Thread: Local lad killed by towball

Originally Posted by slug_burner

year 11 physics

Kenetic Energy (KE)= (1/2)m.v^2 Units are joules (or kg/sec^2)

Force = m.a Units are Newtons (or kg.m/sec^2)

When you use a snatch strap, you have the KE of the snatching vehicle as it has mass and velocity plus some potential energy which is stored in the elastic members of the snatch strap.

Could you please translate this into something that some of us less educated can understand.

Dave.

Mr Whippy (Dave)

I hope your not pulling my leg.

Energy is measured in units of Joules. Work and energy are interchangeable and have the same units e.i., joules. Power is a measure of how quickly you can do work, the units of power are Newtons. Example: Given two vehicles of the same weight, one with a small engine and one with a bigger engine, they can both climb a hill and by the time they get to the top both have done the same amount of work. The one with the bigger engine should be able to get up the hill faster than the one with the small engine because it has a more powerfull motor. See here for another explanation

When people speak of energy, two types are often referred to i.e., Potential Energy and Kenetic Energy. Kenetic Energy is that associated with a moving body.

KE= 1/2 m. v^2 (said as "Kenetic Energy equals half m v squared) "m" is the symbol for mass and has units of kilos and "v" is the symbol for velocity and it has units of metres per second. All this means is that a moving mass has kenetic energy.

Potential Energy is used to express energy in bodies that are not moving. A streched spring or elastic although it might not be moving can store energy and that energy is referred to as potential energy. Energy can also be stored in a body by raising it in hieght (water in a tank raised above the ground, or stored in a dam)

F=m.a (said "Force equals m a") units are Newtons (Newton's Second Law of physics)

So what does that all mean? Power, force and energy are often used interchangeably in common speech but in fact are different. And, when you use a snatch strap not only do you use the kenetic energy of the towing vehicle to do work to pull the bogged vehicle out but you also have stored some potential energy in the snatch strap to help add more energy to pull the vehicle out.






Looking back over the previous equations in the other post I left the metres out of the the units of velocity

Kenetic Energy (KE)= (1/2)m.v^2 Units are joules (or kg m^2/sec^2)





Here is some further explanations


[ame="http://en.wikipedia.org/wiki/Force"] Force [/ame]

Does that mean I'm right??
I thought i sounded smart with all my formulas and numbers but....

Don't suppose any of us who paid attention in physics could do some number crunching and give us the correct answer to my hypothesi? (plural??) Either way I'm glad I got some brains thinking away


Ben

whitakerb,

Are you correct? You ask. Yes and No. I think that most people have a reasonable understanding of what is happening in a recovery situation. School boy physics only takes us so far as there are many factors that we do not have values for. Even the REME or RAEME recovery book has some assumptions in order to determine what the resistance of mud, sand etc are and one lot of mud will be different to another. It them becomes difficult to work things out to any great accuracy. Some of the bits in red below need further clarification which I have tried to do.

Originally Posted by whitakerb

Say for example my Rangie is up to its sills in mud and isn't going to go anywhere easily, suggesting as per US Army manual it will need approx 18T of force to remove from the hole

Snatch strap is attached to my Rangie and recovery vehicle with a 3500kg bow shackles.

If the recovery vehicle moves at walking pace and my maths are correct you will apply the following force on your gear.

For the sake of the argument, my Rangie is an immovable object and we are ignoring any other outside forces.

Mass = Recovery vehicle = 2000kg

Velocity = walking pace = 3km/h = 0.833333m/s

F = X KG F = m.a apart from the mass (m) we need a rate of change of velocity or acceleration (a) to calculate a force.

X= M*V momentum = m.v to change the velocity of a body you need to apply a force (Newton's 1st law)

= 2000*0.83333 = 1666.66

Force = 1666.66KG

So not too much, but to overload the bow shackles we only need 3500kg of force;

F = 3500

M = 2000

V = F/M

3500/2000

1.75m/s = 6.3KM/H

So again if my maths are correct, you need to be doing 6.3km/h to overload a 3.5 T Bow shackle, however you are not likely to break it due to its safety factor which is IIRC 500% so 17.5T

Not much IMHO

Now say the Reece hitch was fully welded, and rule of thumb is 1 inch of weld will hold 1000kg then:

Reece hitch is 50x50mm = 8 inches of weld = 8000kg breaking strain, were not looking at a great deal of speed to create a lethal object.

If anything, make sure you pack your shovel next time so you can do the walking pace recoverys rather than trying warp speed to 'pop' it out of the bog

So condolences out to the victims family and hopefully it wakes us all up when on the tracks

Hi,
I did a fairly extensive search to try and find the ref to 10x vehicle weight and dammed if I can find it.
From memory it was in a table of ground surfaces and rolling resistance factors, so I may have stuffed up with the 'weight'. It may have been 10x rolling resistance, BUT I think force to break suction was mentioned.
Just not sure of it now.
I guess the speed of extraction would have a lot to do with loading the forces required to move a bogged vehicle too.
As mentioned else where here, there would be heaps of variables in trying to get a handle on any maths involved.
cheers

Originally Posted by slug_burner

whitakerb,

Are you correct? You ask. Yes and No. I think that most people have a reasonable understanding of what is happening in a recovery situation. School boy physics only takes us so far as there are many factors that we do not have values for. Even the REME or RAEME recovery book has some assumptions in order to determine what the resistance of mud, sand etc are and one lot of mud will be different to another. It them becomes difficult to work things out to any great accuracy. Some of the bits in red below need further clarification which I have tried to do.

Yes that makes more sence. I was thinking about it last night, and my formula was wrong, so to hypothesize about an answer you would need to work out the acceleration (deceleration) on the recovery vehicle as the strap took strain.

Force = Mass X Acceleration


Cheers

Originally Posted by slug_burner

Mr Whippy (Dave)

I hope your not pulling my leg.

Power is a measure of how quickly you can do work, the units of power are Newtons.
Force

I thought the units of power are Watts??? (1 Watt = 1 Joule/second)

Originally Posted by slug_burner

Mr Whippy (Dave)

I hope your not pulling my leg.

Energy is measured in units of Joules. Work and energy are interchangeable and have the same units e.i., joules. Power is a measure of how quickly you can do work, the units of power are Newtons. Example: Given two vehicles of the same weight, one with a small engine and one with a bigger engine, they can both climb a hill and by the time they get to the top both have done the same amount of work. The one with the bigger engine should be able to get up the hill faster than the one with the small engine because it has a more powerfull motor. See here for another explanation

When people speak of energy, two types are often referred to i.e., Potential Energy and Kenetic Energy. Kenetic Energy is that associated
with a moving body.

KE= 1/2 m. v^2 (said as "Kenetic Energy equals half m v squared) "m" is the symbol for mass and has units of kilos and "v" is the symbol for velocity and it has units of metres per second. All this means is that a moving mass has kenetic energy.

Potential Energy is used to express energy in bodies that are not moving. A streched spring or elastic although it might not be moving can store energy and that energy is referred to as potential energy. Energy can also be stored in a body by raising it in hieght (water in a tank raised above the ground, or stored in a dam)

F=m.a (said "Force equals m a") units are Newtons (Newton's Second Law of physics)

So what does that all mean? Power, force and energy are often used interchangeably in common speech but in fact are different. And, when you use a snatch strap not only do you use the kenetic energy of the towing vehicle to do work to pull the bogged vehicle out but you also have stored some potential energy in the snatch strap to help add more energy to pull the vehicle out.






Looking back over the previous equations in the other post I left the metres out of the the units of velocity

Kenetic Energy (KE)= (1/2)m.v^2 Units are joules (or kg m^2/sec^2)





Here is some further explanations


Force

No I am not pulling your leg.
I do understand all about Kinetic and potential energy.

What I didn't understand and still don't is how to interpret the formula.

Your original post had this,

Kinetic Energy (KE)= (1/2)m.v^2 Units are joules (or kg/sec^2)

Force = m.a Units are Newtons (or kg.m/sec^2)


Ok the (KE) means Kinetic Energy.

(1/2)m I assume means half the mass. Why is the 1/2 bracketed?

Now v is velocity and I have guessed that ^2 means squared.

What is the decimal point between m and v

Dave.