Solving Bullet Speed and Time in Gun Barrel

[Vaalron]

Homework Statement

A bullet leaves the muzzle of a gun at a speed of 400 m/s. The length of the gun barrel is 0.50 m. Assuming the bullet was uniformly accelerated, (a) what was it's average speed inside the barrel? (b) how long was the bullet in the gun after it was fired?

Homework Equations

(b) d= 1/2 at^2 or t^2 = 2d/a

The Attempt at a Solution

d= 0.50 m ... vo=400 m/s ... v= ?

I don't know how to find the velocity!



I can't figure out how to solve the problem if I don't have the velocity, for (b) and No clue how to find the average speed (distance/time) if I don't havea time variable. Please help me!

 

[CaptainZappo]

Try working with Vf^2 - Vi^2 = 2ad

 

[Vaalron]

Want to tell me what those variables stand for? Thanks.

(V, I don't have the velocity, f? wtf does that mean? i? wtf? I can't even get the acceleration b/c i don't know the velocity.

 

[CaptainZappo]

I guess I'll tell you what those variables mean, even though you should already know them if you've so much as opened your physics textbook.

Vf = final velocity
Vi = initial velocity
d = displacement

Now you have Vf, Vi, and d, so you now have the acceleration. Because you have the acceleration, you can then find the time. Because you have the time, you can then get the average velocity.

Is that simple enough for you?

Want to tell me what those variables stand for? Thanks.

(V, I don't have the velocity, f? wtf does that mean? i? wtf? I can't even get the acceleration b/c i don't know the velocity.

 

[Vaalron]

Those variables arn't the same in my textbook. Unfortunately, my book was copyrighted in 1992, I use v as the final velocity, Vo as the final d, don't know what you mean.

PHYSICS SUCKS AND IM GOING TO FAIL SO I QUIT

I can't even do VECTORS.

 

[CaptainZappo]

Don't give up. It will all really make sense once you find a way to relate it to what you already know. You intuitively know that a = (Vf-Vi)/t, etc...

Just don't let the variables overwhelm you; they are really not something that someone just decided to make up one day. They really make sense, if you take the time to figure out why and how they relate.

Those variables arn't the same in my textbook. Unfortunately, my book was copyrighted in 1992, I use v as the final velocity, Vo as the final d, don't know what you mean.

PHYSICS SUCKS AND IM GOING TO FAIL SO I QUIT

I can't even do VECTORS.

 

[Vaalron]

Can someone relate the numbers that go with the variables? That way it'll make the problem easier. I still haven't been able to figure this out, and it's killing me.

My uncle told me...

d= Vot, which algebraically would be distance/velocity

so i got .5/200=.0025 but for some reason I don't think that's right


for a) i got 200 m/s, because the starting speed is 0 m/s, the final is 400 m/s, my uncle, who is a science major and is a science coordinator for k-12, told me to divide it by 2, so i got 200 m/s. is that correct?

 

[Vaalron]

For your info, I'm not asking for the answer, just numbers to go with the variables. Then I can answer it myself.


Thanks

 

[Astronuc]

Unfortunately, different texts/people use different symbols, but CaptainZappo is on the right track.

This is a helpful reference - http://hyperphysics.phy-astr.gsu.edu/hbase/mot.html#motcon
Look at the plate "Description of Motion in One Dimension"

For constant acceleration, a, and initial velocity v₀, the velocity at distance d is given by

v² = v₀² + 2 * a * d

The average velocity is simply the algebraic average (v + v₀)/2.

I think one can find time to leave or travel down the barrel know the distance and the average velocity (or speed).

 

[Vaalron]

Ehh. My head hurts.



I'll ask my teacher, maybe he'll be able to explain it to me.