momentum question from 2018 as physics

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username4167452
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The horizontal runway in Figure 2 has negligible friction and air resistance. As the
truck moves along the runway, it starts to rain. The rain falls vertically and water
collects in the truck.
Discuss whether there are any changes in the momentum of the truck and collected water.

My answer was '
momentum is conserved in the collision between the truck and rain. So momentum of truck before collision + momentum of rain before collision as it hits truck = momentum of truck with water collected. But the momentum of rain is 0 after it hits truck (it gets collected) and because momentum in system has to be conserved, the heavier truck must gain momentum'.

But the correct answer was the momentum of the truck remains the same so its velocity decreases in order to compensate for the greater mass.

Why is my answer wrong? Shouldn't we count the momentum of the rain before hand too because it's included in the system?
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Walnut Prophecy
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Hey so I just came across your question, although I dont know which board you're having, is it cambridge? because mine is edexcel and we just did momentum in A2. So I'm having good a good guess at this, and I hope my answer satisfies your question. Notice that it's specifically mentioning that the rain falling was vertical. As momentum is a vector quantity, this rain has no horizontal component in the direction of motion of the truck. The rain is making no contribution to the total momentum now. So the total momentum of the truck before and after should remain the same according to the law of conservation of momentum, but now the total mass has increased due the rain, so to keep the p constant, v must decrease. As p=mv.
Someone else may correct me if I'm wrong
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mqb2766
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(Original post by Physikoi)
The horizontal runway in Figure 2 has negligible friction and air resistance. As the
truck moves along the runway, it starts to rain. The rain falls vertically and water
collects in the truck.
Discuss whether there are any changes in the momentum of the truck and collected water.

My answer was '
momentum is conserved in the collision between the truck and rain. So momentum of truck before collision + momentum of rain before collision as it hits truck = momentum of truck with water collected. But the momentum of rain is 0 after it hits truck (it gets collected) and because momentum in system has to be conserved, the heavier truck must gain momentum'.

But the correct answer was the momentum of the truck remains the same so its velocity decreases in order to compensate for the greater mass.

Why is my answer wrong? Shouldn't we count the momentum of the rain before hand too because it's included in the system?
Once the rain has been collected by the truck, the horizontal velocity must be the same as the truck (assuming it acts as a single block) whereas it was zero while falling. So for momentum to be conserved, the overall velocity of the truck and water must decrease as the mass increases.

You seem to be arguing that the momentum (velocity) of the rain is zero, relative to the truck? You'd really have to measure the speed of the truck and the rain (combined system) from a stationary point which was away from the truck.
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Eimmanuel
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(Original post by Physikoi)
The horizontal runway in Figure 2 has negligible friction and air resistance. As the
truck moves along the runway, it starts to rain. The rain falls vertically and water
collects in the truck.
Discuss whether there are any changes in the momentum of the truck and collected water.

My answer was '
momentum is conserved in the collision between the truck and rain. So momentum of truck before collision + momentum of rain before collision as it hits truck = momentum of truck with water collected. But the momentum of rain is 0 after it hits truck (it gets collected) and because momentum in system has to be conserved, the heavier truck must gain momentum'.

But the correct answer was the momentum of the truck remains the same so its velocity decreases in order to compensate for the greater mass.

Why is my answer wrong? Shouldn't we count the momentum of the rain before hand too because it's included in the system?
This question stirred up some great discussions among the students in this thread and you may be interested. https://www.thestudentroom.co.uk/sho...239184&page=16
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username4167452
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(Original post by Eimmanuel)
This question stirred up some great discussions among the students in this thread and you may be interested. https://www.thestudentroom.co.uk/sho...239184&page=16
I'll check it out, thanks
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Eimmanuel
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(Original post by Walnut Prophecy)
Hey so I just came across your question, although I dont know which board you're having, is it cambridge? because mine is edexcel and we just did momentum in A2. So I'm having good a good guess at this, and I hope my answer satisfies your question. Notice that it's specifically mentioning that the rain falling was vertical. As momentum is a vector quantity, this rain has no horizontal component in the direction of motion of the truck. The rain is making no contribution to the total momentum now. So the total momentum of the truck before and after should remain the same according to the law of conservation of momentum, but now the total mass has increased due the rain, so to keep the p constant, v must decrease. As p=mv.
Someone else may correct me if I'm wrong
I think OP is doing AQA and this question appear in 2018 AQA paper 1 (I think). And it stirred up some discussions among the students who sat for the exam.
https://www.thestudentroom.co.uk/sho...239184&page=16
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golgiapparatus31
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What happens to momentum vertically?
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