# Easy Physics Problem...

Discussion in 'Community Discussion' started by ipodtouchy333, Oct 2, 2009.

1. ### ipodtouchy333 macrumors 65816

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Nov 15, 2007
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#1
A rocket-driven sled running on a straight,
level track has been used to study the physio-
logical eﬀects of large accelerations on astro-
nauts. One such sled can attain a speed of
395 m/s in 1.6 s starting from rest.
What is the acceleration of the sled, assum-
ing it is constant?

We are starting the chapter tomorrow and I'm not quite sure how to get the final answer. Anyone? Thank you.

2. ### Signal-11 macrumors 65816

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#2
42 blue, with some strawberries.

Acceleration = m/s^2. Everything you need to know to answer was given to you in the question.

3. ### wakwe macrumors newbie

Joined:
Oct 2, 2009
#3
solution

395m/s (want m/s^2)

so divide by 1.6s to achieve that velocity

395/1.6 = 246.875

4. ### mkrishnan Moderator emeritus

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Grand Rapids, MI, USA
#4
And the obvious missing formula was....

v = a * t + c

...

5. ### wakwe macrumors newbie

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Oct 2, 2009
#5
.. a=dx/dt^2?

6. ### mkrishnan Moderator emeritus

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Location:
Grand Rapids, MI, USA
#6
I was giving the benefit of the doubt to the OP... if the OP can already do calculus, I'd be really shocked that he/she couldn't solve this problem on his/her own.

But it's always good to understand the general principle underlying the answer, so that the other three million problems that are exactly the same as this one can be solved in the same way, and in a month or two, when this problem is embedded as just a part of a much larger problem, that part will seem intuitive once one gets there.

7. ### wakwe macrumors newbie

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Oct 2, 2009
#7
agreed

full formula:

x(t) = v(t)*t + 1/2*a(t)*t^2 + x(0)

8. ### ipodtouchy333 thread starter macrumors 65816

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#8
Oh duh! Jeeze thanks guys! Ok one more question...

An electron, starting from rest and moving
with a constant acceleration, travels 4.6 cm in
8.2 ms.
What is the magnitude of this acceleration
in km/s 2 ?

Sorry about this, it just really annoys me when our teacher assigns us homework when we haven't a clue what we're doing yet...

9. ### mkrishnan Moderator emeritus

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Jan 9, 2004
Location:
Grand Rapids, MI, USA
#9
See, that's the point. You solve this with exactly the same equation wakwe posted above. x is the distance, a is the acceleration (well, assuming this is a non-relativistic problem, but again, if you're asking the question, I'm sure it is ). In this case, there's no initial acceleration:

x = v0 * t + 1/2 * a0 *t^2 + x0

V0 is zero. X0 is zero. Plug in X and the time and solve for a0.

If you keep approaching each problem like it's completely unrelated to the one before it, then physics is a disaster... if you approach from general principles it becomes beautiful.

10. ### ipodtouchy333 thread starter macrumors 65816

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#10
Yeaaaa...still can't get it. Maybe I shouldn't have taken AP Physics but thanks for everyone's help anyways!

11. ### Capt Crunch macrumors 6502

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Cleveland, OH
#11
Are you sure you tried what was given to you in the post above yours? It literally gives you the answer.

12. ### ipodtouchy333 thread starter macrumors 65816

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#12
I did try it, yes. I probably sound like the dumbest person ever right now but when I converted to km/s I came out with a reallllyy small number and I just don't think that's right.

13. ### eawmp1 macrumors 601

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#13
Convert the original values to the desired values (cm to KM and ms to s) and plug into the above formula!

14. ### Capt Crunch macrumors 6502

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Cleveland, OH
#14
But everything is already in meters and seconds.

15. ### eawmp1 macrumors 601

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#15
He hasn't got the concept - just trying to make him convert the values to the desired values of the answer so he won't get tripped up by the math.

4.6 cm - 0.000046 km
8.2 ms = 0.0082 sec

16. ### ipodtouchy333 thread starter macrumors 65816

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#16
No, unfortunately. The 4.6 is in centimeters and the 8.2 is in milliseconds.

17. ### eawmp1 macrumors 601

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#17
the answer is between 1 and 2 - just do the math!

18. ### ipodtouchy333 thread starter macrumors 65816

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#18
GOT IT! I forgot to square the t in the equation...wow. Thanks everyone and sorry I sounded like such a dummy, haha.

19. ### Signal-11 macrumors 65816

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#19
Whatever happened to looking in the back of the book and working backwards from there?

20. ### monke macrumors 65816

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#20
The internet.

21. ### Xapphire13 macrumors regular

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Jan 14, 2009
Location:
South Australia
#21
395 m/s in 1.6 seconds
395 = 3 sig fig
1.6 = 2 sig fig
so answer must be two sig fig

∴ 395/1.6 = 246.875 m/s/s (m/s^2 OR ms^-2)

∴ Acceleration ≈ 250 m/s/s (2 sig fig)