Motion in a Plane

  • Thread starter pharm89
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  • #1
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Homework Statement



Hi, I have some answers to the following questions but I would just like to know if Im understanding the concepts correctly.

A firefighter climbs up a 100 m ladder leaning against a verticle wall. The ladder makes an angle of 25 degrees with the wal. The firefighter reaches th eroof in 15 seconds.
(a) what is the height of the wall?
(b) how far is the base of the ladder from the wall?
(c) what is the firefighters average verticle velocity?

Homework Equations


Pythagorean Theorum - (a^2) + (b^2) = c^2
velocity (v) = distance/time


The Attempt at a Solution



(a) COS = adjacent/hypotenuse
Cos25 degrees = X/10 m
0.91 = x/10m
X= 9.06 m

(b) a^2 +b^2 = c^2
a^2 + (9.06)^2 = 10^2
a^2 +82.08 = 100
a^2 = 17.92
a= 4.2 m

(c) given distance up ladder = 10 m
time = 15.0 seconds
Required: velocity
Analysis: velocity = distance/time =10m/15s = 0.666 m/s

what is the difference between average verticle velocity and resultant velocity???
THanks for the help
Pharm 89

Homework Statement





Homework Equations





The Attempt at a Solution

 

Answers and Replies

  • #2
cristo
Staff Emeritus
Science Advisor
8,122
74

Homework Statement



Hi, I have some answers to the following questions but I would just like to know if Im understanding the concepts correctly.

A firefighter climbs up a 100 m ladder leaning against a verticle wall. The ladder makes an angle of 25 degrees with the wal. The firefighter reaches th eroof in 15 seconds.
(a) what is the height of the wall?
(b) how far is the base of the ladder from the wall?
(c) what is the firefighters average verticle velocity?

Since you've used 10m in your calculations, I presume this is a typo.

The Attempt at a Solution



(a) COS = adjacent/hypotenuse
Cos25 degrees = X/10 m
0.91 = x/10m
X= 9.06 m
Method correct.. I dont have a calculator to check your maths though. Also, this is correct, if we make the asumption that the ladder reaches the very top of the building, which we must make here.

(b) a^2 +b^2 = c^2
a^2 + (9.06)^2 = 10^2
a^2 +82.08 = 100
a^2 = 17.92
a= 4.2 m
Again, correct method

(c) given distance up ladder = 10 m
time = 15.0 seconds
Required: velocity
Analysis: velocity = distance/time =10m/15s = 0.666 m/s

what is the difference between average verticle velocity and resultant velocity???

You have calculated the resultant velocity. The vertical velocity will be v=(vertical distance)/(time)
 
  • #3
32
0
Since you've used 10m in your calculations, I presume this is a typo.

Method correct.. I dont have a calculator to check your maths though. Also, this is correct, if we make the asumption that the ladder reaches the very top of the building, which we must make here.


Again, correct method



You have calculated the resultant velocity. The vertical velocity will be v=(vertical distance)/(time)

Thanks for your help. So therefore for part c they are asking for the average verticle velocty...would that be the same answer though??
verticle distance = 10 m/15 seconds = 0.666m/s
 
  • #4
cristo
Staff Emeritus
Science Advisor
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Thanks for your help. So therefore for part c they are asking for the average verticle velocty...would that be the same answer though??
verticle distance = 10 m/15 seconds = 0.666m/s

No, the vertical distance is what you calculated in (a) i.e. the height of the building. Draw a diagram, you'll see that the length of the ladder, 10m, is not a vertical distance!
 
  • #5
32
0
No, the vertical distance is what you calculated in (a) i.e. the height of the building. Draw a diagram, you'll see that the length of the ladder, 10m, is not a vertical distance!

Yes, that makes sense...verticle distance = 9.06 m/15 seconds = 0. 6 m/s
Thanks for the assistance.
Pharm 89
 
  • #6
10m? Wasn't it supposed to be 100m?

I took

[tex]

sin(25)*100m=40m

[/tex]

[tex]

cos(25)*100m=90m

[/tex]

Instead of doing it the hard way without going through Pythogarous and keeping the idea of significant digits in my head.

And no tough work involved here...

[tex]

40m/15s = 3 m/s

[/tex]

Got 3 m/s, which isn't the same as you'lls (The Texan accent triumphs again)

But pharm89, where did the 10m instead of the 100m come from?

Please be careful in calculations.
 
Last edited:
  • #7
Hootenanny
Staff Emeritus
Science Advisor
Gold Member
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:bugeye: A 100m ladder :bugeye:
 
  • #8
radou
Homework Helper
3,120
7
[tex]

sin(25)*100m=40m

[/tex]

There is no reason to round off 100*sin(25) = 42.26 to 40.
 
  • #9
cristo
Staff Emeritus
Science Advisor
8,122
74
10m? Wasn't it supposed to be 100m?

I took

[tex]

sin(25)*100m=40m

[/tex]

[tex]

cos(25)*100m=90m

[/tex]

Instead of doing it the hard way without going through Pythogarous and keeping the idea of significant digits in my head.

And no tough work involved here...

[tex]

40m/15s = 3 m/s

[/tex]

Got 3 m/s, which isn't the same as you'lls (The Texan accent triumphs again)

But pharm89, where did the 10m instead of the 100m come from?

Please be careful in calculations.

I presumed (and was apparently right, as the poster didn't correct me) that there was a typo in the original post, and it should in fact be 10m. And, as Hootenanny points out, 100m is a little long for a ladder!
 
  • #10
DaveC426913
Gold Member
20,080
3,379
At first I wondered why he shrunk the ladder from 100m to 10m in his calcs.

But 10m has to be the correct size. Can you imagine climbing 100m in 15s?
 

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