Another kinematics problem.

  • Thread starter -Dragoon-
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  • #1
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Thanks for all the help on the previous question, everyone. I appreciate it. :smile: I've got just another problem that has confused me.

Homework Statement


Marian, who is standing on her balcony, is surprised by a pigeon and throws a flowerpot up in the air at 2.1m/s. It takes 3.0 seconds for the flowerpot to smash to the ground. The flowerpot experiences acceleration due to gravity of 9.81m/s^2 [down].
a) how high is Marian's balcony?
b) How fast was the flowerpot moving just before it smashed to the ground?

Homework Equations


Delta d = V1(delta t) + 1/2(a)(delta t^2)
Delta d = V2(delta t) - 1/2(a)(delta t^2)
V2 ^2 = V1^2+2(a)(delta d)

The Attempt at a Solution


a) part really confused me. I applied the first equation to find the distance:
d = 2.1m/s(3.0 seconds) +1/2(-9.81m/s^2 [down])(3.0^2)
d = 6.3 - 44.145
Now, unless she lives underground then I am clearly doing something wrong. If I were to apply the second equation, I would get a positive value but that only applies when there is a velocity final. This question has me completely stumped, I would like it if answerers could guide me through it by giving hints and not giving me the full answer.

For b part, I know that all I would need to apply is the third equation after finding the distance. But since the question states just before, would I use 2.9 as my change in time instead of 3.0 seconds which is when the pot hit the ground?

Thanks in advance for all the help. :biggrin:
 

Answers and Replies

  • #2
kuruman
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Where is your origin? It seems that you have picked it at the point where she throws the flowerpot, i.e above ground. Therefore, it is no surprise that the flowerpot hits the ground below that point.
 
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  • #3
gneill
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Draw a diagram of the scenario. Be sure to include your assumed coordinate system. You may be smarter than you think!
 
  • #4
309
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Where is your origin? It seems that you have picked it at the point where she throws the flowerpot, i.e above ground. Therefore, it is no surprise that the flowerpot hits the ground below that point.

I think I know what I did wrong. I need to use the second equation and forgot one important equation: V2 = V1 + a(delta t).
V2 = 2.1m/s + (-9.81m/s^2)(3.0)
V2 = -27.33 m/s
Is this correct? If so, I can now find distance by using this equation:
Delta d = V2(delta t) - 1/2(a)(delta t^2)
 
  • #5
309
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Draw a diagram of the scenario. Be sure to include your assumed coordinate system. You may be smarter than you think!

I just think I forgot an important equation that I posted in my earlier one, but I'll definitely start drawing diagrams from now on. Thanks for the suggestion!
 
  • #6
309
7
Where is your origin? It seems that you have picked it at the point where she throws the flowerpot, i.e above ground. Therefore, it is no surprise that the flowerpot hits the ground below that point.
Ah, I completely forgot about direction.

So, distance is: 6.3 - 44.145 = -37.85 m [down] which is also 37.85m[up], correct?

Just new to vectors, directions, etc. XD
 
  • #7
326
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the -37.8 is the displacement which has a direction but the height of the balcony is just a scalar so you don't put directions
 
  • #8
309
7
the -37.8 is the displacement which has a direction but the height of the balcony is just a scalar so you don't put directions

Really? I always thought displacement was the total distance traveled by the object. And since Marian was startled, she threw the pot in the air before it started falling, no? I have no idea how to find that distance, but after finding that we would that to the height of the balcony to find displacement?

My apologies if these observations seem noobish, I am self-teaching as I am doing the course online. >_<
 
  • #9
326
1
the displacement is the just change of position independent of the path just where it was and it is now anything else doesn't matter.
 

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