A bullet from speeding to full stop.

[chawki]

Homework Statement

A bullet (mass 15 g) moving at a speed of 425 m/s is brought to a stop in 3.5 cm in a block of wood.

Homework Equations

Find the force on the bullet, assuming that it is constant.

The Attempt at a Solution

what law should we use?
0 - 1/2*m*V^2 = m*g*0.035 + F*0.035 ?

 

[PhanthomJay]

Homework Statement

A bullet (mass 15 g) moving at a speed of 425 m/s is brought to a stop in 3.5 cm in a block of wood.

Homework Equations

Find the force on the bullet, assuming that it is constant.

The Attempt at a Solution

what law should we use?
0 - 1/2*m*V^2 = m*g*0.035 + F*0.035 ?

Explain why you think that the gravity force does work.

 

[chawki]

It's a law..isn't ? HELP

 

[chawki]

Wow. I think i was wrong...it doesn't have work because the motion is on X axis, not Y axis.
so i think we will have only: - 1/2*m*V^2 = F*0.035 ?

 

[tiny-tim]

so i think we will have only: - 1/2*m*V^2 = F*0.035 ?

yup!

 

[chawki]

But then we will get F=-38705.35N
How is possible to have a negative value!

 

[PhanthomJay]

Force is a vector, that has both magnitude and direction. The negative sign implies that if the bullet is moving from left to right, in the positive x direction, then the force acting on it is in the opposite (negative x) direction, from right to left, acting to slow it down.

 

[chawki]

Ok, so F=-38705.35N is correct ?
And thank you everyone, I'm learning new things :)

 

[tiny-tim]

Find the force on the bullet, assuming that it is constant.

Ok, so F=-38705.35N is correct ?

no, the question is either asking for the magnitude of F, which is always positive, or it's asking for a "vector" description of F, which means a magnitude and a direction

(oh, and far too many significant figures )

 

[chawki]

ok, so it is 38705.35N!

 

[tiny-tim]

yeees …

but what did i say about the significant figures?

 

[chawki]

38705?

 

[tiny-tim]

hmm …

how many significant figures are there in the question?

so how many do you think there should be in the answer?

 

[chawki]

I never ever used these things of significant figures...that's why I'm all confused

 

[tiny-tim]

ok …*look at signifcant figures in the PF Library

 

[chawki]

38705 Yes or NO

 

[chawki]

hmm …

how many significant figures are there in the question?

so how many do you think there should be in the answer?

There are three? so the answer would be 38705.357N?

 

[tiny-tim]

There are three?

no, the question gives m v and d to 3 and 2 significant figures …

so the answer is probably best in only 2 sig figs

… so the answer would be 38705.357N?

are you even trying?

how would you write 38705.357 to 3 sig figs?

how would you write 38705.357 to 2 sig figs?

(if you don't know how, then look it up)

 

[chawki]

39000n?

 

[tiny-tim]

yes, to 3 significant figures it's 38700, to 2 it's 39000

 

[chawki]

we can write also: - 1/2*m*V^2 = -F*0.035 knowing that F will be from right to left, opposite the bullet direction and then we will find F the same. no need to introduce the magnitude.