
#1
Nov2806, 04:31 PM

P: 106

1. The problem statement, all variables and given/known data
A 600 kg ice boat moves on runners on essentially frictionless ice. A steady wind blows, applying a constant force to the sail. At the end of an 7 s run, the acceleration is 0.4 m/s2. What was the acceleration at the beginning of the run? Answer: 0.4m/s What was the force due to the wind? Answer:240N What retarding force must be applied at the end of 4 s to bring the ice boat to rest by the end of the next 4 s? (The wind is still blowing. Assume the boat was at rest at time t = 0.) Answer:??? 2. Governing equations Vf=Vi+at 3. The attempt at a solution I understand the first two questions i have them figured out but the last one i can't seem to figure out. i used the equation 0.4=0+a(4) and i get a=0.1 which i multiply by 600kg using the formula EF=ma and get EF=60N i don't understand why this is wrong any help would be appreciated. 



#2
Nov2806, 04:56 PM

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If you apply the same force as the wind, then what will the velocity do? What force did the boat feel for the first 4s? If that force stopped suddenly at 4s, what force would it take to stop the boat in the next 4s? What if the wind force stays there  how much total reverse force is required to stop the boat in that next 4s?




#3
Nov2806, 05:02 PM

P: 106

i reread the question and realize that the finally velocity needs to be zero would that make Vi=0.4




#4
Nov2806, 05:15 PM

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P: 39,723

Ice Boat Problem
It says the boat was at rest at t=0, so Vi=0.
Reread my hints about what the situation is if the wind stops at t=4s. 



#5
Nov2806, 05:18 PM

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#6
Nov2806, 05:21 PM

P: 106

If you apply the same force as the vind the velocity slows down,would it not feel a force of 60 for the first four seconds?




#7
Nov2806, 05:23 PM

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P: 39,723





#8
Nov2806, 05:24 PM

P: 106





#9
Nov2806, 05:26 PM

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#10
Nov2806, 05:27 PM

P: 106





#11
Nov2806, 05:28 PM

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#12
Nov2806, 05:35 PM

P: 106

Just before 4s the boat was moving with a constant velocity




#13
Nov2806, 05:38 PM

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P: 39,723

Vf = Vi + at expresses. Now, Tell us what would happen if the wind stopped at t=4 and you applied the 240N backwards for 4s. Then tell us the solution to this problem. 



#14
Nov2806, 05:43 PM

P: 106

the boat would then slow down




#16
Nov2806, 05:54 PM

P: 106

i don't understand what you mean by the change in velocity must be zero does this mean the Vf and Vi are the same number, and i don't understand how to apply 240N to this equation.




#17
Nov2806, 05:58 PM

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P: 39,723

And then the next step to get to the final solution for 3 is to keep the 240N wind force blowing for t>4s, and ask yourself what the total reverse force is that you must apply on the bow to stop in the next 4s. 



#18
Nov2806, 06:04 PM

P: 106

ok the acceleraton would be 0.4 the final velocity would be zero because you want to find t when the boat is stopped so to find the vi would be how fast the boat is going with the wind pushing right. so i get that it take 4s for the boat to stop.



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