My parting question!

Classical Mechanics Level 3

You’re stuck in a boat in the middle of a lake. Luckily, you brought your physics book! You decide to use your book to propel you back to the shore. You throw your 1 kg book overboard with a speed of 10 m/s to propel yourself back towards the shore. Assume the combined mass of you and the boat is 100 kg.How long would it take you to reach the shore after throwing your book? (Ignore friction between the water and the boat.) The shore is 60 m away. Give your answer in minutes.


The answer is 10.

Mardokay Mosazghi by Mardokay Mosazghi , 22, USA

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2 solutions

The speed of the boat can be find by law conservative momentum where the book stores momentum to the boat.

m v 1 = m v 2 mv1=mv2 1.10 = 100. v 1.10=100.v v = 0.1 m / s v=0.1m/s And we know that, t = d / v t=d/v t = 60 / 0.1 t=60/0.1 t = 600 s t=600s t = 10 m i n u t e s t=10minutes

3 Helpful 0 Interesting 1 Brilliant 0 Confused
Sravanth C.
Feb 4, 2015

we know that, m1u1+m2u2=m1v1+m2v2

here, the initial velocities are, u1=0 and u2=0 respectively.

therefore,
m1v1+m2v2=0

or, 1 10+100 v2=0

or, v2 = -10/100 =-1/10

or, 1/10m/s inthe opposite direction.

therefore time taken to reach the shore = distance/speed = 60/1/10 = 60*10 = 600seconds

or, 600/60 min = 10minutes

2 Helpful 0 Interesting 0 Brilliant 0 Confused

I also did something similae to that although kinda different to be crystal. used formula for momentum (mv)-(mu) and (1x10)-(100v) 10/100 = 0.1 which is v.

t=d/v distance given as 60, v= 0.1 t=d/v therefore 60/0.1 =600 seconds

Zeyad Eldesouki - 1 year, 7 months ago

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