The tires of a car make 75 revolutions as the car reduces its speed uniformly from 95 km/h to 55 km/h. The tires have a diameter of 0.80 m. (a) What was the angular acceleration of the tires? If the car continues to decelerate at this rate, (b) how much more time is required for it to stop, and (c) how far does it go?

Short Answer

Expert verified

The results for parts (a), (b), and (c) are\( - 3.1\;{\rm{rad/}}{{\rm{s}}^2}\), \(12.2\,{\rm{s}}\), and \(282.4\;{\rm{m}}\),respectively.

Step by step solution

01

Given data

The number of revolutions is\(\theta = 75\;{\rm{rev}}\).

The initial speed of the car is\({v_1} = 95\;{\rm{km/h}}\).

The final speed of the car is\({v_2} = 55\;{\rm{km/h}}\).

The diameter of the tire is\(D = 0.80\;{\rm{m}}\).

02

Understanding angular velocity

In this problem, first, the angular velocity of the wheel needs to be calculated by dividing the linear velocity with time. Thereafter, the equation of kinematics needs to be used to evaluate the angular acceleration.

03

Determine the angular acceleration

The angular acceleration can be calculated as:

\(\begin{aligned}{l}\alpha &= \frac{{{\omega _2}^2 - {\omega _1}^2}}{{2\theta }}\\\alpha &= \frac{{{v_2}^2 - {v_1}^2}}{{2\theta {r^2}}}\\\alpha &= \frac{{{v_2}^2 - {v_1}^2}}{{2\theta {{\left( {\frac{D}{2}} \right)}^2}}}\end{aligned}\)

On plugging the values in the above relation, you get:

\(\begin{aligned}{l}\alpha &= \left( {\frac{{{{\left( {55\;{\rm{km/h}}} \right)}^2} - {{\left( {95\;{\rm{km/h}}} \right)}^2}{{\left( {\frac{{1\;{\rm{m/s}}}}{{3.6\;{\rm{km/h}}}}} \right)}^2}}}{{2\left( {75\;{\rm{rev}} \times \frac{{2\pi \;{\rm{rad}}}}{{1\;{\rm{rev}}}}} \right){{\left( {\frac{{0.8\;{\rm{m}}}}{2}} \right)}^2}}}} \right)\\\alpha &= - 3.1\;{\rm{rad/}}{{\rm{s}}^2}\end{aligned}\)

Thus, \(\alpha = - 3.1\;{\rm{rad/}}{{\rm{s}}^2}\) is the required angular acceleration.

04

Determine the time to stop the car

The relation to obtain the required time is given by:

\(t = \frac{{\omega - {\omega _0}}}{\alpha }\)

Here, \({\omega _0}\) and \(\omega \) are the initial and final angular velocities, where the final angular velocity is equal to zero.

On plugging the values in the above relation, you get:

\(\begin{aligned}{l}t &= \frac{{0 - {v_2}}}{{\alpha r}}\\t &= - \frac{{{v_2}}}{{\alpha \left( {\frac{D}{2}} \right)}}\\t &= - \left( {\frac{{\left( {55\;{\rm{km/h}}} \right)\left( {\frac{{1\;{\rm{m/s}}}}{{3.6\;{\rm{km/h}}}}} \right)}}{{\left( { - 3.1\;{\rm{rad/}}{{\rm{s}}^2}} \right)\left( {\frac{{0.8\;{\rm{m}}}}{2}} \right)}}} \right)\\t = 12.2\,{\rm{s}}\end{aligned}\)

Thus, \(t = 12.2\,{\rm{s}}\) is the required time.

05

Determine the total angular displacement

The relation to obtain the required time is given by:

\({\omega ^2} = {\omega _0}^2 + 2\alpha \Delta \theta \)

On plugging the values in the above relation, you get:

\(\begin{aligned}{l}{\left( 0 \right)^2} &= {\left( {\frac{{{v_2}}}{{\left( {\frac{D}{2}} \right)}}} \right)^2} + 2\alpha \Delta \theta \\\Delta \theta &= \frac{{{{\left( {\frac{{\left( {55\;{\rm{km/h}}} \right)\left( {\frac{{1\;{\rm{m/s}}}}{{3.6\;{\rm{km/h}}}}} \right)}}{{\left( {\frac{{0.8\;{\rm{m}}}}{2}} \right)}}} \right)}^2}}}{{2\left( { - 3.1\;{\rm{rad/}}{{\rm{s}}^2}} \right)}}\\\Delta \theta &= 235.2\;{\rm{rad}}\end{aligned}\)

06

Determine the total distance

The relation to obtain the distance traveled by the car is given by:

\(\begin{aligned}{l}{d_1} &= r\Delta \theta \\{d_1} &= \left( {\frac{D}{2}} \right)\Delta \theta \\{d_1} &= \left( {\frac{{0.8\;{\rm{m}}}}{2}} \right)\left( {235.2\;{\rm{rad}}} \right)\\{d_1} &= 94.08\;{\rm{m}}\end{aligned}\)

The relation to obtain the distance covered by the car during 75 revolutions is given by:

\(\begin{aligned}{l}{d_2} &= r\theta \\{d_2} &= \left( {\frac{D}{2}} \right)\theta \\{d_2} &= \left( {\frac{{0.8\;{\rm{m}}}}{2}} \right)\left( {75\;{\rm{rev}} \times \frac{{2\pi \;{\rm{rad}}}}{{1\;{\rm{rev}}}}} \right)\\{d_2} &= 188.4\;{\rm{m}}\end{aligned}\)

The total distance traveled by the car is calculated as:

\(\begin{aligned}{l}{d_{{\rm{total}}}} &= {d_1} + {d_2}\\{d_{{\rm{total}}}} &= \left( {94.08\;{\rm{m}}} \right) + \left( {188.4\;{\rm{m}}} \right)\\{d_{{\rm{total}}}} &= 282.4\;{\rm{m}}\end{aligned}\)

Thus, \({d_{{\rm{total}}}} = 282.4\;{\rm{m}}\) is the total distance.

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Most popular questions from this chapter

A uniform rod of mass M and length l can pivot freely (i.e., we ignore friction) about a hinge attached to a wall, as in Fig. 8–63. The rod is held horizontally and then released. At the moment of release, determine (a) the angular acceleration of the rod, and (b) the linear acceleration of the tip of the rod. Assume that the force of gravity acts at the center of mass of the rod, as shown. [Hint: See Fig. 8–20g.]

A cyclist accelerates from rest at a rate of \({\bf{1}}{\bf{.00}}\;{\bf{m/}}{{\bf{s}}^{\bf{2}}}\). How fast will a point at the top of the rim of the tire (diameter = 0.80 cm) be moving after 2.25 s? [Hint: At any moment, the lowest point on the tire is in contact with the ground and is at rest — sees Fig. 8–57.]

FIGURE 8-57 Problem 79

Two wheels having the same radius and mass rotate at the same angular velocity (Fig. 8–38). One wheel is made with spokes so nearly all the mass is at the rim. The other is a solid disk. How do their rotational kinetic energies compare?

(a) They are nearly the same.

(b) The wheel with spokes has about twice the KE.

(c) The wheel with spokes has higher KE, but not twice as high.

(d) The solid wheel has about twice the KE.

(e) The solid wheel has higher KE, but not twice as high.

FIGURE 8-38

MisConceptual Question 7.

Calculate the moment of inertia of an align of point objects, as shown in Fig. 8–47 about (a) the y axis and (b) the x-axis. Assume two masses, \(m = 2.2\;{\rm{kg}}\)and \(M = 3.4\;{\rm{kg}}\), and the objects are wired together by very light, rigid pieces of wire. The align is rectangular and split through the middle by the x-axis. (c) About which axis would it be harder to accelerate this align?

FIGURE 8-47

Problem 39

Question:(I) (a) What is the angular momentum of a 2.8-kg uniform cylindrical grinding wheel of radius 28 cm when rotating at 1300 rpm? (b) How much torque is required to stop it in 6.0 s?

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