Hit Again!!!!

[Horace42]

Here are some figures to illustrate what Scholar Gypsy has said:

 

KE = 1/2 x m x v x v

 

KE is measured in joules

m is measured in kg

v is measured in m/s

 

In an isolated system where you can consider the narrow boat and all its contents and the scull and all its contents to be unaffected by any forces except those between the two craft, then you can take the momentum before the collision to be equal to the momentum immediately after the collision.

 

P = mv

 

P = momentum in Kg-m/s

m = mass in kg.

 

If the scull remains in contact with the narrow boat after the collision rather than bouncing off then, as SG says, they can be considered to be one body with the total mass of nb + scull

 

Using all that lot to work out the final velocity of Halsall and scull:

 

 

Shall we say that Halsall and all that's in it has a mass of 20T or 20,000kg and the scull and its occupant and oars 200kg (that might well be a bit too much but it might make the sums easier.

 

Halsall was stationary so had no momentum before the collision.

 

The scull might have been moving at 15mph, about 7m/s

 

Momentum before collision = 20,000 x 0 + 200 x 7 = 1,400kg-m/s

Momentum after collision is the same so

 

(20,000+200) x final velocity = 1400

 

final velocity = 1400/(20000+200) = 0.07m/s which is not a lot (0.2mph)

 

If we feed these numbers into the energy formula we get:

 

KE before the collision = 1/2 x 200 x 7x7 = 4900 joules

 

KE after the collision = 1/2 x 20200 x 0.07 x 0.07 = 49.5 joules

 

So using these figures it seems that we have lost 99% of the KE.

 

I just though that some figures might help.

 

N

Thanks to Scholar Gypsy and Theo. You have gone to a lot of trouble on my behalf. It is appreciated.

I have a few more questions, but will gen up on-line and come back if I need to.

Scalar or vector, the result is the same.

I guess the vector bit comes in when assessing the mode of impact, using a rear end collision in the example - but they could have collided head on - then it gets interesting - the points of the bows would need to be dead on and in a perfect straight line ......quite an achievement. And likely not to happen - so there would be a sliding deflection sideways at the bows .- complicated further by the curvature of the bows........

I think it better to avoid collisions.

[bizzard]

I suppose its just as interesting looking at where you've been, instead of looking at where your going.

[FadeToScarlet]

Here are some figures to illustrate what Scholar Gypsy has said:

 

KE = 1/2 x m x v x v

 

KE is measured in joules

m is measured in kg

v is measured in m/s

 

In an isolated system where you can consider the narrow boat and all its contents and the scull and all its contents to be unaffected by any forces except those between the two craft, then you can take the momentum before the collision to be equal to the momentum immediately after the collision.

 

P = mv

 

P = momentum in Kg-m/s

m = mass in kg.

 

If the scull remains in contact with the narrow boat after the collision rather than bouncing off then, as SG says, they can be considered to be one body with the total mass of nb + scull

 

Using all that lot to work out the final velocity of Halsall and scull:

 

 

Shall we say that Halsall and all that's in it has a mass of 20T or 20,000kg and the scull and its occupant and oars 200kg (that might well be a bit too much but it might make the sums easier.

 

Halsall was stationary so had no momentum before the collision.

 

The scull might have been moving at 15mph, about 7m/s

 

Momentum before collision = 20,000 x 0 + 200 x 7 = 1,400kg-m/s

Momentum after collision is the same so

 

(20,000+200) x final velocity = 1400

 

final velocity = 1400/(20000+200) = 0.07m/s which is not a lot (0.2mph)

 

If we feed these numbers into the energy formula we get:

 

KE before the collision = 1/2 x 200 x 7x7 = 4900 joules

 

KE after the collision = 1/2 x 20200 x 0.07 x 0.07 = 49.5 joules

 

So using these figures it seems that we have lost 99% of the KE.

 

I just though that some figures might help.

 

N

15mph is probably a bit fast, unless he was absolutely flat out racing.

 

Rowing speeds are normally expressed in splits, I.e. Time taken to cover 500m, and 15mph is I think 1:15 / 500. That's pretty quick, the kind of speed a decent or very good sculler might get off of a start, briefly, but they wouldn't be able to maintain it.

 

10mph is more likely.

[Theo]

I suppose its just as interesting looking at where you've been, instead of looking at where your going.

Don't economists have an oarsman's view of the world?

[Theo]

Thanks to Scholar Gypsy and Theo. You have gone to a lot of trouble on my behalf. It is appreciated.

I have a few more questions, but will gen up on-line and come back if I need to.

Scalar or vector, the result is the same.

I guess the vector bit comes in when assessing the mode of impact, using a rear end collision in the example - but they could have collided head on - then it gets interesting - the points of the bows would need to be dead on and in a perfect straight line ......quite an achievement. And likely not to happen - so there would be a sliding deflection sideways at the bows .- complicated further by the curvature of the bows........

I think it better to avoid collisions.

 

If you start considering more than one dimension (colinear collisions) and branch out into two then the momentum is conserved in each dimension. Same in three but boats don't do much moving in the vertical except for locks, boat lifts and sinking. Ah! The people who do it on wide expanses of salty water do a lot of three dimensional stuff all the time.

[Jrtm]

I wonder if i can claim too for sleepless nights over what happened think ill take it to an american court

[MtB]

If the scull remains in contact with the narrow boat after the collision rather than bouncing off then, as SG says, they can be considered to be one body with the total mass of nb + scull

 

 

Which of course it doesn't, and my physics teacher at skool always used to duck this issue too when working out momentum questions!

 

Can you re-run the figures, showing us how they differ when the two objects bounce off each other instead of continuing as one please, Sir?

 

Thanks ever so...

[Scholar Gypsy]

 

 

Which of course it doesn't, and my physics teacher at skool always used to duck this issue too when working out momentum questions!

 

Can you re-run the figures, showing us how they differ when the two objects bounce off each other instead of continuing as one please, Sir?

 

Thanks ever so...

 

This is a little trickier. The two extreme cases are

 

• if the bodies stick together as one - see above
• if they bounce off each other perfectly, like billiard balls

In the latter case the closing speed before the collision is equal to the separation speed afterwards (coefficient of restitution = 1 ) and kinetic energy is conserved.

 

If the NB is intiallly stationary, and ratio of the masses is 100:1 (20 tonnes to 200 kg), and the closing speed is 5 m/sec (taking FtS advice!) then after the collision the narrow boat will move forwards at 0.09901 m/sec, and the scull will bounce off backwards at 4.90099 m/sec,

 

The truth is somewhere inbetween these two extremes of course - and is impossible to model without assuming more about how the collision actually works out.

 

Warning: Algebra follows

 

initially scull moves at speed u to the left with mass m

afterwards NB moves at speed V to the left with mass M, scull moves at speed u-V to the right

(this is what happens with a perfect collision - separation speed = u)

 

then conservation of momentum gives the two speeds as

 

V = 2mu/(M+m)

and

u-V = (M-m)u/(M+m)

 

this gives the results shown above.

 

Furthermore

 

MV² + m(u-V)²

 

= [ 4Mm² + m(M-m)² ] u ² / (M+m)²

 

= (exercise for the reader)

 

= mu²

 

which proves that kinetic energy is conserved.

 

https://en.wikipedia.org/wiki/Coefficient_of_restitution

Edited August 29, 2016 by Scholar Gypsy

[MtB]

Many thanks! I can see why my physics teacher avoided it.

 

Now, about electrical machine trigonometry... That I NEVER properly understood...

[Scholar Gypsy]

By the way, the above result does not rely on the NB being at rest initially, as Newton's laws still apply in a frame of reference moving at the same speed as the NB is initially.

[MtB]

By the way, the above result does not rely on the NB being at rest initially, as Newton's laws still apply in a frame of reference moving at the same speed as the NB is initially.

 

 

Yes all movement can only ever be relative, as any fule kno.

 

As a small child, bored in my parents' car I used to imagine the possibility that the car was remaining stationary and the wheels were making the world beneath us rotate, creating the illusion we were moving when we weren't... It's very strange being me

[dogless]

Yes all movement can only ever be relative, as any fule kno.

 

As a small child, bored in my parents' car I used to imagine the possibility that the car was remaining stationary and the wheels were making the world beneath us rotate, creating the illusion we were moving when we weren't... It's very strange being me

It's a lot stranger reading you !

 

At least you know what to expect

 

Rog

[Scholar Gypsy]

 

 

Yes all movement can only ever be relative, as any fule kno.

 

As a small child, bored in my parents' car I used to imagine the possibility that the car was remaining stationary and the wheels were making the world beneath us rotate, creating the illusion we were moving when we weren't... It's very strange being me

 

Not strange at all. I am trying to find the scientist who allegedly asked when boarding a train "Does Crewe stop at this train". Instead I found this thread on another forum ... http://www.pprune.org/archive/index.php/t-205887.html

[X Alan W]

Even when they are faster, lighter, more maneuverable and approaching at speed from behind?

Does a river/canal not qualify for a Clause 64 if that's the the correct number ( in restricted waters it's every vessel for itself) rater than powered giving way to sail/unpowered, would be interesting to get the legal answer.

[Jerra]

Does a river/canal not qualify for a Clause 64 if that's the the correct number ( in restricted waters it's every vessel for itself) rater than powered giving way to sail/unpowered, would be interesting to get the legal answer.

I was asking because I have seen diagrams showing the following boat as the "Give way boat" and the boat ahead as the "Stand to boat". Which suggested to me the boat behind has to give way i.e. not run into the boat in front.

[Theo]

 

 

Yes all movement can only ever be relative, as any fule kno.

 

As a small child, bored in my parents' car I used to imagine the possibility that the car was remaining stationary and the wheels were making the world beneath us rotate, creating the illusion we were moving when we weren't... It's very strange being me

 

Funny that. I used to imagine that very thing, too. Picturing the car a pushing the earth around with is wheels.

 

You can consider the Earth as stationary with all the other planets and stars moving but the complex cycloids that this involves and the difficulty of reconciling this to the gravitational forces involved means that it is not a very sensible way to look at things.

[David Mack]

 

 

Yes all movement can only ever be relative, as any fule kno.

 

As a small child, bored in my parents' car I used to imagine the possibility that the car was remaining stationary and the wheels were making the world beneath us rotate, creating the illusion we were moving when we weren't... It's very strange being me

But what about the cars travelling in the opposite direction?

[ditchcrawler]

 

 

Yes all movement can only ever be relative, as any fule kno.

 

As a small child, bored in my parents' car I used to imagine the possibility that the car was remaining stationary and the wheels were making the world beneath us rotate, creating the illusion we were moving when we weren't... It's very strange being me

I am glad I wasn't the only one, but in my case it was a bike, we didn't have a car

[dogless]

I am glad I wasn't the only one, but in my case it was a bike, we didn't have a car

For no obvious reason, that reminds me of family picnics when I was a boy.

 

We'd go to Hickleton from the pit houses at Bolton on Dearne.

 

My Dad would take my Mum and the picnic on his Honda 50, and us six kids would set off walking.

 

Dad would return and collect the youngest and ferry them to the picnic site. The rest of us would carry on walking.

 

By the time my eldest brother was picked up, he was only a couple of hundred yards away !

 

Character building.......or something.

 

Rog

[bizzard]

I had a vision of what would happen if my dads cars rear wheels were imprisoned in concrete when he tried to drive off. Later I realized that if the engine had been powerful enough, the front of the car would rear up into the air, doing a stationary wheely because the pinion had climbed up the differentials crown wheel.

[MtB]

I had a vision of what would happen if my dads cars rear wheels were imprisoned in concrete when he tried to drive off. Later I realized that if the engine had been powerful enough, the front of the car would rear up into the air, doing a stationary wheely because the pinion had climbed up the differentials crown wheel.

 

I disagree, I bet the clutch would have slipped