- #1

_N3WTON_

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## Homework Statement

The position function of objects A and B describe different motion along the same path for t >= 0.

A: [itex] r(t) = cos(t)i + sin(t)j [/itex]

B: [itex] r(t) = cos(3t)i + sin(3t)j [/itex]

a) Sketch the path followed by A and B

b) Find the velocity and acceleration of A and B and discuss the distance

c) Express the acceleration of A and B in terms of the tangential and normal components and discuss the difference

## Homework Equations

T(t) = velocity/speed (unit tangent vector)

N(t) = T'(t)/ absolute value of T'(t) (unit normal vector)

[itex] a_T = v' [/itex]

[itex] a_N = kv^{2} [/itex]

[itex] a = v'T + kV^{2}N [/itex] (k is curvature)

## The Attempt at a Solution

Ok, this was a problem for my multivariable calculus class but if the mods feel that it belongs in a physics subforum then by all means move it there :).

a) I know that both functions are circles with radius r = 1, traced out in a counterclockwise direction

b) [itex] v(t)_A = \int r(t) dt = -sin(t)i + cos(t)j [/itex]

[itex] v(t)_B = \int r(t) dt = -3sin(3t)i + 3cos(3t)j [/itex]

[itex] a(t)_A = \int v(t) dt = -cos(t)i - sin(t)j [/itex]

[itex] a(t)_B = \int v(t) dt = -9cos(3t)i - 9sin(3t)j [/itex]

The difference between the two is that the 'B' function is traced out much faster than the 'A' function

c) For A:

[itex] T(t) = -sin(t)i + cos(t)j [/itex]

[itex] N(t) = -cos(t)i - sin(t)j [/itex]

The magnitude of T, N is equal to 1, as is the curvature k

Tangential acceleration is:

[itex] a_T = -cos(t)i - sin(t)j [/itex]

Normal acceleration is:

[itex] a_N = sin^2(t)i + cos^2(t)j [/itex]

For B:

[itex] T(t) = -sin(3t)i + cos(3t)j [/itex]

[itex] N(t) = -cos(3t)i - sin(3t)j [/itex]

The magnitude of T, N is equal to 3 and the curvature k is equal to 1

[itex] a_T = -9cos(3t)i - 9sin(3t)j [/itex]

[itex] a_N = 9sin^2(3t)i + 9cos^2(3t)j [/itex]

The tangential component shows the rate of change of velocity and the normal component shows the rate of change of direction. In this case the tangential components of the two functions are equal. However B has a larger normal component, therefore it changes direction more quickly. This problem was worth 20 points; however, I only received 16... I am allowed to make a correction for full credit so I was hoping somebody could tell me where I went wrong :)