PHYS101: Introduction to Mechanics

Example 10.3 Calculating the Acceleration of a Fishing Reel

A deep-sea fisherman hooks a big fish that swims away from the boat pulling the fishing line from his fishing reel. The whole system is initially at rest and the fishing line unwinds from the reel at a radius of 4.50 cm from its axis of rotation. The reel is given an angular acceleration of $110 \, \mathrm{rad} / \mathrm{s}^{2}+$ for 2.00 s as seen in Figure 10.8.

(a) What is the final angular velocity of the reel?

(b) At what speed is fishing line leaving the reel after 2.00 s elapses?

(c) How many revolutions does the reel make?

(d) How many meters of fishing line come off the reel in this time?

Strategy

In each part of this example, the strategy is the same as it was for solving problems in linear kinematics. In particular, known values are identified and a relationship is then sought that can be used to solve for the unknown.

Solution for (a)

Here $\alpha$ and $t$ are given and $\omega$ needs to be determined. The most straightforward equation to use is $\omega=\omega_{0}+\alpha t$ because the unknown is already on one side and all other terms are known. That equation states that

$\omega=\omega_{0}+\alpha t.$

We are also given that $\omega_{0}=0$ (it starts from rest), so that

$\omega=0+\left(110 \, \mathrm{rad} / \mathrm{s}^{2}\right)(2.00 \mathrm{~s})=220 \, \mathrm{rad} / \mathrm{s}.$

Solution for (b)

Now that $\omega$ is known, the speed $v$ can most easily be found using the relationship

$v=r \omega,$

where the radius $r$ of the reel is given to be $4.50 \mathrm{~cm}$; thus,

$v=(0.0450 \mathrm{~m})(220 \, \mathrm{rad} / \mathrm{s})=9.90 \mathrm{~m} / \mathrm{s}.$

Note again that radians must always be used in any calculation relating linear and angular quantities. Also, because radians are dimensionless, we have $\mathrm{m} \times \mathrm{rad}=\mathrm{m}$.

Solution for (c)

Here, we are asked to find the number of revolutions. Because $1 \mathrm{rev}=2 \pi \mathrm{rad}$, we can find the number of revolutions by finding $\theta$ in radians. We are given $\alpha$ and $t$, and we know $\omega_{0}$ is zero, so that $\theta$ can be obtained using $\theta=\omega_{0} t+\frac{1}{2} \alpha t^{2}$.

$\begin{aligned} \theta &=\omega_{0} t+\frac{1}{2} \alpha t^{2} \\ &=0+(0.500)\left(110 \, \mathrm{rad} / \mathrm{s}^{2}\right)(2.00 \mathrm{~s})^{2}=220 \, \mathrm{rad}. \end{aligned}$

Converting radians to revolutions gives

$\theta=(220 \, \mathrm{rad}) \frac{1 \, \mathrm{rev}}{2 \pi \, \mathrm{rad}}=35.0 \, \mathrm{rev}.$

Solution for (d)

The number of meters of fishing line is $x$, which can be obtained through its relationship with $\theta$:

$x=r \theta=(0.0450 \mathrm{~m})(220 \, \mathrm{rad})=9.90 \mathrm{~m}.$

Discussion

This example illustrates that relationships among rotational quantities are highly analogous to those among linear quantities. We also see in this example how linear and rotational quantities are connected. The answers to the questions are realistic. After unwinding for two seconds, the reel is found to spin at 220 rad/s, which is 2100 rpm. (No wonder reels sometimes make high-pitched sounds). The amount of fishing line played out is 9.90 m, about right for when the big fish bites.

Example 10.4 Calculating the Duration When the Fishing Reel Slows Down and Stops

Now let us consider what happens if the fisherman applies a brake to the spinning reel, achieving an angular acceleration of $-300 \mathrm{rad} / \mathrm{s}^{2}$. How long does it take the reel to come to a stop?

Strategy

We are asked to find the time $t$ for the reel to come to a stop. The initial and final conditions are different from those in the previous problem, which involved the same fishing reel. Now we see that the initial angular velocity is $\omega_{0}=220 \mathrm{rad} / \mathrm{s}$ and the final angular velocity $\omega$ is zero. The angular acceleration is given to be $\alpha=-300 \mathrm{rad} / \mathrm{s}^{2}$. Examining the available equations, we see all quantities but $t$ are known in $\omega=\omega_{0}+\alpha t$, making it easiest to use this equation.

Solution

The equation states 

$\omega=\omega_{0}+\alpha t.$

We solve the equation algebraically for $t$, and then substitute the known values as usual, yielding

$t=\frac{\omega-\omega_{0}}{\alpha}=\frac{0-220 \, \mathrm{rad} / \mathrm{s}}{-300 \, \mathrm{rad} / \mathrm{s}^{2}}=0.733 \mathrm{~s}.$

Discussion

Note that care must be taken with the signs that indicate the directions of various quantities. Also, note that the time to stop the reel is fairly small because the acceleration is rather large. Fishing lines sometimes snap because of the accelerations involved, and fishermen often let the fish swim for a while before applying brakes on the reel. A tired fish will be slower, requiring a smaller acceleration.

Example 10.5 Calculating the Slow Acceleration of Trains and Their Wheels

Large freight trains accelerate very slowly. Suppose one such train accelerates from rest, giving its $0.350$ m-radius wheels an angular acceleration of $0.250 \, \mathrm{rad} / \mathrm{s}^{2}$. After the wheels have made 200 revolutions (assume no slippage): (a) How far has the train moved down the track? (b) What are the final angular velocity of the wheels and the linear velocity of the train?

Strategy

In part (a), we are asked to find $x$, and in (b) we are asked to find $\omega$ and $v$. We are given the number of revolutions $\theta$, the radius of the wheels $r$, and the angular acceleration $\alpha$.

Solution for (a)

The distance $x$ is very easily found from the relationship between distance and rotation angle:

$\theta=\frac{x}{r}.$

Solving this equation for $x$ yields

$x=r \theta.$

Before using this equation, we must convert the number of revolutions into radians, because we are dealing with a relationship between linear and rotational quantities:

$\theta=(200 \text { rev }) \frac{2 \pi \, \mathrm{rad}}{1 \, \mathrm{rev}}=1257 \, \mathrm{rad}$

Now we can substitute the known values into $x=r \theta$ to find the distance the train moved down the track:

$x=r \theta=(0.350 \mathrm{~m})(1257 \, \mathrm{rad})=440 \mathrm{~m}.$

Solution for (b)

We cannot use any equation that incorporates $t$ to find $\omega$, because the equation would have at least two unknown values. The equation $\omega^{2}=\omega_{0}^{2}+2 \alpha \theta$ will work, because we know the values for all variables except $\omega$:

$\omega^{2}=\omega_{0}^{2}+2 \alpha \theta.$

Taking the square root of this equation and entering the known values gives

$\begin{aligned}\omega &=\left[0+2\left(0.250 \mathrm{rad} / \mathrm{s}^{2}\right)(1257 \mathrm{rad})\right]^{1 / 2} \\ &=25.1 \mathrm{rad} / \mathrm{s}.\end{aligned}$

We can find the linear velocity of the train, $v$, through its relationship to $\omega$ :

$v=r \omega=(0.350 \mathrm{~m})(25.1 \mathrm{rad} / \mathrm{s})=8.77 \mathrm{~m} / \mathrm{s}.$

The distance traveled is fairly large and the final velocity is fairly slow (just under 32 km/h).

Example 10.6 Calculating the Distance Traveled by a Fly on the Edge of a Microwave Oven Plate

A person decides to use a microwave oven to reheat some lunch. In the process, a fly accidentally flies into the microwave and lands on the outer edge of the rotating plate, and remains there. If the plate has a radius of 0.15 m and rotates at 6.0 rpm, calculate the total distance traveled by the fly during a 2.0-min cooking period. (Ignore the start-up and slow-down times).

Strategy

First, find the total number of revolutions $\theta$, and then the linear distance $x$ traveled. $\theta=\bar{\omega} t$ can be used to find $\theta$ because $\bar{\omega}$ is given to be $6.0$ rpm.

Solution

Entering known values into $\theta=\bar{\omega} t$ gives

$\theta=\bar{\omega} t=(6.0 \, \mathrm{rpm})(2.0 \, \mathrm{~min})=12 \, \mathrm{rev}.$

As always, it is necessary to convert revolutions to radians before calculating a linear quantity like $x$ from an angular quantity like $\theta$:

$\theta=(12 \text { rev })\left(\frac{2 \pi \, \mathrm{rad}}{1 \, \mathrm{rev}}\right)=75.4 \, \mathrm{rad}.$

Now, using the relationship between $x$ and $\theta$, we can determine the distance traveled:

$x=r \theta=(0.15 \mathrm{~m})(75.4 \, \mathrm{rad})=11 \mathrm{~m}.$

Discussion

Quite a trip (if it survives)! Note that this distance is the total distance traveled by the fly. Displacement is actually zero for complete revolutions because they bring the fly back to its original position. The distinction between total distance traveled and displacement was first noted in One-Dimensional Kinematics.