Go Back   Team-BHP > Under the Hood > Technical Stuff


Reply
 
Thread Tools Search this Thread
Old 6th February 2010, 16:44   #61
BHPian
 
r@CYR@y's Avatar
 
Join Date: Oct 2009
Location: Pride of Deccan - Hyderabad
Posts: 78
Thanked: Once
Question

Quote:
Originally Posted by Shan2nu View Post
Somehow im not getting the relation between zero friction and the weight of the car.

Shan2nu
Didnt you heard about hovercrafts.

I hope somebody will use those principles for making a car. It sounds so fascinating just by thinking that future cars will have no tyres and will float in air ( ZERO friction you see)

Concept is very old but what the mechanical engineers and automobile engineers are thinking I dont know.
r@CYR@y is offline   Reply With Quote
Old 6th February 2010, 17:05   #62
Senior - BHPian
 
McLaren Rulez's Avatar
 
Join Date: Dec 2007
Location: Mysore
Posts: 2,375
Thanked: 474 Times
Default

Quote:
Originally Posted by Shan2nu View Post
Yes, it is alien with no friction or aero drag involved. Need to find out what this is all about.

But when you say levitation has normal force = 0. There will only be mass but no weight involved since there is no force acting on the object.

Shan2nu
There is weight actually. Weight is the force that exists when a mass is in the presence of gravity. As long as gravity is there and the body has mass, weight cannot disappear. But there is a second equal and opposite force to the weight for any levitating body. The understanding that weight is a force and measured in Newtons while mass is, well mass, and measured in kg is important. Mass determines quantities like acceleration due to a force.

I guess you need a refresh of your physics and all of it will click easily. Its easy to lose touch I guess!
McLaren Rulez is offline   Reply With Quote
Old 6th February 2010, 19:32   #63
BHPian
 
anku94's Avatar
 
Join Date: Dec 2009
Location: Bangalore
Posts: 225
Thanked: 27 Times
Default

Wow. There's a lot of posts I have to catch up on, but there's the basic concept of force and mass a few people are still confused.

The acceleration of a body does depend on the mass, for a constant force. That's the Newton's Second Law. Force is the change in momentum over time, which brings us the equation,

f=ma

The resistive forces are dependent on other factors as well. Friction between two bodies is the product of the friction co-efficient and the force that one body is applying on the other, usually it's weight in simple systems.

So if you were to pull a cube with a force F, on a surface with frictional co-efficient 'u',

Resistive force=u*normal force=umg

So net force on the block is F-umg

Acceleration is (F-umg)/m=F/m - ug

So you can see that acceleration in this case depends on the mass.

What happens in free fall is that when a heavier body is thrown, the force on it is automatically increased.

According to the law of gravitation:-

Gravitational force = GMm/d^2 where G is a constant, M is the mass of the larger body, earth in this case, d is the distance between the centre of the earth and the body. Since in normal cases, even if you raise an object to 1km height, the distance becomes radius of earth [about 6000km] + 1 km which is approximately the radius of the earth.

So acceleration of the falling body is force/mass=GMm/d^2 * 1/m = GM/d^2 which is independent of the mass.

Although I think the basic query has been answered, it does take a lot of power to power a 10-sec car. But this topic is becoming so much fun.

And thanks McLaren for saving my back....you're absolutely right with what I wanted to say.

@Shan2nu

Acceleration does depend on mass. The basic behind a physics model is that you create an ideal world, without any of the complicated factor. And then you slowly start adding other factors.

In this case, the car doesn't have to have wheels. It's simply a highly polished box on a highly polished surface. Friction is negligible. And weight and mass are different things. Mass is the quantity of matter in the body. It's constant. If you're made up of 100 molecules, that's it, no one can change that. But weight is the force with which you're pulled by the planet. Friction depends on weight but net acceleration depends on mass. It can be a bit confusing, but that's it.

Last edited by anku94 : 6th February 2010 at 19:46. Reason: Some things missing
anku94 is offline   Reply With Quote
Old 7th February 2010, 00:22   #64
BHPian
 
FanaticOnWheels's Avatar
 
Join Date: May 2009
Location: Chennai
Posts: 296
Thanked: 68 Times
Default

Like the above poster said, you are assuming that the car has a constant acceleration, which is not true in most of the cases. You have to find out the acceleration that the car offers as a function of time, and integrate that function appropriately to determine the force and other parameters.

Regarding friction, there is actually negligible friction when you drive a car. This is because, the wheels of the car undergo pure rolling, and hence, at any instant, the relative motion of the point of contact of the wheel and the earth is 0, hence friction doesn't act (It acts only when there is a relative motion between two objects, and acts in order to null that motion). Here, friction aids rotational motion (without friction, the car wouldn't move forward, it would slip in the same place), and doesn't affect translational motion. I know it's a difficult concept to digest.

All of the three equations of motion can be used only for constant acceleration, and not for variable acceleration, which is the case with most of the cars.

Last edited by FanaticOnWheels : 7th February 2010 at 00:23.
FanaticOnWheels is offline   Reply With Quote
Old 7th February 2010, 01:40   #65
BHPian
 
anku94's Avatar
 
Join Date: Dec 2009
Location: Bangalore
Posts: 225
Thanked: 27 Times
Default

Yeah. That too. Although we were assuming frictionless sliding, even if you're assuming rolling, friction, that opposes translatory motion is ultra less. All it does is provide a torque to the wheels to turn. [It's not the same torque you talk about when describing engine power]
anku94 is offline   Reply With Quote
Old 7th February 2010, 04:32   #66
Senior - BHPian
 
Shan2nu's Avatar
 
Join Date: Feb 2004
Location: Hubli - Karnata
Posts: 5,525
Thanked: 87 Times
Default

Quote:
@Shan2nu

Acceleration does depend on mass. The basic behind a physics model is that you create an ideal world, without any of the complicated factor. And then you slowly start adding other factors.

In this case, the car doesn't have to have wheels. It's simply a highly polished box on a highly polished surface. Friction is negligible. And weight and mass are different things. Mass is the quantity of matter in the body. It's constant. If you're made up of 100 molecules, that's it, no one can change that. But weight is the force with which you're pulled by the planet. Friction depends on weight but net acceleration depends on mass. It can be a bit confusing, but that's it.
I got the mass vs weight part of it. A 1600kg object has a fixed mass whether it is on earth or on the moon, but the weight of this car will vary with change in gravitational pull. And for this 1600kg object to weigh the same on the moon, will need to increase its mass. Is that right?

But for mass to affect acceleration, it has to resist change in velocity, in some form, right? So there has to be some sort of resistance involved.

Quote:
Although I think the basic query has been answered, it does take a lot of power to power a 10-sec car. But this topic is becoming so much fun.
And informative too. This is prob the first time such a topic has been brought up on the forum and its interesting to see things from a diff perspective, as opposed to what we generally discuss in real world conditions.

Shan2nu

Last edited by Shan2nu : 7th February 2010 at 04:51.
Shan2nu is offline   Reply With Quote
Old 7th February 2010, 05:01   #67
BHPian
 
anku94's Avatar
 
Join Date: Dec 2009
Location: Bangalore
Posts: 225
Thanked: 27 Times
Default

Quote:
Originally Posted by Shan2nu View Post
I got the mass vs weight part of it. A 1600kg object has a fixed mass whether it is on earth or on the moon, but the weight of this car will vary with change in gravitational pull. And for this 1600kg object to weigh the same on the moon, will need to increase its mass. Is that right?

But for mass to affect acceleration, it has to resist change in velocity, in some form, right? So there has to be some sort of resistance involved.

Shan2nu
You're right about the first part. 6 times more mass will weigh the same on moon.

But for the second part, you don't need any resistive forces to resist change in velocity. The very presence of mass is a resistance to change in velocity.

In this case, a daily example will provide a lot of help. You take an empty bucket, cover it with some sort of lid. and you move it. Pretty easy. You fill it with water, cover with the same lid and try to move it. Requires more effort=more force. There has been more or less no change in shape. Friction also remained same approximately. There was no appreciable increase in any resistive force. Still it became [very] difficult to move it.

And thanks for the compliment. The number of posts have gone a long way in increasing my post count as well.
anku94 is offline   Reply With Quote
Old 7th February 2010, 05:02   #68
Senior - BHPian
 
Shan2nu's Avatar
 
Join Date: Feb 2004
Location: Hubli - Karnata
Posts: 5,525
Thanked: 87 Times
Default

So according to the calculations, it would take 0.42875hp to move 1kg over a distance of 400mtrs in 10secs in an ideal situation.

So my OHC only needs 422bhp to do the same. Wish it was that easy in real life. Rocam's OHC does close to 400hp at the crank and he's still in the 13s'.

Shan2nu
Shan2nu is offline   Reply With Quote
Old 7th February 2010, 09:32   #69
BHPian
 
esteemer's Avatar
 
Join Date: Nov 2007
Location: New Delhi
Posts: 135
Thanked: Once
Default

Wow! Great thread.

I've got my exams this month, and Motion is the physics chapter I had planned of revising today.

All the discussion here, refreshed my knowledge, and the revision will now be much quicker.

Awesome
esteemer is offline   Reply With Quote
Old 7th February 2010, 11:18   #70
BHPian
 
Ford Rocam's Avatar
 
Join Date: Feb 2005
Location: Mumbai-India
Posts: 880
Thanked: 16 Times
Default

Quote:
Originally Posted by Shan2nu View Post

So my OHC only needs 422bhp to do the same. Wish it was that easy in real life. Rocam's OHC does close to 400hp at the crank and he's still in the 13s'.

Shan2nu
Its no more at the crank now, its at the wheels & i never got in to 13's could barely kept the car on track last time as it went haywire. Putting all that power to ground is a different ball game all together especially in a F.W.D car.
lets see how i fare on upcoming drag in Hyd. ****Fingers Crossed****

Last edited by Ford Rocam : 7th February 2010 at 11:20.
Ford Rocam is offline   Reply With Quote
Old 7th February 2010, 17:01   #71
BHPian
 
anku94's Avatar
 
Join Date: Dec 2009
Location: Bangalore
Posts: 225
Thanked: 27 Times
Default

Lol...makes my dad's 62HP Spark look like a baby turtle....
anku94 is offline   Reply With Quote
Old 7th February 2010, 18:50   #72
Senior - BHPian
 
Shan2nu's Avatar
 
Join Date: Feb 2004
Location: Hubli - Karnata
Posts: 5,525
Thanked: 87 Times
Default

Quote:
Its no more at the crank now, its at the wheels & i never got in to 13's could barely kept the car on track last time as it went haywire. Putting all that power to ground is a different ball game all together especially in a F.W.D car.
lets see how i fare on upcoming drag in Hyd. ****Fingers Crossed****
400whp, thats more like 450-460hp at the crank. Insane!!!

High time you considered making it an AWD with sticky tyres. Then you should be able to do a sub 11sec run for sure.

Shan2nu
Shan2nu is offline   Reply With Quote
Old 7th February 2010, 20:50   #73
BHPian
 
anku94's Avatar
 
Join Date: Dec 2009
Location: Bangalore
Posts: 225
Thanked: 27 Times
Default

Is there a difference between Wheel Horsepower, Brake Horsepower and Crank Horsepower ?? And why should there be a difference between crank HP and Wheel HP ? They're connected to each other, right ? It's just the flywheel in between..
anku94 is offline   Reply With Quote
Old 8th February 2010, 11:48   #74
Senior - BHPian
 
Shan2nu's Avatar
 
Join Date: Feb 2004
Location: Hubli - Karnata
Posts: 5,525
Thanked: 87 Times
Default

Brake Horse power is the power/torque produced by the engine without having to take the load of other 2ndry components around it like A/C, waterpump, powersteering, alternator, diff, transmission etc.

Crank Horse power is the power/torque measured at the crank with all the 2ndry components except diff, transmission and wheels.

Wheel Horsepower is the final or real world power/torque measured at the wheels. In real world conditions, this figure actually decides the power of the car.

So this is why the power/torque figures vary between the 3 setups.

Shan2nu

Last edited by Shan2nu : 8th February 2010 at 11:54.
Shan2nu is offline   Reply With Quote
Old 8th February 2010, 20:09   #75
Team-BHP Support
 
Rehaan's Avatar
 
Join Date: Feb 2004
Location: Bombay
Posts: 22,381
Thanked: 22,610 Times
Default

Quote:
Originally Posted by Shan2nu View Post
....Would the mass still affect acceleration?
Yes ofcourse.

The more mass there is the more energy it will take to make it move -- even if you take friction and gravity out of the equation.

Say you were floating in space and you squeezed yourself in between an astral potato-chip and the Starship Enterprise.

You try to make some room for yourself by pushing the two objects apart. Which one(s)/what will move ?


** You can re-read the question in bold above as Which one would be easier to accelerate?


Quote:
Originally Posted by anku94 View Post
....And why should there be a difference between crank HP and Wheel HP ? They're connected to each other, right ?..
In one word = FRICTION!

There's an entire gearbox + differential along the way between the crank and wheels, which accounts for the drivetrain losses.

cya
R

Last edited by Rehaan : 8th February 2010 at 20:13.
Rehaan is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Car Wash, Polishing & more - Shell Car Care (Sec-34, Noida) Grr7 Delhi NCR 7 5th July 2016 19:22
0 to 60mph in 1 sec (!) Street Legal (!) Car hydrashok Int'l Motorsport 2 20th October 2006 17:51
Mod : Converting a 500 SEC into a SLR. v12 Modifications & Accessories 14 1st July 2006 13:11
0-60 mph- 1.97 sec, 0-100 mph- 4.33 sec BRABUS The International Automotive Scene 5 8th October 2004 01:24


All times are GMT +5.5. The time now is 16:01.

Copyright 2000 - 2017, Team-BHP.com
Proudly powered by E2E Networks