Jump to content

Hit Again!!!!


junior

Featured Posts

The one that hit me was a single, so really he either needs a lookout or to turn round and face the way he is going.

 

 

This seems very sensible. By having to push the oars the rower would find himself going slower, and he could also see where he's going. Wins all around!

Edited to add that there are mirrors but these are generally regarded as needed for old people. In rowing that's anyone over 27!

 

Ah, a bit of a 'pride' thing then. Anyone using a mirror is a wuss!

 

A bit like bow thrusters on narrow boats then :)))

Link to comment
Share on other sites

Dont forget to add how worried you are about boating on a river incase this happens again and your getting sleepless nights over it!

Dont forget to mention that he was going as fast as possable and was aiming for you. Dont forget his words of sorry were words of anger in your eyes. And had you not have had a rear fender that brushed him off and slowed him as he hit your paint your boat would have had a big hole in it

Link to comment
Share on other sites

Dont forget to add how worried you are about boating on a river incase this happens again and your getting sleepless nights over it!

Dont forget to mention that he was going as fast as possable and was aiming for you. Dont forget his words of sorry were words of anger in your eyes. And had you not have had a rear fender that brushed him off and slowed him as he hit your paint your boat would have had a big hole in it

 

Whiplash section will require a calculation of the relative momentum of the two boats. Let us say 15 tonnes at 1 mph which is about 0.44 m/sec. Momentum = 7000 kgm/sec.

 

For the sculler shall we say 100 kg ? Even if he was travelling 10 times faster, momentum still only 400 kgm/sec, much less than the narrow boat. So the change in speed of the narrow boat will be very small... Sorry.

Link to comment
Share on other sites

 

Whiplash section will require a calculation of the relative momentum of the two boats. Let us say 15 tonnes at 1 mph which is about 0.44 m/sec. Momentum = 7000 kgm/sec.

 

For the sculler shall we say 100 kg ? Even if he was travelling 10 times faster, momentum still only 400 kgm/sec, much less than the narrow boat. So the change in speed of the narrow boat will be very small... Sorry.

 

 

I see you ducked the more complex calcluation required to determine the NB change in velocity!

 

(Which I suspect requires more information given the two masses did not join togeher and continue as a single mass..)

Link to comment
Share on other sites

 

 

I see you ducked the more complex calcluation required to determine the NB change in velocity!

 

(Which I suspect requires more information given the two masses did not join togeher and continue as a single mass..)

 

Yes, well assuming the two boats are travelling in the same direction, and stick together, then their joint speed after the collision would increase to 1.059 mph.

 

Interestingly, some 32% of the initial kinetic energy is lost, converted into other forms of energy (eg the noise of splintering woodwork, cursing by those involved etc). The scull in this case has 2/3rds of the energy of the boat (1/2 m v2).

Link to comment
Share on other sites

Yes, well assuming the two boats are travelling in the same direction, and stick together, then their joint speed after the collision would increase to 1.059 mph.

 

Interestingly, some 32% of the initial kinetic energy is lost, converted into other forms of energy (eg the noise of splintering woodwork, cursing by those involved etc). The scull in this case has 2/3rds of the energy of the boat (1/2 m v2).

Depends if it was an African scull or a European scull, and is always proportional to the coconut.

  • Greenie 2
Link to comment
Share on other sites

 

Interestingly, some 32% of the initial kinetic energy is lost, converted into other forms of energy (eg the noise of splintering woodwork, cursing by those involved etc). The scull in this case has 2/3rds of the energy of the boat (1/2 m v2).

 

 

Is it? Where does that figure of 32% originate? I'd have thought it would vary widely.

Link to comment
Share on other sites

 

 

Is it? Where does that figure of 32% originate? I'd have thought it would vary widely.

 

Yes, it will vary a lot, depending on the ratios of mass and of speed.

 

Using the ratios above - narrow boat weighing 150 times the scull; scull going 10 times faster, then

  • the velocity of the narrowboat increases by about 6% when the scull crashes into it (and sticks to it)
  • the energy of the narrowboat beforehand is 1480, and of the scull 990 (Joules), total 2470
  • the kinetic energy afterwards is 1670, so 800 of energy is converted to other modes. (32%).
Link to comment
Share on other sites

 

 

Yes, it will vary a lot, depending on the ratios of mass and of speed.

 

Using the ratios above - narrow boat weighing 150 times the scull; scull going 10 times faster, then

  • the velocity of the narrowboat increases by about 6% when the scull crashes into it (and sticks to it)
  • the energy of the narrowboat beforehand is 1480, and of the scull 990 (Joules), total 2470
  • the kinetic energy afterwards is 1670, so 800 of energy is converted to other modes. (32%).

 

 

 

Ohh I see. I misunderstood, thinking you meant 32% of the kinetic energy was dissipated in the crushing and snapping of the wood!

Link to comment
Share on other sites

Surely if the oars were alternated then it wasn't sculling. As I remember from the 60s sculling required each person to have 2 oars.

Absolutely. I should have read it before hitting send. I will edit is now.

 

Nick

Link to comment
Share on other sites

 

Whiplash section will require a calculation of the relative momentum of the two boats. Let us say 15 tonnes at 1 mph which is about 0.44 m/sec. Momentum = 7000 kgm/sec.

 

For the sculler shall we say 100 kg ? Even if he was travelling 10 times faster, momentum still only 400 kgm/sec, much less than the narrow boat. So the change in speed of the narrow boat will be very small... Sorry.

 

But sir! But sir!! Please sir, excuse me.

 

Isn't momentum a vector quantity? Shouldn't you be talking about velocity? Do I get a star for knowing that?

Link to comment
Share on other sites

 

But sir! But sir!! Please sir, excuse me.

 

Isn't momentum a vector quantity? Shouldn't you be talking about velocity? Do I get a star for knowing that?

 

I can't give you a gold star, but I would give you a green one if I had any left today. Sorry, especially as you have just given me one on another thread.

 

You are right, that momentum is a vector, while energy is scalar. However in an earlier post I did say I was assuming the two boats were travelling in the same direction, so I can measure velocity using a single number. Of course those numbers could be both positive (in the case here, of one boat crashing into the other) or one positive and one negative (a head-on collision) -- these two cases would give different results...

Link to comment
Share on other sites

 

I can't give you a gold star, but I would give you a green one if I had any left today. Sorry, especially as you have just given me one on another thread.

 

You are right, that momentum is a vector, while energy is scalar. However in an earlier post I did say I was assuming the two boats were travelling in the same direction, so I can measure velocity using a single number. Of course those numbers could be both positive (in the case here, of one boat crashing into the other) or one positive and one negative (a head-on collision) -- these two cases would give different results...

 

You can't give mods greenies anyway. I suppose it is to prevent corruption.

Link to comment
Share on other sites

Ah, a bit of a 'pride' thing then. Anyone using a mirror is a wuss!

 

Not really. They're not that helpful, and do give a false sense of security. Even with a mirror, there's a blind spot, albeit smaller, and when using one the user would be tempted to look around less.

 

It's recommended that Scullers and steerers of coxless boats look around every 3-5 strokes, in alternate directions- unless doing something like racing on a rowing lake, with no other traffic to worry about.

 

Me, I stick to coxing mostly, I prefer to see where I'm going.

Link to comment
Share on other sites

 

But sir! But sir!! Please sir, excuse me.

 

Isn't momentum a vector quantity? Shouldn't you be talking about velocity? Do I get a star for knowing that?

Does the vector v scalar knowledge matter at this basic level of maths (a sincere question based on limited understanding).

My simple formula gives the same answer regardless of definition of the quantities.

If the definition does matter then my lack of appreciation of this point might explain why I had trouble grasping some important points of advanced engineering science.

For instance, would it help explain where the kinetic energy has gone.?

Link to comment
Share on other sites

Does the vector v scalar knowledge matter at this basic level of maths (a sincere question based on limited understanding).

My simple formula gives the same answer regardless of definition of the quantities.

If the definition does matter then my lack of appreciation of this point might explain why I had trouble grasping some important points of advanced engineering science.

For instance, would it help explain where the kinetic energy has gone.?

 

It does matter. In the made up example above, the mass of the narrowboat is 15 times that of the scull, but the scull is travelling 10 times faster before the collision.

 

The two boats are then assumed to form a single object. It does matter whether the initial velocities are both going in the same direction, 180o apart (head on collision), or some other angle in between - this is the vector bit. I chose the first example.

 

Roughly, the scull loses 98.9% of its initial kinetic energy (in essence as its speed reduces by nearly 90%, and 0.1 * 0.1 = 0.01), of this 18.4% is added to the narrowboat which speeds up a small amount, and 80.5% is converted to other forms of energy - crunching wood, a slightly squashed narrowboat (elastic energy), noise, heat, water turbulence etc,

Link to comment
Share on other sites

 

It does matter. In the made up example above, the mass of the narrowboat is 15 times that of the scull, but the scull is travelling 10 times faster before the collision.

 

The two boats are then assumed to form a single object. It does matter whether the initial velocities are both going in the same direction, 180o apart (head on collision), or some other angle in between - this is the vector bit. I chose the first example.

 

Roughly, the scull loses 98.9% of its initial kinetic energy (in essence as its speed reduces by nearly 90%, and 0.1 * 0.1 = 0.01), of this 18.4% is added to the narrowboat which speeds up a small amount, and 80.5% is converted to other forms of energy - crunching wood, a slightly squashed narrowboat (elastic energy), noise, heat, water turbulence etc,

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

Edited by Theo
Link to comment
Share on other sites

well the specific heat capacity of steel is 420 J per kg per degree C (source).

 

So if by some miracle all the lost kinetic energy went to heat up the boat then each collision would raise the temperature by approx (4850/420)/20,000 degrees, about 1/2000th of a degree.

 

So the answer is about 2,000 collisions. This ignores heat loss into the water of course, and also assumes that all of the mass of the boat is steel, even the fake rivets.

  • Greenie 1
Link to comment
Share on other sites

How hard would it be to fit a small mirror so that they can see what is behind them?

 

Steve

He doesn't need to see that way (he's facing that way) It's seeing what he's about to run into that he needs the mirror for.clapping.gif

Link to comment
Share on other sites

 

It does matter. In the made up example above, the mass of the narrowboat is 15 times that of the scull, but the scull is travelling 10 times faster before the collision.

 

The two boats are then assumed to form a single object. It does matter whether the initial velocities are both going in the same direction, 180o apart (head on collision), or some other angle in between - this is the vector bit. I chose the first example.

 

Roughly, the scull loses 98.9% of its initial kinetic energy (in essence as its speed reduces by nearly 90%, and 0.1 * 0.1 = 0.01), of this 18.4% is added to the narrowboat which speeds up a small amount, and 80.5% is converted to other forms of energy - crunching wood, a slightly squashed narrowboat (elastic energy), noise, heat, water turbulence etc,

Thanks. I think I follow you.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.