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Planet X rotates in the same manner as the earth, around an axis through its north and south poles, and is perfectly spherical. An astronaut who weighs \(943.0 \mathrm{~N}\) on the earth weighs \(915.0 \mathrm{~N}\) at the north pole of Planet \(X\) and only \(850.0 \mathrm{~N}\) at its equator. The distance from the north pole to the equator is \(18,850 \mathrm{~km}\), measured along the surface of Planet X. (a) How long is the day on Planet X? (b) If a \(45,000 \mathrm{~kg}\) satellite is placed in a circular orbit \(2000 \mathrm{~km}\) above the surface of Planet \(\mathrm{X}\). what will be its orbital period?

Short Answer

Expert verified
The day on Planet X is approximately 48 hours long and the orbital period of the satellite is around 18 hours.

Step by step solution

01

Calculate the gravitational acceleration at the North Pole

To find the gravitational acceleration at the north pole, divide the weight of the astronaut by his mass. Use the relationship between weight and mass, which is weight = mass * acceleration. From the problem, it is known that an astronaut who weighs 943.0 N on the earth weighs 915.0 N at the north pole of Planet X. Using this information, it can be determined that the astronaut's mass is 96.20 kg (943.0 N / 9.81 m/s^2). Consequently, the gravitational acceleration at the north pole is 9.50 m/s^2 (915.0 N / 96.20 kg).
02

Calculate the Gravitational Pull at the Equator

Using the same method as in step 1, calculate the gravitational acceleration at the equator. The astronaut weighs 850.0 N at the equator. So, the gravitational acceleration there is 8.84 m/s^2 (850.0 N / 96.20 kg). The difference between the gravitational pull at the pole and the equator is due to the centrifugal force resulting from the planet's rotation.
03

Find the Radius of Planet X

The distance from the north Pole to the equator is given as 18,850 km. Since this is half the diameter, the radius of Planet X is 18,850 km.
04

Calculate the Angular Speed

Using the known values of gravitational acceleration at the pole and the equator, and the radius of Planet X, solve for the angular speed. Use the formula for centrifugal acceleration, which is \(a_c = \omega^2 * r\), where \(a_c\) is the centrifugal acceleration (difference in gravitational acceleration at the pole and equator, 9.50 m/s^2 - 8.84 m/s^2 = 0.66 m/s^2), \(r\) is the radius of Planet X (18,850 km = 1.885 x 10^7 m), and \(\omega\) is the angular speed. Solve for \(\omega\) to get 1.2 x 10^-4 rad/s.
05

Calculate the Length of the Day on Planet X

After obtaining the angular speed, calculate the length of the day on Planet X. That is the time it takes to make one full rotation (2Ï€ rad). Therefore, the time \(T\) for one full rotation is given by the formula \(T = \frac{2\pi}{\omega}\). Substituting the computed value of angular speed (\(\omega = 1.2 x 10^-4 rad/s\)), we acquire \(T = 1.73 x 10^5 s\). Coverting seconds into hours, the length of the day is approximately 48 hours.
06

Calculate the Gravitational Constant of Planet X

To find the gravitational constant of Planet X (GM), use the formula for gravitational acceleration: \(g = \frac{GM}{r^2}\). Substituting for \(g\) at the pole (9.5 m/s^2) and \(r\) (radius of Planet X, 1.885 x 10^7 m), solve for GM to get GM = 3.38 x 10^24 m^3/s^2.
07

Calculate the Orbital Period of the Satellite

Finally, calculate the orbital period of the satellite. The formula for the orbital period is \(T = 2\pi \sqrt{\frac{r^3}{GM}}\). In this instance, \(r\) is the distance from the center of Planet X to the satellite (which is planet's radius + the height above the surface = 2.085 x 10^7 m). GM is the gravitational constant. Therefore, the orbital period is approximately 6.3 x 10^4 s, or around 18 hours.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Centrifugal Force
When we talk about centrifugal force, we're referring to the apparent force that draws a rotating object away from the center of rotation. It is not a 'real' force but rather an effect of inertia - the tendency of an object to resist any change in its state of motion. On a rotating planet like Planet X, this effect causes objects at the equator to experience a 'lighter' weight than at the poles.

Imagine you're swinging a ball on a string; the ball pulls outward as it spins around - that pull is similar to the centrifugal force. Likewise, the astronaut on Planet X weighs less at the equator than at the pole due to the centrifugal force acting away from the axis of rotation, which effectively reduces the 'felt' gravity.

Let's delve into the mathematics of this effect. The centrifugal acceleration can be calculated using the formula:
\[a_c = \omega^2 \times r\],
where \(a_c\) is the centrifugal acceleration, \(\omega\) is the angular speed, and \(r\) is the radius of the planet at the equator. By comparing the weight of the astronaut at the poles and equator and having gravity in mind, we deduced the change due to centrifugal force, which informed us about Planet X's rotation dynamics.
Angular Speed
Angular speed is an essential concept in understanding how celestial bodies like Planet X rotate. It is a measure of the rate at which an object rotates around an axis, indicating how quickly it completes a rotation, usually expressed in radians per second (rad/s).

Calculating the angular speed, \(\omega\), of Planet X involves understanding the physical influence of its rotation on the gravitational force felt at different locations. One of the steps to derive angular speed utilized the difference in gravitational pull at the pole and equator, accounting for the centrifugal force. Here's the equation used:
\[\omega = \sqrt{\frac{a_c}{r}}\],
where \(r\) is the radius of Planet X and \(a_c\) is the centrifugal acceleration.

By determining the angular speed, we can understand how quickly Planet X spins and subsequently calculate other rotational properties, such as the length of the day on the planet.
Orbital Period
The orbital period is the time it takes for an object to make one complete orbit around another object. When discussing satellites or planets, the orbital period is crucial as it determines the 'year' for that satellite or celestial body. In this exercise, we calculated the orbital period of a satellite orbiting Planet X.

To calculate this, the gravitational constant of Planet X is used along with the formula:
\[T = 2\pi \times \sqrt{\frac{r^3}{GM}}\],
where \(T\) is the orbital period, \(r\) is the radius from the center of Planet X to the satellite, and \(GM\) is the gravitational constant of Planet X. From this equation, we can understand that the orbital period depends on the distance from the center of the planet (radius) and the planet's mass.

The concept of orbital period helps in planning satellite trajectories, designing communication networks, and can even relate to how we measure time, all of which are based on the fundamental physics of gravitation and rotational motion witnessed with celestial objects.

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Most popular questions from this chapter

Tidal Forces near a Black Hole. An astronaut inside a spacecraft, which protects her from harmful radiation, is orbiting a black hole at a distance of \(120 \mathrm{~km}\) from its center. The black hole is 5.00 times the mass of the sun and has a Schwarzschild radius of \(15.0 \mathrm{~km} .\) The astronaut is positioned inside the spaceship such that one of her \(0.030 \mathrm{~kg}\) ears is \(6.0 \mathrm{~cm}\) farther from the black hole than the center of mass of the spacecraft and the other ear is \(6.0 \mathrm{~cm}\) closer. (a) What is the tension between her ears? Would the astronaut find it difficult to keep from being torn apart by the gravitational forces? (Since her whole body orbits with the same angular velocity, one ear is moving too slowly for the radius of its orbit and the other is moving too fast. Hence her head must exert forces on her cars to keep them in their orbits.) (b) Is the center of gravity of her head at the same point as the center of mass? Explain.

A narrow uniform rod has length \(2 a\). The linear mass density of the rod is \(\rho,\) so the mass \(m\) of a length \(l\) of the rod is \(\rho l\). (a) A point mass is located a perpendicular distance \(r\) from the center of the rod. Calculate the magnitude and direction of the force that the rod exerts on the point mass. (Hint: Let the rod be along the \(y\) -axis with the center of the rod at the origin, and divide the rod into infinitesimal segments that have length \(d y\) and that are located at coordinate \(y\). The mass of the segment is \(d m=\rho d y\). Write expressions for the \(x\) - and \(y\) -components of the force on the point mass, and integrate from \(-a\) to \(+a\) to find the components of the total force. Use the integrals in Appendix B.) (b) What does your result become for \(a \gg r ?\) (Hint: Use the power series for \((1+x)^{n}\) given in Appendix B.) (c) For \(a \gg r,\) what is the gravitational field \(g=\boldsymbol{F}_{g} / m\) at a distance \(r\) from the rod? (d) Consider a cylinder of radius \(r\) and length \(L\) whose axis is along the rod. As in part (c), let the length of the rod be much greater than both the radius and length of the cylinder. Then the gravitational ficld is constant on the curved side of the cylinder and perpendicular to it, so the gravitational flux \(\Phi_{g}\) through this surface is cqual to \(g A\), where \(A=2 \pi r L\) is the area of the curved side of the cylinder (see Problem 13.59 ). Calculate this flux. Write your result in terms of the mass \(M\) of the portion of the rod that is inside the cylindrical surface. How does your result depend on the radius of the cylindrical surface?

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