Interpretation of an impulse graph

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  • #1
lovelyrwwr
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Which of the following is true if the mass is at rest at t=0 ?
A. Velocity of the mass is decreasing btwn 10 and 20 seconds.
B. The accel. of the mass is constant between 0 and 10 seconds
C. At 40 seconds the mass is stationary.
D. The distance trsveled by the mass bten 0 and 10 seconds is equal to that traveled btwn 10 and 20 seconds.

The answer is C. I understand why I can rule out answer B. But I do not see why A and D are not true.

From the graph, I see that force decreases during 10 < t < 20 seconds. Thus accel gets smaller but is still positive, which means that velocity should still be increasing. However what I do not understand is what the inflection point at t=10 (and t=30 for that matter) represents...I mean between 0-20 seconds, velocity is always increasing (according to the solution, but then what does this inflection point at t=10 seconds represent?

This problem is driving me insane, plz help! :)
 

Answers and Replies

  • #2
Dick
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View attachment 62919
Which of the following is true if the mass is at rest at t=0 ?
A. Velocity of the mass is decreasing btwn 10 and 20 seconds.
B. The accel. of the mass is constant between 0 and 10 seconds
C. At 40 seconds the mass is stationary.
D. The distance trsveled by the mass bten 0 and 10 seconds is equal to that traveled btwn 10 and 20 seconds.

The answer is C. I understand why I can rule out answer B. But I do not see why A and D are not true.

From the graph, I see that force decreases during 10 < t < 20 seconds. Thus accel gets smaller but is still positive, which means that velocity should still be increasing. However what I do not understand is what the inflection point at t=10 (and t=30 for that matter) represents...I mean between 0-20 seconds, velocity is always increasing (according to the solution, but then what does this inflection point at t=10 seconds represent?

This problem is driving me insane, plz help! :)

From what you've just said, you can already rule out A. The points at x=10 and x=30 are just the points of maximum acceleration and deceleration respectively. Now you just have to rule out D. I'm not sure what's driving you insane.
 
  • #3
lovelyrwwr
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So btwn t = 0 and 10 seconds the mass is accelerating, right? And from 10 to 20 sec it is still accelerating but acceleration is declining...so shouldnt answer A be true?
 
  • #4
Dick
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So btwn t = 0 and 10 seconds the mass is accelerating, right? And from 10 to 20 sec it is still accelerating but acceleration is declining...so shouldnt answer A be true?

If it's still accelerating that means velocity is still increasing, just more slowly. Declining acceleration doesn't mean that the velocity is decreasing. Just as you've already said. Acceleration and velocity are two different things.
 
  • #5
lovelyrwwr
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Wow im such a dope! Thanks for all of ur help!!! :)))))
 

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