How fast is the light sweeping along the 2 km shoreline?

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SUMMARY

The discussion focuses on calculating the speed at which light from a lighthouse, located 1 km from a beach shore and revolving at 10π radians/minute, sweeps across a shoreline 2 km away. The key formula involved is \(v = \dot{\theta} r\), where \(\dot{\theta}\) represents the angular velocity in radians per unit time. The participants emphasize the need to understand the relationship between the angle and the distance along the shore, specifically using trigonometric principles such as the tangent function to determine the rate of change of the light's position on the shoreline.

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  • Familiarity with trigonometric functions, particularly tangent
  • Knowledge of the Pythagorean theorem
  • Basic calculus concepts related to rates of change
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hatelove
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A lighthouse 1 km from a beach shore revolves at 10\pi radians/minute. How fast is the light sweeping across the shore 2 km from the lighthouse?

I drew a diagram here to try and help:

KokY2.png


I think it will have something to do with a triangle. So just to find the other side, I'll use the Pythagorean theorem and get \sqrt{3}. I already knew this because I recognized the 30-60-90 triangle, so I know the angles too, but I don't know if we need them yet.

What I don't understand is what the question is asking. I thought the light was going around at a constant speed of 10\pi radians/minute? What speed do they want?
 
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A lighthouse 1 km from a beach shore revolves at 10\pi radians/minute. How fast is the light sweeping across the shore 2 km from the lighthouse?

1. Don't put the quetion in a quote, it makes it more difficult to construct a coherent reply.

2. \(v=\dot{\theta}r\), \(\dot{\theta}\) in units of radians per unit time, is probably what they are looking for, but there is really insufficient information in what you posted to be sure.

CB
 
daigo said:
I drew a diagram here to try and help:

KokY2.png


I think it will have something to do with a triangle. So just to find the other side, I'll use the Pythagorean theorem and get \sqrt{3}. I already knew this because I recognized the 30-60-90 triangle, so I know the angles too, but I don't know if we need them yet.

What I don't understand is what the question is asking. I thought the light was going around at a constant speed of 10\pi radians/minute? What speed do they want?
You are asked how fast the light is moving along the shoreline- that would be a speed in km per min or km per hr. What you are given is how fast the light is rotating. Yes, at the moment shown, the section along the shortline that the light reaches is 2 times \sqrt{3}. But the light is moving so that right triangle is changing. If you call the angle theta, do you see that opposite leg of the triangle is given by tan(theta)? What is the rate of change of that length relative to the rate of change of the angle.
 

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