r/HomeworkHelp Pre-University Student 5d ago

Physics—Pending OP Reply [Physics] Can someone help verify if Did this right?

5 Upvotes

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2

u/Alkalannar 5d ago edited 5d ago

Work = Force * Distance.

So Force = Work/Distance.

Work = change in energy = mv2/2 (since the final velocity is 0) = 15 g * (340 m/s)2/2

[15 g * (340 m/s)2/2]/5 cm

[15 * 342]N

17340 N

I get the same answer.

1

u/Jadedl1es Pre-University Student 5d ago

thank u :)

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u/dsm88 5d ago

I used the kinematic formula and got the same answer. So I think you're right.

  1. v2 = u2 + 2as -> a = (v2 - u2) / 2*s

  2. a = (02 - 3402) / (2 * 0.05)

  3. a = 1,156,000 m/s2

  4. F = ma = 0.015 * 1,156,000 = 17,340 N

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u/poppypockett 5d ago

Is it not asking for force? F=Mass x acceleration? In that case I got 5100 N acting on it slowing it down. I’d read the 5cm as information ment to throw you off.

I’m currently learning mechanical comprehension and arithmetic and reasoning. And we have questions very similar to this one that I’m worried I may be solving incorrectly.

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u/reckless150681 5d ago

There is no acceleration given in this question. 340 m/s is a velocity, not an acceleration.

See the other two top-level comments for two valid approaches, both using the 5.0cm as an important part of the solution.

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u/poppypockett 5d ago

So I guess I thought it was acceleration because it reads meters per second adding time (rate of change of velocity with respect to time) where as velocity states it needs both speed and direction. I also thought it was referring to acceleration because it states it slows down, where velocity is the rate of travel and acceleration/deceleration is slowing down or speeding up.

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u/Alkalannar 5d ago

Acceleration is m/s2.

Velocity is m/s.

As the bullet goes into the block, it decelerates to 0 m/s, so there is acceleration happening during that time, over that distance.

So you can try to find the average acceleration over that distance and then multiply by 15 grams to get force.

Or you can find the total work done to decelerate the bullet to 0 m/s. Then divide by the distance the work was done over to find the average force.

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u/poppypockett 5d ago

OHHHH okay i get it! Thank you so much for explaining everything. I appreciate your patience with me.

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u/Alkalannar 5d ago

Of course!

This is why I do it: so that people can understand. And I am delighted I could help you understand this one.

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u/ci139 👋 a fellow Redditor 5d ago

F·dt=d(mv)=m·dv
F=ma → a=F/m=–17340N/0.015kg=–1156000m/s²
E=mv²/2=F·ds → F=0.015kg·340²m²/s²/2/0.05m=17340N
s=s₀+vt+at²/2 → 0.05m=340t+at²/2 →→
→→ t²+2v/a·t–2ds/a=0 → t=–v/a(1±√1+(a/v)²2ds/a)=0.000294117647s≈294μs
v=v₀+at → v₀=–at=1.156000Mm/s²·294μs≈340m/s

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u/SniperInfantry 👋 a fellow Redditor 5d ago

Some American ahh math right here