# Physics: Can you help with number 9?

How did he get 12.5m/s?

(Number 8 ties in with number 9)

### 3 Answers

- 冷眼旁觀Lv 61 month agoFavorite Answer
Assign all quantities towards Sally to be negative, and in the direction of shooting out from Sally to be positive.

Assign the asteroid to be "1", and the cable to be "2".

m₁ = 85 kg, m₂ = 10 kg

Initial: u₁ = -17.0 m/s, u₂ = 25 m/s

FinalL v₁ = v₂ = v

Total initial momentum = Total final momentum

m₁u₁ + m₂u₂ = (m₁ + m₂)v

85×(-17.0) + 10×25 = (85 + 10)v

v = [85×(-17.0) + 10×25] / (85 + 10)

Final velocity v = -12.6 m/s

(The negative sign shows that the direction is towards Sally.)

The momentum of the asteroid-cable after the cable impacts with the asteroid

= (85 + 10) × (-12.6)

= -1200 kg•m/s

(The negative sign shows that the direction is towards Sally.)

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- Anonymous1 month ago
8)

Vs = 25*10/55 = 50/11 of m/sec

9)

final momentum M = 85*17-10*25 = 1195 kg*m/sec

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- DylanLv 61 month ago
Question 9 asks for the momentum of the asteroid + cable after the impact so 12.5 m/s is actually wrong, it is asking for momentum not velocity.

Momentum before the collision is the same as after the collision.

Ma*Va + Mc*Vc

The cable is moving at 25 m/s away from sally and the asteroid is moving towards sally at 17 m/s, the weight of both is given.

85*17 + 10*(-25)

= 1195 kg m/s

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