Sheila weighs 60 kg and is riding a bike. her momentum on the bike is 340 kg • m/s. the bike hits a rock, which stops it completely and throws sheila forward onto the pavement. if there is no net force on the system, what is sheila’s velocity immediately after she is thrown from the bike? 1.8 m/s 2.0 m/s 5.0 m/s 5.7 m/s

5.7 m/s is the correct answer.

5.7 m/s

Explanation:

1) Physical principle: conservation of momentum.

2) In consequence, considering that there is no net force on the system, Sheila and the bike’s momentum before and after the collision is the same.

So, now do the calculations:

         p₁ = (mass of Sheila + mass of the bike)×velocity

         p₁ = (60kg + m) × v₁ = 340 kg • m/s

        p₂ = mass of sheila × velocity of Sheila + mass of the bike × velocity of the bike

        p₂ = 60kg × v₂ + m×0 = 60kg × v₂

        p₁ = p₂ ⇒ (60kg + m) × v₁ = 60kg × v₂ ⇒ 340 kg • m/s = 60kg × v₂

        ⇒ v₂ = 340 kg • m/s / 60 kg ≈ 5.7 m/s ← answer

Final reflection: since the momentum is conserved and the bike stops completely after the collision, Sheila wil keep the total momentum, and you can calculate her velocity dividing the total momentum by her mass:  v = 340 kg • m/s / 60 kg ≈ 5.7 m/s

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