and intial v. And what about t? This isn’t happening over x seconds, it’s happening instantaneously, yeah?

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Prioritise. Persevere. No pain, no gain.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,621

Re: Momentum

If you are travelling with a constant momentum then it is a timeless measurement.  If you hit something there is an impulse as momentum is transferred. This takes place over time.

Have a look at this MIF page as it gives a good account of this:

https://www.mathsisfun.com/physics/momentum.html

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

paulb203

Member

Registered: 2023-02-24

Posts: 313

Re: Momentum

Thanks, Bob.

I thought I'd replied to this already. Must have dreamt it

Anyway. I look forward to checking out the MIF page, although those pages, for me at this stage, tend to go beyond GCSE level quite quickly, I think (?).

In the meantime, some basics.

With GCSE physics questions (such as, A man with a mass of 90kg is travelling at 3m/s; what is his momentum?) do we assume that the 3m/s is his instantaneous velocity? And by instantaneous momentum I meant; as opposed to the average momentum that we would get if the 3m/s was his avg v.

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Prioritise. Persevere. No pain, no gain.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,621

Re: Momentum

do we assume that the 3m/s is his instantaneous velocity?

Yes, that's all you can do.  Momentum must be a vector measure as it has velocity as a component. If the man is accelerating then his momentum is going up.

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob