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# Motion in a circle Watch

1. 1. A fast jet flies at 400 ms-1, and is designed to undergo accelerations up to 12g. What is the smallest radius turn it can execute at this speed ?

2. You may have been taught that there is no such thing as a “centrifugal force” but rather that there can be “an absence of a centripetal force”. Use the example of water leaving clothes through the holes in the drum of a spin dryer to explain this statement.

3. A bobsleigh has mass 390 kg and the track is banked at 60 degrees ona bend of a radius 26.0m.
a. How fast is the bobsleigh able to go?
b. What angle of banking would be needed for the sleigh to be able to reach 130 kmh-1
c. What would be the centripetal force required for the situation in b?

...........................

I am stuck on these questions. Could somebody please explain to me how to solve these questions.

Many Thanks
2. 1. Centripetal acceleration = v²/r
3. There is a formula for angle of banking, it starts tan (theta) =
where theta is the banking angle. On the other side of the formula is the speed, radius of curvature and g. Do you have this formula in your book or notes?
3. (Original post by H.R)
1. A fast jet flies at 400 ms-1, and is designed to undergo accelerations up to 12g. What is the smallest radius turn it can execute at this speed ?

2. You may have been taught that there is no such thing as a “centrifugal force” but rather that there can be “an absence of a centripetal force”. Use the example of water leaving clothes through the holes in the drum of a spin dryer to explain this statement.

3. A bobsleigh has mass 390 kg and the track is banked at 60 degrees ona bend of a radius 26.0m.
a. How fast is the bobsleigh able to go?
b. What angle of banking would be needed for the sleigh to be able to reach 130 kmh-1
c. What would be the centripetal force required for the situation in b?

...........................

I am stuck on these questions. Could somebody please explain to me how to solve these questions.

Many Thanks

2)

You either have a centrifugal force OR a centripedal force, depending on whether you are in the reference frame of the centre of the circle, or of the object being spun round.

In the reference frame of the object, all you can feel is being pushed away from the centre of the circle, hence you have centrifugal force. In the reference frame of the centre of the circle, all you can see is the object constantly spinning, hence you have centripedal force, which is being provided by whatever it is holding the spinning object in a circle and stopping it flying away in a straight line.

One explanation of centripedal force I heard which was useful was to think about was to imagine a spinning object in 2D cartesian co-ordinates.

the centre of the circle is (0,0) Imagine the object at the point (0,1) The object is moving in the +y direction. The acceleration of the object is towards the centre of the circle - effectively it is falling straight down. However, at the same time it is still moving sideways, so that by the time it has reached the x axis it has moved 1 unit in the x direction - so it is now at (1,0) and moving in the -y direction, and so the story continues ad infinitum. So, basically you can think of an orbiting object as falling towards the earth but always missing to the side by the same amount. It keeps falling, but never gets any bloody closer.

Anyway, in answer to your question. If you imagine a man swinging round a hammer (like the olympics). the man initially throws the hammer in one direction, but then keeps pulling it back towards him as he spins round to stop it flying away - he is providing the centripedal force. Once he stops providing the force, the hammer simply carries on in the same direction it was last travelling, and zooms away from the man. It is the sudden absence of the centripedal force provided by the guys arm that frees the hammer from spinning in a circle and allows it to simply travel away in a straight line until it hits the ground. If you were the hammer, this would feel like a centrifugal force pushing you away from the man.

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Updated: December 1, 2010
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