f=ma ?

[Janus]

I was thinking in terms of power dissipation in watts upon the projectile hitting the target' date=' some will be sound, some heat, some in projectile fragmentation etc...

 

so watts is a good unit to use (I think?).[/quote']

 

You are not going to like this, but, The dissipation in watts also depends on the distance over which the projectile stops.

 

What does not depend on the stopping distance is the energy of projectile. The energy of your projectile is 125000 ergs, which is equal to .0125 watt-sec or .000003 watt-hours. This is about the amount of energy it takes to light a 100w light bulb for 125/1000000 of a sec.

 

A watt-sec is equivalent to one watt for one sec. In your example, if the projectile stops in 1 millisec it would need to disipate at a rate of 12.5 watts for 1 sec.

 

If it took .5 millisec to stop, it would disipate at a rate of 25 watts for .5 millisec.

 

So if you are interested in the property of the impact that does not change with stopping distance, then you need to deal with energy, which is measured in ergs, joules, watt-sec, kilowatt-hrs, calories, therms, BTUs, horsepower-hours, electron-volts etc.

[J.C.MacSwell]

You are not going to like this' date=' but, The dissipation in watts also depends on the distance over which the projectile stops.

 

What does not depend on the stopping distance is the energy of projectile. The energy of your projectile is 125000 ergs, which is equal to .0125 watt-sec or .000003 watt-hours. This is about the amount of energy it takes to light a 100w light bulb for 125/1000000 of a sec.

 

A watt-sec is equivalent to one watt for one sec. In your example, if the projectile stops in 1 millisec it would need to disipate at a rate of 12.5 watts for 1 sec.

 

If it took .5 millisec to stop, it would disipate at a rate of 25 watts for .5 millisec.

 

So if you are interested in the property of the impact that does not change with stopping distance, then you need to deal with energy, which is measured in ergs, joules, watt-sec, kilowatt-hrs, calories, therms, BTUs, horsepower-hours, electron-volts etc.[/quote']

 

As much as I may have botched up what I did I think 125 newton metres is right. Is that the same as 125 joules or 125 watt seconds? One of us is off by a few decimal places.

[Janus]

As much as I may have botched up what I did I think 125 newton metres is right. Is that the same as 125 joules or 125 watt seconds? One of us is off by a few decimal places.

 

Its me, I forgot to convert the meter/sec to cm/sec before plugging the numbers in.

 

It would be 125 watt-sec, .03 watt-hrs, run a 100w bulb for 1.25 secs, etc.

[rahuldandekar]

WEll, in reply to the original question, i may give some reasons why f=ma and not mv :

 

First, a body having no net external force which acts on travels with const. velocity where the constant can be zero. So we need something called 'Force' to explain changes in motion, not the motion itself. It's acceleration that is the measure of changes in motion. We define force such that it is proportional to the acceleration. The constant differs from object to object, and we denote it by 'm' and call it 'mass. So, we write f=ma .

[5614]

f=ma uses a and not v because it can be used to calculate a and cannot be used to calculate v!!!

 

For example a=f/m

 

so if a 10kg thing is pushed with 100N then it will accelerate at 10m/s^2.... it will not have a velocity of 10m/s because it has to accelerate first. Using f=ma acceleration is dependant on the force. After all using Newton's other laws of motion we know that if no force is applied it will not accelerate, it will maintain it's current speed, be it 0m/s or 100m/s.

 

If it were f=mv then a 100kg object moving at 10m/s would have a force of 1000N acting on it... but if it were maintaing the speed of 10m/s we know that a force cannot be acting on it (or the net force would be 0 anyway), because if there was a net unblanced force it'd be accelerating (or deccelerating, which is only technically accelerating in a different direction).

 

With f=ma if there's no force there's no acceleration, the velocity of the object may be anything from 0 to infinity.

[bascule]

Inertia opposes acceleration, not velocity. You need to exert a force to change velocity, but you don't to keep it constant (unless you're opposing a different force such as gravity)

[jcarlson]

We were taught force = mass X acelleration

The thing that gets me about that equation is what if the acceleration is so great that it accelerates up to 99.999c dosn't the mass change? I thought that this equation assumes that mass not should be variable on the value of a. Thats what bugs me about it...

 

~Scott

 

[math]\sum F_{n} = ma[/math] is classical physics, based on newtons laws. While these laws are fairly accurate when modeling the real world, for extreme situations like what you mentioned above there are modified equations from Einstein's theory of relativity that take things like the speed of light into account.

[sciencedummy]

wow, this is so hard to understand... lol, i just got confused by going through the page... lol

 

inertia is opposes acceleration? i thought that when something is moving at a specific velocity, it keeps going at that velocity.