Given that force equates to mass times acceleration, so how come massive
objects travelling at a constant velocity (acceleration = zero) can cause
damage when they hit something?
After all, if I'm knocked over by a car doing a constant 70mph, I'm going
to end up a mess, aren't I? I'm going to experience force!
I'm a bit rusty on all this stuff. Can some clever-arse ******* disabuse
me of my ignorance? Cheers.

On 11/11/14 15:26, Cursitor Doom wrote:
Given that force equates to mass times acceleration, so how come massive
objects travelling at a constant velocity (acceleration = zero) can cause
damage when they hit something?
After all, if I'm knocked over by a car doing a constant 70mph, I'm going
to end up a mess, aren't I? I'm going to experience force!
I'm a bit rusty on all this stuff. Can some clever-arse ******* disabuse
me of my ignorance? Cheers.

The problem is that the car when it hits you accelerates (bits of) YOU.


--
Everything you read in newspapers is absolutely true, except for the
rare story of which you happen to have first-hand knowledge. €“ Erwin Knoll

On 11/11/2014 15:26, Cursitor Doom wrote:
Given that force equates to mass times acceleration, so how come massive
objects travelling at a constant velocity (acceleration = zero) can cause
damage when they hit something?
After all, if I'm knocked over by a car doing a constant 70mph, I'm going
to end up a mess, aren't I? I'm going to experience force!
I'm a bit rusty on all this stuff. Can some clever-arse ******* disabuse
me of my ignorance? Cheers.

Because when it hits you it decelerates (change in velocity) and that hurts.

Another Dave

--
Change nospam to gmx in e-mail.

On 11/11/2014 15:26, Cursitor Doom wrote:
Given that force equates to mass times acceleration, so how come massive
objects travelling at a constant velocity (acceleration = zero) can cause
damage when they hit something?
After all, if I'm knocked over by a car doing a constant 70mph, I'm going
to end up a mess, aren't I? I'm going to experience force!
I'm a bit rusty on all this stuff. Can some clever-arse ******* disabuse
me of my ignorance? Cheers.


If the car stops exactly as it hits you then nothing will happen to you.
The force is what accelerates you to ~70mph when it does hit you.

Its conservation of momentum that you should read next.

On Tue, 11 Nov 2014 15:40:07 +0000, Dennis@home wrote:

Given that force equates to mass times acceleration, so how come
massive objects travelling at a constant velocity (acceleration = zero)
can cause damage when they hit something?
After all, if I'm knocked over by a car doing a constant 70mph, I'm
going to end up a mess, aren't I? I'm going to experience force!
I'm a bit rusty on all this stuff. Can some clever-arse *******
disabuse me of my ignorance? Cheers.

If the car stops exactly as it hits you then nothing will happen to you.
The force is what accelerates you to ~70mph when it does hit you.

Then the bit that REALLY hurts is when you hit the wall/tree/whatever and
decelerate from 70mph to zero.

Adrian wrote:

If the car stops exactly as it hits you then nothing will happen to you.
The force is what accelerates you to ~70mph when it does hit you.

Then the bit that REALLY hurts is when you hit the wall/tree/whatever and
decelerate from 70mph to zero.

Maybe that's why ambulances go so fast. If the ambulance picks you up
when it's parked and speeds up to 70mph when it takes you to the ozzie
it will restore the deceleration. That's why you feel so much better
when to get to A 'n E.

Bill

On 11/11/14 15:26, Cursitor Doom wrote:
Given that force equates to mass times acceleration, so how come massive
objects travelling at a constant velocity (acceleration = zero) can cause
damage when they hit something?
After all, if I'm knocked over by a car doing a constant 70mph, I'm going
to end up a mess, aren't I? I'm going to experience force!
I'm a bit rusty on all this stuff. Can some clever-arse ******* disabuse
me of my ignorance? Cheers.


Because:

1) The large object in it's constant speed state and you in your at-rest
(standing around whistling) state both experience zero force (excepting
gravity which is not relevant here)

2) When the lump hits you at 70mpg either it stops or you go or
something in between.

You going from 0-70mph (more or less) is going to require a force
depending on how quickly you get to 70mph.

Lets assume you can be "squished" by 1cm before you hit bone - so
between the object touching you and the object moving 1cm forward, you
need to accelerate 0-70mph.

That's a lot of force!

In the most pathological case, if you are 30cm "thick" and we go for
full splattage, your mass needs to accelerate 0-70mph as the original
large object moves through 30cm. At 0cm it starts imparting force. At
30cm it has by definition contacted all of your material mass so that is
now moving at nearly 70mph.



In the real world, it would hit your legs, break them, you'd buckle and
fold around the front of the object and things get more complicated. But
it's still Bad (TM)

Does that help?

On 11/11/2014 15:44, Tim Watts wrote:

In the real world, it would hit your legs, break them, you'd buckle and
fold around the front of the object and things get more complicated. But
it's still Bad (TM)

Being hit by a flat fronted HGV could be fairly bad in that respect...

Still to those of us with more generous padding, its nice to know we are
not fat, we just have better crumple zones ;-)


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

"Cursitor Doom" wrote in message
...
Given that force equates to mass times acceleration, so how come massive
objects travelling at a constant velocity (acceleration = zero) can cause
damage when they hit something?
After all, if I'm knocked over by a car doing a constant 70mph, I'm going
to end up a mess, aren't I? I'm going to experience force!
I'm a bit rusty on all this stuff.

Can some clever-arse ******* disabuse
me of my ignorance?

Newton?

http://en.wikipedia.org/wiki/Newton's_laws_of_motion



--
Adam

In message , ARW
writes
"Cursitor Doom" wrote in message
...
Given that force equates to mass times acceleration, so how come massive
objects travelling at a constant velocity (acceleration = zero) can cause
damage when they hit something?
After all, if I'm knocked over by a car doing a constant 70mph, I'm going
to end up a mess, aren't I? I'm going to experience force!
I'm a bit rusty on all this stuff.

Can some clever-arse ******* disabuse
me of my ignorance?

Newton?

http://en.wikipedia.org/wiki/Newton's_laws_of_motion

Consider.... a tennis ball bouncing off the front of a moving train. Did
the train stop momentarily:-)

--
Tim Lamb

"Tim Lamb" wrote in message
...
In message , ARW
writes
"Cursitor Doom" wrote in message
...
Given that force equates to mass times acceleration, so how come massive
objects travelling at a constant velocity (acceleration = zero) can
cause
damage when they hit something?
After all, if I'm knocked over by a car doing a constant 70mph, I'm
going
to end up a mess, aren't I? I'm going to experience force!
I'm a bit rusty on all this stuff.

Can some clever-arse ******* disabuse
me of my ignorance?

Newton?

http://en.wikipedia.org/wiki/Newton's_laws_of_motion

Consider.... a tennis ball bouncing off the front of a moving train. Did
the train stop momentarily:-)


No. But it slowed down.

--
Adam

On 11/11/14 20:30, ARW wrote:
"Tim Lamb" wrote in message
...
In message , ARW
writes
"Cursitor Doom" wrote in message
...
Given that force equates to mass times acceleration, so how come
massive
objects travelling at a constant velocity (acceleration = zero) can
cause
damage when they hit something?
After all, if I'm knocked over by a car doing a constant 70mph, I'm
going
to end up a mess, aren't I? I'm going to experience force!
I'm a bit rusty on all this stuff.

Can some clever-arse ******* disabuse
me of my ignorance?

Newton?

http://en.wikipedia.org/wiki/Newton's_laws_of_motion

Consider.... a tennis ball bouncing off the front of a moving train.
Did the train stop momentarily:-)


No. But it slowed down.


How fast is the tennis ball going?

In article ,
Tim Lamb wrote:

Consider.... a tennis ball bouncing off the front of a moving train. Did
the train stop momentarily:-)

It's possible that a few molecules on the front of it did, resulting
in them becoming closer to the rest of the train, perhaps leaving a
dent.

-- Richard

On Tue, 11 Nov 2014 19:14:18 +0000, Tim Lamb
wrote:

In message , ARW
writes
"Cursitor Doom" wrote in message
...
Given that force equates to mass times acceleration, so how come massive
objects travelling at a constant velocity (acceleration = zero) can cause
damage when they hit something?
After all, if I'm knocked over by a car doing a constant 70mph, I'm going
to end up a mess, aren't I? I'm going to experience force!
I'm a bit rusty on all this stuff.

Can some clever-arse ******* disabuse
me of my ignorance?

Newton?

http://en.wikipedia.org/wiki/Newton's_laws_of_motion

Consider.... a tennis ball bouncing off the front of a moving train. Did
the train stop momentarily:-)

I prefer the analogy that substitutes the tennis ball with a fly
where the primary question concerns "What's the last thing that goes
through that fly's mind?". The answer, of course, being its arsehole.
:-)
--
J B Good

In article ,
Cursitor Doom wrote:
Given that force equates to mass times acceleration, so how come massive
objects travelling at a constant velocity (acceleration = zero) can cause
damage when they hit something?

If you somehow contrive to have it hit something without decelerating,
it won't cause any damage. But that's not possible.

After all, if I'm knocked over by a car doing a constant 70mph, I'm going
to end up a mess, aren't I? I'm going to experience force!

If it hits you, it won't continue doing a constant 70mph. It will
experience an equal but opposite reaction force to the force it
exerts on you. But that force will decelerate it much less than
it accelerates you, because its mass is much bigger than yours.

-- Richard

On 11/11/14 20:35, Richard Tobin wrote:
In article ,
Cursitor Doom wrote:
Given that force equates to mass times acceleration, so how come massive
objects travelling at a constant velocity (acceleration = zero) can cause
damage when they hit something?

If you somehow contrive to have it hit something without decelerating,
it won't cause any damage. But that's not possible.

course it is. I've hit many a speed limit without decelerating.



After all, if I'm knocked over by a car doing a constant 70mph, I'm going
to end up a mess, aren't I? I'm going to experience force!

If it hits you, it won't continue doing a constant 70mph. It will
experience an equal but opposite reaction force to the force it
exerts on you. But that force will decelerate it much less than
it accelerates you, because its mass is much bigger than yours.

-- Richard



--
Everything you read in newspapers is absolutely true, except for the
rare story of which you happen to have first-hand knowledge. €“ Erwin Knoll

On 11/11/2014 15:26, Cursitor Doom wrote:

Given that force equates to mass times acceleration, so how come massive
objects travelling at a constant velocity (acceleration = zero) can cause
damage when they hit something?

Because you apply a force to make them accelerate - so you are doing
work on the object. If the object is free to move (i.e. you are not
dissipating much of that work overcoming friction, then you are in
effect storing that energy in the momentum of the object.

Once moving it has kinetic energy, which has to be dissipated when it
hits something.

Where the Energy E = 1/2 m v^2

After all, if I'm knocked over by a car doing a constant 70mph, I'm going
to end up a mess, aren't I? I'm going to experience force!
I'm a bit rusty on all this stuff. Can some clever-arse ******* disabuse
me of my ignorance? Cheers.

You could even do the sums to work out how hard it would hit you ;-)





--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Because some of that force put into the car that hits you is used to
accelerate you!
Brian

--
From the Sofa of Brian Gaff Reply address is active
"Cursitor Doom" wrote in message
...
Given that force equates to mass times acceleration, so how come massive
objects travelling at a constant velocity (acceleration = zero) can cause
damage when they hit something?
After all, if I'm knocked over by a car doing a constant 70mph, I'm going
to end up a mess, aren't I? I'm going to experience force!
I'm a bit rusty on all this stuff. Can some clever-arse ******* disabuse
me of my ignorance? Cheers.

Tim Streater wrote:

Also why a bullet does so much damage. Small mass but very large
velocity.


And the heads of small masonry nails (those in cable clips) if they
break off when bashed with a big hammer. They will enter your eye and go
through to your brain. Always wear eye protection.

Bill

On Tue, 11 Nov 2014 15:37:39 +0000, Tim Streater wrote:

Still missing something.

It's that force which does the damage

OK, but force = mass x acceleration; f=ma

So according to the equation, it doesn't matter what the mass of the
object is, if it's traveling at constant velocity, then it's not
accelerating, so that factor is zero and anything times zero (force in
this case) is zero. Doesn't make sense.

On 11/11/14 18:16, Cursitor Doom wrote:
On Tue, 11 Nov 2014 15:37:39 +0000, Tim Streater wrote:

Still missing something.

It's that force which does the damage

OK, but force = mass x acceleration; f=ma

So according to the equation, it doesn't matter what the mass of the
object is, if it's traveling at constant velocity, then it's not
accelerating, so that factor is zero and anything times zero (force in
this case) is zero. Doesn't make sense.



It's not travelling at constant velocity WHEN it hits you. It's
decelerating and you are accelerating (fast) - so lots of force.

On 11/11/14 18:23, Tim Watts wrote:
On 11/11/14 18:16, Cursitor Doom wrote:
On Tue, 11 Nov 2014 15:37:39 +0000, Tim Streater wrote:

Still missing something.

It's that force which does the damage

OK, but force = mass x acceleration; f=ma

So according to the equation, it doesn't matter what the mass of the
object is, if it's traveling at constant velocity, then it's not
accelerating, so that factor is zero and anything times zero (force in
this case) is zero. Doesn't make sense.



It's not travelling at constant velocity WHEN it hits you. It's
decelerating and you are accelerating (fast) - so lots of force.

exactly. Speef doesn't kill. Acceleration does.


--
Everything you read in newspapers is absolutely true, except for the
rare story of which you happen to have first-hand knowledge. €“ Erwin Knoll

On 11/11/2014 18:26, The Natural Philosopher wrote:


exactly. Speef doesn't kill. Acceleration does.



The classical speeder excuse!

Its actually energy that kills.

On 11/11/14 19:24, Dennis@home wrote:
On 11/11/2014 18:26, The Natural Philosopher wrote:


exactly. Speef doesn't kill. Acceleration does.



The classical speeder excuse!

The classic socialist nonsense.


Its actually energy that kills.

actually, it isn't.

consider a car travelling at 100mph.

It has a certain energy.

It can be stopped safely or slammed into a concrete wall. The energy
(loss) is the same.

The acceleration is dramatically different.

Peak force is what smashes bones and beaks blood vesse4ls


--
Everything you read in newspapers is absolutely true, except for the
rare story of which you happen to have first-hand knowledge. €“ Erwin Knoll

On Tue, 11 Nov 2014 18:23:41 +0000, Tim Watts wrote:

It's not travelling at constant velocity WHEN it hits you. It's
decelerating and you are accelerating (fast) - so lots of force.

Ah! Right. Got it now. Cheers.

On 11/11/14 18:40, Cursitor Doom wrote:
On Tue, 11 Nov 2014 18:23:41 +0000, Tim Watts wrote:

It's not travelling at constant velocity WHEN it hits you. It's
decelerating and you are accelerating (fast) - so lots of force.

Ah! Right. Got it now. Cheers.


Don't be put off I did my degree in physics and concepts like these
often had WTF? moments trying to get my head around them!

On 11/11/14 18:16, Cursitor Doom wrote:
On Tue, 11 Nov 2014 15:37:39 +0000, Tim Streater wrote:

Still missing something.

It's that force which does the damage

OK, but force = mass x acceleration; f=ma

So according to the equation, it doesn't matter what the mass of the
object is, if it's traveling at constant velocity, then it's not
accelerating, so that factor is zero and anything times zero (force in
this case) is zero. Doesn't make sense.

if it doesn't hit anything it is travelling at constant velocity.

The moment it does, then that equation has non zero acceleration.


--
Everything you read in newspapers is absolutely true, except for the
rare story of which you happen to have first-hand knowledge. €“ Erwin Knoll

On Tue, 11 Nov 2014 18:16:54 +0000, Cursitor Doom wrote:

Still missing something.

You are.

It's that force which does the damage

OK, but force = mass x acceleration; f=ma

So according to the equation, it doesn't matter what the mass of the
object is, if it's traveling at constant velocity, then it's not
accelerating, so that factor is zero and anything times zero (force in
this case) is zero. Doesn't make sense.

You have an object - you - at zero velocity and known mass.
You have an object coming towards you, with velocity and mass.

What happens when those two objects meet? Energy is transferred from the
moving object to the static object. Some of the energy is absorbed by the
two objects deforming. Some of the energy causes the static object to
accelerate. Those energies are determined by the relative masses, and by
the deformability.

If you stand there and are hit with a 1kg pillow at 20m/s, the pillow
deforms, and absorbs much of the energy, so little is transferred. It
doesn't hurt.

If you stand there and are hit with a 1kg brick at 20m/s, the brick
doesn't deform, and more energy is transferred. It hurts.

If you stand there and are hit with a 1000kg pallet of bricks at 20m/s,
the bricks don't deform, a ****load of energy is transferred. It _REALLY_
hurts. But not for long.

"Cursitor Doom" wrote in message
...
On Tue, 11 Nov 2014 15:37:39 +0000, Tim Streater wrote:

Still missing something.

It's that force which does the damage

OK, but force = mass x acceleration; f=ma

So according to the equation, it doesn't matter what the mass of the
object is, if it's traveling at constant velocity, then it's not
accelerating, so that factor is zero and anything times zero (force in
this case) is zero. Doesn't make sense.

You're using the wrong formula.

Its mv2 that matters, velocity not acceleration.

On 11/11/2014 18:16, Cursitor Doom wrote:
On Tue, 11 Nov 2014 15:37:39 +0000, Tim Streater wrote:

Still missing something.

It's that force which does the damage

OK, but force = mass x acceleration; f=ma

So according to the equation, it doesn't matter what the mass of the
object is, if it's traveling at constant velocity, then it's not
accelerating, so that factor is zero and anything times zero (force in
this case) is zero. Doesn't make sense.

When you get hit by a car, you don't travel at constant velocity. You
get accelerated to the same speed as the car, then you get decelerated
back down to zero when you hit the road.

On 12/11/14 00:10, Clive George wrote:
On 11/11/2014 18:16, Cursitor Doom wrote:
On Tue, 11 Nov 2014 15:37:39 +0000, Tim Streater wrote:

Still missing something.

It's that force which does the damage

OK, but force = mass x acceleration; f=ma

So according to the equation, it doesn't matter what the mass of the
object is, if it's traveling at constant velocity, then it's not
accelerating, so that factor is zero and anything times zero (force in
this case) is zero. Doesn't make sense.

When you get hit by a car, you don't travel at constant velocity. You
get accelerated to the same speed as the car, then you get decelerated
back down to zero when you hit the road.


Well in my experience, its actually a whole lot more complicated than
that, at least in the two case where I have seen someone killed in front
of my own eyes..

--
Everything you read in newspapers is absolutely true, except for the
rare story of which you happen to have first-hand knowledge. €“ Erwin Knoll

On Wed, 12 Nov 2014 00:32:27 +0000, The Natural Philosopher wrote:

at least in the two case where I have seen someone killed in front of
my own eyes..

Yet you still have a licence?

In article , Adrian wrote:
On Wed, 12 Nov 2014 00:32:27 +0000, The Natural Philosopher wrote:

at least in the two case where I have seen someone killed in front of
my own eyes..

Yet you still have a licence?

No reason to assume he was driving. Besides which, 1 in 5 motorists
convicted of causing death by dangerous or careless driving get away
without a ban, and those banned are usually back on the road after a
couple of years. And most deaths don't even result in any conviction.

http://www.ctc.org.uk/news/1-5-motor...-causing-death

On 12/11/14 09:24, Adrian wrote:
On Wed, 12 Nov 2014 00:32:27 +0000, The Natural Philosopher wrote:

at least in the two case where I have seen someone killed in front of
my own eyes..

Yet you still have a licence?

I was not driving in either case.

I was on foot at the road side, and in the second case, track side.

First was when I was 13 years old...

--
Everything you read in newspapers is absolutely true, except for the
rare story of which you happen to have first-hand knowledge. €“ Erwin Knoll

On Tue, 11 Nov 2014 15:37:39 +0000, Tim Streater wrote:

In article , Cursitor Doom
wrote:

Given that force equates to mass times acceleration, so how come
massive objects travelling at a constant velocity (acceleration = zero)
can cause damage when they hit something?

Conservation of momentum (momentum = mass x velocity). Momentum before
collision = momentum after, measuring yours and the object's together.
So some gets transferred to you, meaning you end up with some velocity,
and the object with less. Further, for the object to therefore have less
velocity after, and you to have some, a force must be applied to each
object. You supply the force to change the velocity of the object,
and vice versa. Good luck if the object is a car, or planet.

It's that force which does the damage - oh, and any acceleration of the
object at the moment of impact is not relevant. It's the velocity that
counts. If you fall off a chair, the damage the Earth does to you and
vice versa is minimal. Not so if you fall out of a plane, but (in
principle and ignoring air resistance), your acceleration at the moment
of impact is the same in both cases.

Also why a bullet does so much damage. Small mass but very large
velocity.

I've never quite 'got' conservation of momentum, either. Is that the one
that says for every action there's an equal and opposite reaction? If so
I have a problem with that too.
Imagine a 9mm pistol mounted on model train being fired. The energy of
the bullet leaving the gun's muzzle seems to me to be vastly greater than
the resulting recoil which makes the model jump backwards on the
tracks. :-/

On 11/11/14 18:35, Cursitor Doom wrote:

I've never quite 'got' conservation of momentum, either. Is that the one
that says for every action there's an equal and opposite reaction? If so
I have a problem with that too.
Imagine a 9mm pistol mounted on model train being fired. The energy of
the bullet leaving the gun's muzzle seems to me to be vastly greater than
the resulting recoil which makes the model jump backwards on the
tracks. :-/


That's because the engergies are NOT the same. The momentum is

OK - with semi realistic figures:

Train mass = 0.1kg
Bullet mass = 0.01kg (it's a small bullet).

Bullet speed = 100m/s on exit

So the bullet's momentum is:

100x0.01 = 1 kg.m/s

So the train's momentum must be -1 kg.m/s (- means backwards)
and thus it's speed is -1 / 0.1 = 10m/s

----

Bullet's kinetic energy = 1/2 m.v2 = 1/2 x 0.01 x 100x100 = 50 J (joules)

Train's kinetic energy = 1/2 x 0.1 x 10x10 = 5 J


So the bullet ends up with 10 times as much energy

On 11/11/2014 18:44, Tim Watts wrote:
On 11/11/14 18:35, Cursitor Doom wrote:

I've never quite 'got' conservation of momentum, either. Is that the one
that says for every action there's an equal and opposite reaction? If so
I have a problem with that too.
Imagine a 9mm pistol mounted on model train being fired. The energy of
the bullet leaving the gun's muzzle seems to me to be vastly greater than
the resulting recoil which makes the model jump backwards on the
tracks. :-/


That's because the engergies are NOT the same. The momentum is

OK - with semi realistic figures:

Train mass = 0.1kg
Bullet mass = 0.01kg (it's a small bullet).

Bullet speed = 100m/s on exit

So the bullet's momentum is:

100x0.01 = 1 kg.m/s

So the train's momentum must be -1 kg.m/s (- means backwards)
and thus it's speed is -1 / 0.1 = 10m/s

----

Bullet's kinetic energy = 1/2 m.v2 = 1/2 x 0.01 x 100x100 = 50 J (joules)

Train's kinetic energy = 1/2 x 0.1 x 10x10 = 5 J


So the bullet ends up with 10 times as much energy


Oops, that means what I said isn't quite correct.

I just did some sums and the energy in a 9mm bullet is about 500 joules.
The gun weighs about a kilo and gets an energy of about 10 joules.

On 11/11/2014 18:35, Cursitor Doom wrote:
On Tue, 11 Nov 2014 15:37:39 +0000, Tim Streater wrote:

In article , Cursitor Doom
wrote:

Given that force equates to mass times acceleration, so how come
massive objects travelling at a constant velocity (acceleration = zero)
can cause damage when they hit something?

Conservation of momentum (momentum = mass x velocity). Momentum before
collision = momentum after, measuring yours and the object's together.
So some gets transferred to you, meaning you end up with some velocity,
and the object with less. Further, for the object to therefore have less
velocity after, and you to have some, a force must be applied to each
object. You supply the force to change the velocity of the object,
and vice versa. Good luck if the object is a car, or planet.

It's that force which does the damage - oh, and any acceleration of the
object at the moment of impact is not relevant. It's the velocity that
counts. If you fall off a chair, the damage the Earth does to you and
vice versa is minimal. Not so if you fall out of a plane, but (in
principle and ignoring air resistance), your acceleration at the moment
of impact is the same in both cases.

Also why a bullet does so much damage. Small mass but very large
velocity.

I've never quite 'got' conservation of momentum, either. Is that the one
that says for every action there's an equal and opposite reaction? If so
I have a problem with that too.
Imagine a 9mm pistol mounted on model train being fired. The energy of
the bullet leaving the gun's muzzle seems to me to be vastly greater than
the resulting recoil which makes the model jump backwards on the
tracks. :-/


Its the same.
The gun is heavy so it recoils slower for the same energy (e=mv2).
Its easy to get confused if you watch hollywood films as the bullet
knocks someone over. Real bullets can't impart anymore force than the
recoil of the gun and shooter.

On 11/11/14 19:30, Dennis@home wrote:
On 11/11/2014 18:35, Cursitor Doom wrote:
On Tue, 11 Nov 2014 15:37:39 +0000, Tim Streater wrote:

In article , Cursitor Doom
wrote:

Given that force equates to mass times acceleration, so how come
massive objects travelling at a constant velocity (acceleration = zero)
can cause damage when they hit something?

Conservation of momentum (momentum = mass x velocity). Momentum before
collision = momentum after, measuring yours and the object's together.
So some gets transferred to you, meaning you end up with some velocity,
and the object with less. Further, for the object to therefore have less
velocity after, and you to have some, a force must be applied to each
object. You supply the force to change the velocity of the object,
and vice versa. Good luck if the object is a car, or planet.

It's that force which does the damage - oh, and any acceleration of the
object at the moment of impact is not relevant. It's the velocity that
counts. If you fall off a chair, the damage the Earth does to you and
vice versa is minimal. Not so if you fall out of a plane, but (in
principle and ignoring air resistance), your acceleration at the moment
of impact is the same in both cases.

Also why a bullet does so much damage. Small mass but very large
velocity.

I've never quite 'got' conservation of momentum, either. Is that the one
that says for every action there's an equal and opposite reaction? If so
I have a problem with that too.
Imagine a 9mm pistol mounted on model train being fired. The energy of
the bullet leaving the gun's muzzle seems to me to be vastly greater than
the resulting recoil which makes the model jump backwards on the
tracks. :-/


Its the same.
The gun is heavy so it recoils slower for the same energy (e=mv2).
Its easy to get confused if you watch hollywood films as the bullet
knocks someone over. Real bullets can't impart anymore force than the
recoil of the gun and shooter.

More ******** from the man who knows nothing.

The force is dependent on how fasts the bullet is STOPPED or STARTED.
Not how fast its going.


--
Everything you read in newspapers is absolutely true, except for the
rare story of which you happen to have first-hand knowledge. €“ Erwin Knoll

On 11/11/14 19:30, Dennis@home wrote:

Its the same.
The gun is heavy so it recoils slower for the same energy (e=mv2).
Its easy to get confused if you watch hollywood films as the bullet
knocks someone over. Real bullets can't impart anymore force than the
recoil of the gun and shooter.

Well, technically they can if the exhaust gas is used to balance the
recoil. But your point is sound

Unless you were including the gas ;-)