- #1
Arbitrary
- 11
- 0
Homework Statement
A powerful, 100-kg man is kneeling in the middle of a frozen lake, holding two large rocks. One rock has a mass of 25 kg, and the other a mass of 15 kg. The man wants to quickly get to the south edge of the lake, and he knows that he needs to throw his rocks to the north to propel himself there. He always throws each rock away from himself at 20 m/s.
What will be his final speed if he
a) throws both rocks simultaneously?
b) throws the heavy rock and then the lighter rock?
c) throws the lighter rock and then the heavy rock?
Homework Equations
m[tex]\Delta[/tex]v[tex]_{initial}[/tex] = m[tex]\Delta[/tex]v[tex]_{final}[/tex]
The Attempt at a Solution
(a)
m[tex]_{man}[/tex]v = (m[tex]_{rock1}[/tex]+m[tex]_{rock2}[/tex])v[tex]_{rock}[/tex]
100v = (20+15)20
v = 8 m/s
(b)
mv = m[tex]_{rock1}[/tex]v[tex]_{rock}[/tex]
100v = 25*20
v = 5
mv[tex]_{final}[/tex]-(m+m[tex]_{rock2}[/tex])v[tex]_{initial}[/tex] = m[tex]_{rock2}[/tex]v
100v[tex]_{final}[/tex]-(100+15)*5 = 15*20
v[tex]_{final}[/tex] = 8.75 m/s
(c) I get the same answer as (b), using the same methods, which is quite weird.
The correct answers are 5.71 m/s for (a), 6.18 m/s for (b), and 6.14 m/s for (c). I'm not sure what I'm doing wrong for each part...