# 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 ?

Homework Helper
Gold Member

## 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 ?

Homework Helper
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!

Homework Helper
Gold Member
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 :)

Homework Helper
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!

Homework Helper
yeees …

but what did i say about the significant figures?

chawki
38705?

Homework Helper
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

Homework Helper
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?

Homework Helper
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?