quick physics question

[Semidevil]

so lets say a car with a mass of 1,000kg starts from rest and accerlates to a velocity of 27m/s in a time interval of delta 9 seconds. assuming that the accerlation is constant, what force is needed for this accerlation(as in how many newtons)?

lets ignore friction...

I have no idea on how to start.......

also, if the time it takes to reach the final velocity doubles, what has happened to the force on the car? did it increase? decrease? by how much?

I know, Im lazy..........plesae help though

 

[edro]

Don't you have a formula that shows what F equals?

This is the EASIEST Physics can get, but I forget the formulas....

 

[Legendary]

vf = vi + at
27 = 0 + 9a
a = 3

F = ma
F = 1000kg 3m/s2
F = 3000 N

edit - 2nd part
27 = 0 + 18a
a = 1.5 m/s2

F = 1500 N
so it's halved.

 

[Heisenberg]

Find the acceleration and then use F = ma.

Use delta t of 18 seconds and do as above.

Edit: Jeebus Legendary you could have left some of the problem for him to do.

 

[Anubis]

im just wondering if we get credited for doing other peoples homework for them?

 

[Darien]

edit: was beaten to answering question.

gah, you all type too fast.

 

[TNTrulez]

Originally posted by: TheEvil1
im just wounderin if we get credited for dooing other peoples homework for them?

Maybe you should ask us for help in English homework.

 

[Anubis]

thats why im a physics major not an english major

 

[glenn1]

I'm going to piggyback on this thread with an unrelated question of my own, hope Semidevil doesn't mind.

In terms of flight and ballistics, what's the current limiting factor for air travel speeds? There seems to be an absolute threshhold over the last couple of decades that about mach 5 is the fastest i've seen reports of any object reaching (rocket propelled artillery rounds and spy aircraft). Is it a function of wind resistance of the travelling object, that propulsion methods can't be refined any more, or something else entirely?

 

[Anubis]

its pretty simple. we do have the technology to propel something faster then mach 5 or so but it comes with a price. The technology is very expensive and the average human body can only take about 6-8G's before you pass out and at speeds that high the G factor is out of controll. That new Scram Jet i think its called can do like mach 8 I think with that crazy engine its got

 

[Heisenberg]

Originally posted by: glenn1
I'm going to piggyback on this thread with an unrelated question of my own, hope Semidevil doesn't mind.

In terms of flight and ballistics, what's the current limiting factor for air travel speeds? There seems to be an absolute threshhold over the last couple of decades that about mach 5 is the fastest i've seen reports of any object reaching (rocket propelled artillery rounds and spy aircraft). Is it a function of wind resistance of the travelling object, that propulsion methods can't be refined any more, or something else entirely?

IMHO, it's a combination of engineering difficulties and economics (I'm mainly talking about aircraft here). First off, the propulsion systems needed to do Mach 5+ are still new. Although things like pulse-jet and ram-jet engines have been around for a while, they've never been developed that far. They're certainly not as well refined for aircraft as conventional jet engines are. Second, a major concern is the heating of the leading surfaces of the aircraft from the friction of the air. You need materials that can withstand extreme temperatures but are still suitable to flight. Third, it's a matter of cost vs. need. I think the cost required to go much faster outweighs the need for such speed.

 

[da loser]

i'm guessing its two factors. the skin heats up a lot by air friction, so you need exotic materials. blah this article states that ramjets can't exceed mach 5 because of heat effects, which probably means we haven't found metals capable of that high temperature for the engines.

 

[glenn1]

Although things like pulse-jet and ram-jet engines have been around for a while, they've never been developed that far. They're certainly not as well refined for aircraft as conventional jet engines are. Second, a major concern is the heating of the leading surfaces of the aircraft from the friction of the air. You need materials that can withstand extreme temperatures but are still suitable to flight. Third, it's a matter of cost vs. need. I think the cost required to go much faster outweighs the need for such speed.

What if we confined it to a straight question of ballistics then? With enough powder and a proper projectile, could i create an artillery piece which fired a projectile at say mach 20? This way we don't have to worry about G-forces and such, as the inanimate projectile won't care how many G's it pulls, and temperatures won't be too much of an issue either

 

[Heisenberg]

What if we confined it to a straight question of ballistics then? With enough powder and a proper projectile, could i create an artillery piece which fired a projectile at say mach 20?

I suppose. Assuming you had a projectile which wouldn't melt down from the resistive heating from the air, and a weapon than could withstand the pressure created from using that much propellant. I can't think of any purely theoretical limitation off the top of my head - it would mainly be an engineering problem. Again, I think it goes back to cost vs. need. I'm no expert though, so I could have overlooked some factor.

 

[glenn1]

I suppose. Assuming you had a projectile which wouldn't melt down from the resistive heating from the air, and a weapon than could withstand the pressure created from using that much propellant. I can't think of any purely theoretical limitation off the top of my head - it would mainly be an engineering problem. Again, I think it goes back to cost vs. need. I'm no expert though, so I could have overlooked some factor.

So a hypersonic weapon using rail gun technology wouldn't be impossible from a ballistics and physics POV then? That's kinda where i was going with this

 

[Heisenberg]

So a hypersonic weapon using rail gun technology wouldn't be impossible from a ballistics and physics POV then? That's kinda where i was going with this

Not that I know of. Actually, I'm pretty sure the military is working on railgun (fancy name for eletro-magnetic propulsion) technology now. It's mostly engineering problems. For example, the magnetic field strength required inside a weapon that could accelerate a projectile to hypersonic speeds in only a meter or two of length would be freakin' huge. The only possible problem I can think of right now is that because drag force depends on the velocity of the object, you could reach a point where the velocity is so huge the drag force would be large enough to stop the projectile almost immediately after it exits the barrel of the weapon.

Edit: This is actually a pretty complicated question because it involves air resistance. The nature of a fluid's resistance depends on what it is. In the case of air, the velocity dependence is a combination of a linear and quadratic term. For low speeds, the linear term dominates; for high speeds the quadratic. For the speeds here, the quadratic would certainly be the most important. I don't think I'm really fully qualified to give you an answer other than there may be some upper limit on a projectile's velocity since the force pushing it stops after it leaves the barrel and it is then only subjected to the drag force.

 

[Semidevil]

Originally posted by: Heisenberg

So a hypersonic weapon using rail gun technology wouldn't be impossible from a ballistics and physics POV then? That's kinda where i was going with this

Not that I know of. Actually, I'm pretty sure the military is working on railgun......

haha, I thought you said Nailgun....

but ok, thanx for everyone's help..I got it now.