A bullet from speeding to full stop.

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In summary, the conversation discusses a problem involving a bullet (mass 15 g) with a velocity of 425 m/s being brought to a stop in a block of wood. The task is to find the force on the bullet, assuming it is constant. The conversation also touches on the concept of significant figures and the direction of force acting on the bullet. Ultimately, the correct solution is found to be 38705.35 N, with 2 significant figures.
  • #1
chawki
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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 ?
 
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  • #2
chawki said:

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.
 
  • #3
It's a law..isn't ? HELP
 
  • #4
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 ?
 
  • #5
chawki said:
so i think we will have only: - 1/2*m*V^2 = F*0.035 ?

yup! :smile:
 
  • #6
But then we will get F=-38705.35N
How is possible to have a negative value!
 
  • #7
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.
 
  • #8
Ok, so F=-38705.35N is correct ?
And thank you everyone, I'm learning new things :)
 
  • #9
chawki said:
Find the force on the bullet, assuming that it is constant.
chawki said:
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 :redface:)
 
  • #10
ok, so it is 38705.35N!
 
  • #11
yeees …

but what did i say about the significant figures?
 
  • #12
38705?
 
  • #13
hmm …

how many significant figures are there in the question?

so how many do you think there should be in the answer? :smile:
 
  • #14
I never ever used these things of significant figures...that's why I'm all confused
 
  • #15
ok …*look at signifcant figures in the PF Library :smile:
 
  • #16
38705 Yes or NO
 
  • #17
tiny-tim said:
hmm …

how many significant figures are there in the question?

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

There are three? so the answer would be 38705.357N?
 
  • #18
chawki said:
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? :redface:

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)
 
  • #19
39000n?
 
  • #20
yes, to 3 significant figures it's 38700, to 2 it's 39000
 
  • #21
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.
 

1. What is the velocity of a bullet when it is fired?

The velocity of a bullet when it is fired can vary depending on the type of gun and ammunition being used. On average, a bullet can leave the barrel of a handgun at a velocity of 1,000 feet per second (305 meters per second) and a rifle bullet can travel at a velocity of 3,000 feet per second (914 meters per second).

2. How does a bullet slow down to a full stop?

A bullet slows down to a full stop due to a combination of factors. The most significant factor is air resistance, which creates drag on the bullet as it travels through the air. Additionally, the bullet's kinetic energy is gradually transferred to the surrounding air, causing it to slow down. Finally, gravity also plays a role in slowing down the bullet.

3. What is the average distance a bullet can travel before coming to a full stop?

The average distance a bullet can travel before coming to a full stop depends on various factors such as the initial velocity, air resistance, and the weight and shape of the bullet. On average, a bullet fired from a handgun can travel between 1,000 to 2,000 feet (305 to 610 meters) before coming to a full stop.

4. Can a bullet come to a full stop in mid-air?

No, a bullet cannot come to a full stop in mid-air. As mentioned before, a bullet slows down gradually due to various factors. It is impossible for a bullet to stop instantaneously in mid-air without any external forces acting on it.

5. How long does it take for a bullet to come to a full stop?

The time it takes for a bullet to come to a full stop can vary depending on the factors mentioned before. On average, a bullet can take anywhere from 1 to 5 seconds to come to a full stop.

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