# Gcse physics help

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#1
The roads were very icy. An accident was recorded by a security camera.Car A was waiting at a road junction. Car B, travelling at 10 m/s, went into the back of car A. This reduced car B’s speed to 4 m/s and caused car A to move forward.The total mass of car A was 1200 kg and the total mass of car B was 1500 kg.Calculate the change in momentum of car B in this accident.Give your answer in kilogram metres per second.
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6 months ago
#2
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6 months ago
#3
(Original post by abdulwk16x)
The roads were very icy. An accident was recorded by a security camera.Car A was waiting at a road junction. Car B, travelling at 10 m/s, went into the back of car A. This reduced car B’s speed to 4 m/s and caused car A to move forward.The total mass of car A was 1200 kg and the total mass of car B was 1500 kg.Calculate the change in momentum of car B in this accident.Give your answer in kilogram metres per second.
Think about the initial momentum of Car A and Car B. Then after the collision think about how these momentums change. Have you made a start on this question?
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#4
(Original post by TSRTD)
Think about the initial momentum of Car A and Car B. Then after the collision think about how these momentums change. Have you made a start on this question?
is it the same during and after the crash?
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6 months ago
#5
(Original post by abdulwk16x)
is it the same during and after the crash?
Yes, total momentum before the crash = the total momentum after the crash
Now try and calculate the different momentums
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#6
(Original post by TSRTD)
Yes, total momentum before the crash = the total momentum after the crash
Now try and calculate the different momentums
15000kg m/s?
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6 months ago
#7
(Original post by abdulwk16x)
The roads were very icy. An accident was recorded by a security camera.Car A was waiting at a road junction. Car B, travelling at 10 m/s, went into the back of car A. This reduced car B’s speed to 4 m/s and caused car A to move forward.The total mass of car A was 1200 kg and the total mass of car B was 1500 kg.Calculate the change in momentum of car B in this accident.Give your answer in kilogram metres per second.
Momentum= mass x velocity
Car A=nothing required
Car B=1500x10=15000 before accident
Car B=1500x4= 6000 after accident
15000-6000= 9000 is change in momentum
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6 months ago
#8
(Original post by abdulwk16x)
15000kg m/s?
Yep thats the momentum of car B before the crash. What about after?
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#9
(Original post by TSRTD)
Yep thats the momentum of car B before the crash. What about after?
the same
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6 months ago
#10
(Original post by abdulwk16x)
the same
No, the momentum of car B isn't the same before and after. I think you are getting confused by conservation of momentum; that applies to the whole system. As car B impacted upon car A, some of its momentum was transferred to car A.

What conservation of momentum tells us is:
momentum of car B before + momentum of car A before = momentum of car B after + momentum of car A after

The momentum of A has increased, and the momentum of B has decreased.

The momentum of car B after = mass x velocity = 4 x 1500 = 6000kgms-1.

Its change of momentum is therefore 15000-6000=9000kgms-1.

The change in momentum of car A will be equal and opposite to that of car B, so total momentum is conserved.
Last edited by Zuvio; 6 months ago
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