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Have just been doing Physics Jan04 Unit4 paper and I'm confused on something (it's question2 if anyonhe has the paper).

It's talking about a car travelling over a hump-backed bridge in the road. When the car is driven over the bridge it follows part of a vertical circle of radius (25m) at the point O below the bridge. At the top of the bridge when the car is at rest (ie. equil.) it has normal reaction force acting upwards and the weight downwards.

The part I'm stuck on is: "If the car is driv en across the bridge repeatedly at gradually inc speeds it is found that above a critical speed the car loses contact with the road at A and "takes off". Explain why this happens."

I would have thought this would occur only when R is greater than the weight. The markscheme says "The centripetal force required is greater than the weight, which is impossible". Could someone explain this 2 me (tonight!!!).

Thank you sooooo much!

It's talking about a car travelling over a hump-backed bridge in the road. When the car is driven over the bridge it follows part of a vertical circle of radius (25m) at the point O below the bridge. At the top of the bridge when the car is at rest (ie. equil.) it has normal reaction force acting upwards and the weight downwards.

The part I'm stuck on is: "If the car is driv en across the bridge repeatedly at gradually inc speeds it is found that above a critical speed the car loses contact with the road at A and "takes off". Explain why this happens."

I would have thought this would occur only when R is greater than the weight. The markscheme says "The centripetal force required is greater than the weight, which is impossible". Could someone explain this 2 me (tonight!!!).

Thank you sooooo much!

Hoofbeat

Have just been doing Physics Jan04 Unit4 paper and I'm confused on something (it's question2 if anyonhe has the paper).

It's talking about a car travelling over a hump-backed bridge in the road. When the car is driven over the bridge it follows part of a vertical circle of radius (25m) at the point O below the bridge. At the top of the bridge when the car is at rest (ie. equil.) it has normal reaction force acting upwards and the weight downwards.

The part I'm stuck on is: "If the car is driv en across the bridge repeatedly at gradually inc speeds it is found that above a critical speed the car loses contact with the road at A and "takes off". Explain why this happens."

I would have thought this would occur only when R is greater than the weight. The markscheme says "The centripetal force required is greater than the weight, which is impossible". Could someone explain this 2 me (tonight!!!).

Thank you sooooo much!

It's talking about a car travelling over a hump-backed bridge in the road. When the car is driven over the bridge it follows part of a vertical circle of radius (25m) at the point O below the bridge. At the top of the bridge when the car is at rest (ie. equil.) it has normal reaction force acting upwards and the weight downwards.

The part I'm stuck on is: "If the car is driv en across the bridge repeatedly at gradually inc speeds it is found that above a critical speed the car loses contact with the road at A and "takes off". Explain why this happens."

I would have thought this would occur only when R is greater than the weight. The markscheme says "The centripetal force required is greater than the weight, which is impossible". Could someone explain this 2 me (tonight!!!).

Thank you sooooo much!

I would say a higher velocity when the car reaches the peak means more upward momentum when the road turns - as the car's weight is constant, and there is no road, the resultant force is the car's weight, but as this is a constant, larger upward momentum takes longer to be changed to downward momentum. Therefore above a higher velocity, when the road slopes downward the car's wheels leave the road - below that velocity, the car still has upward momentum, but not enough for the car to leave the road before it is reduced by gravity..

I think. What's centripetal?

mik1a

I think. What's centripetal?

I think. What's centripetal?

We started it today, it is unit 4

correct me if i'm wrong but the velocity required for circular motion can be found by v=root(gr) where g is the acceleration due to gravity and r is the radius of the circle (in this case 25m). i may be wrong. meaning that the required velocity is 15.66m/s. any greater than this and it will take off (lets be honest, thats what we all want!). like i say i could be wrong

Rustyk1

correct me if i'm wrong but the velocity required for circular motion can be found by v=root(gr) where g is the acceleration due to gravity and r is the radius of the circle (in this case 25m). i may be wrong. meaning that the required velocity is 15.66m/s. any greater than this and it will take off (lets be honest, thats what we all want!). like i say i could be wrong

So the centripetal force is gravity?

How does that work, since gravity is always acting downwards on the car. And the answer states: "The centripetal force required is greater than the weight"... which makes me think they are two seperate things?

mik1a

So the centripetal force is gravity?

How does that work, since gravity is always acting downwards on the car. And the answer states: "The centripetal force required is greater than the weight"... which makes me think they are two seperate things?

How does that work, since gravity is always acting downwards on the car. And the answer states: "The centripetal force required is greater than the weight"... which makes me think they are two seperate things?

friction can cause centripetal force too...so that probably helps

The centripetal force is whatever force is keeping the object moving in a circle - in this case the weight of the car is the centripetal force.

The centripetal force required to keep the car moving in a circle depends on the velocity of the car (F=V^2/r). When it says "the centripetal force becomes greater than the weight" what it means is more force than is provided by the weight of the car would be required to keep the car following its circular path, therefore there is not enough weight to act as a centripetal force to the circular motion and so the circular motion is broken.

The centripetal force required to keep the car moving in a circle depends on the velocity of the car (F=V^2/r). When it says "the centripetal force becomes greater than the weight" what it means is more force than is provided by the weight of the car would be required to keep the car following its circular path, therefore there is not enough weight to act as a centripetal force to the circular motion and so the circular motion is broken.

Rustyk1

correct me if i'm wrong but the velocity required for circular motion can be found by v=root(gr) where g is the acceleration due to gravity and r is the radius of the circle (in this case 25m). i may be wrong. meaning that the required velocity is 15.66m/s. any greater than this and it will take off (lets be honest, thats what we all want!). like i say i could be wrong

thanks every1 it makes sense now. And yes 15.6m/s is the correct answer for the subsequent part of the question!

Good Luck to anyone doing Edexcel A2 Physics this morning!

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- Subject change and repeats
- How to solve this question?
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- Car
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