AP Physics 1 : Period and Frequency

Study concepts, example questions & explanations for AP Physics 1

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Example Questions

Example Question #171 : Circular, Rotational, And Harmonic Motion

A bowling ball of mass \displaystyle 5kg and radius \displaystyle r=0.12m is thrown with a rotational kinetic energy of \displaystyle 20J. Find its period.

Possible Answers:

\displaystyle 0.39s

\displaystyle 0.90s

\displaystyle 0.17s

\displaystyle 0.52s

\displaystyle 0.77s

Correct answer:

\displaystyle 0.17s

Explanation:

The expression for rotational kinetic energy is:

\displaystyle K = \frac{1}{2}Iw^2

Where I is the moment of inertia for a solid sphere:

\displaystyle I = \frac{2}{5}mr^2

And w is the angular velocity:

\displaystyle w = \frac{v}{r}

Plugging these in, we get:

\displaystyle K = \frac{1}{5}mv^2

Rearranging for linear velocity:

\displaystyle v=\sqrt{\frac{5K}{m}}

The expression for period is the circumference of the ball divided by the linear velocity of the ball:

\displaystyle P = \frac{c}{v}

Plugging in expressions, we get:

\displaystyle P = \frac{2\pi r}{\sqrt{\frac{5K}{m}}}

\displaystyle P = 2\pi r \sqrt{\frac{m}{5K}}

We have all of these values, so time to plug and chug:

\displaystyle P = 2\pi (0.12m)\sqrt{\frac{5kg}{5(20J)}}

\displaystyle P = 0.17s

Example Question #1081 : Newtonian Mechanics

A ferris wheel has a diameter of 60 meters. The carriages on the outside of the wheel are traveling at an instantaneous velocity of \displaystyle 3\frac{m}{s}. What is the period of rotation of the wheel?

Possible Answers:

\displaystyle 14\ \text{seconds}

\displaystyle 31\ \text{seconds}

\displaystyle 63\ \text{seconds}

\displaystyle 81\ \text{seconds}

\displaystyle 20\ \text{seconds}

Correct answer:

\displaystyle 63\ \text{seconds}

Explanation:

We can calculate the circumference of the wheel using the given radius:

\displaystyle C = 2\pi r = 2\pi \cdot30 = 60\pi\ m

We can use this and the given velocity to find the period:

\displaystyle P = \frac{C}{v} = \frac{60\pi}{3} = 63\ \text{seconds}

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