Chapter 3 - Astronomy

Which has more mass: an armful of feathers or an armful of lead? Which has more volume: a kilogram of feathers or a kilogram of lead? Which has higher density: a kilogram of feathers or a kilogram of lead?

1) armful of lead (mass is related to weight) 2) Kg of feathers has more volume as it is less dense. 3) Lead has higher density

Write out Newton's three laws of motion in terms of what happens with the momentum of objects.

1. An object at rest remains at rest and object in motion remains in motion unless something impacts the object 2. The rate of change or momentum is equal to the force that produced the change 3. For every action there is an equal reaction

To calculate the angular momentum of an object, which properties of an object do you need to know?

1. Mass of the rotating object 2. Velocity of the rotating object 3. Radius of path in which the object moves 4. The angle subtended by the path

To calculate the momentum of an object, which properties of an object do you need to know?

1.Mass of the body 2. velocity of the body

Which major planet has the largest . . . A. semimajor axis? B. average orbital speed around the Sun? C. orbital period around the Sun? D. eccentricity?

A. Neptune has the largest semi-major axis. B. Mercury has the highest average orbital speed around the sun. C. Neptune has the highest orbital period around the sun. D. Mercury has the highest eccentricity.

As friction with our atmosphere causes a satellite to spiral inward, closer to Earth, its orbital speed increases. Why?

According to Kepler's third law, P2 ∝ a3, where a is the semimajor axis (essentially the distance from the center of Earth in our case). If the satellite spirals in (because of air friction), a decreases and, hence, so does P. Thus, a decrease in a means an increase in V.

Evil space aliens drop you and your fellow astronomy student 1 km apart out in space very far from any star or planet. Discuss the effects on gravity on each of you

Both of us would be attracted towards each other due to the force of gravity with a specific magnitude of 2.4 x 10^-13 N-that number is calculated using the formula for gravitational force and inputting 1KM for r^2

The gas pedal, the brakes, and the steering wheel all have the ability to accelerate a car—how?

Gas pedal - increase speed Brake - decrease speed Steering wheel - drives direction and acceleration of speed

Compare the density, weight, mass, and volume of a pound of gold to a pound of iron on the surface of Earth.

Gold is denser than iron, both have the same weight (a pound each), both have the same mass, but iron has a greater volume than gold.

Why was Brahe reluctant to provide Kepler with all his data at one time?

He feared Kepler will come out with a universal theory for planetary motion and thereby taking all the credit by himself.

Why is Tycho Brahe often called "the greatest naked-eye astronomer" of all time?

He made many discoveries without a telescope.

How does the mass of an astronaut change when she travels from Earth to the Moon? How does her weight change?

Her mass does not change because that measures the amount of "stuff" that makes up her body. Her weight will decrease because that is a measure of the amount of gravitational force she is experiencing; on the Moon, her weight would be about 1/6 what it is on Earth.

If there is gravity where the International Space Station (ISS) is located above Earth, why doesn't the space station get pulled back down to Earth?

If the space station were not moving, it would do just that, but when the space station was first assembled at a certain height above Earth, it was provided an appropriate speed for that height in a direction parallel to the surface of Earth. As such, instead of falling toward Earth, it is continuously falling around Earth. Since friction with Earth's atmosphere slows down the ISS, it requires occasional upward pushes, or boosts, to stay in orbit.

Explain how Kepler was able to find a relationship (his third law) between the orbital periods and distances of the planets that did not depend on the masses of the planets or the Sun.

In the year 1619, he came out with a relation between orbit of the planet and their relative distances from the sun. The relation was as follows P^2=a^3. Here P represents the orbital period and a represents The average distance from the sun. The relationship is now known as Kepler's third law of planetary motion. The planet's mass is so small that it doesn't count in the equation so only the mass of the Sun matters.

Is it possible to escape the force of gravity by going into orbit around Earth? How does the force of gravity in the International Space Station (orbiting an average of 400 km above Earth's surface) compare with that on the ground?

It is not possible to escape the force of gravity by going into orbit. The space station is about 6800 km from the center of Earth, only about 86% farther away than the surface of our planet. The force of gravity in the International Space Station (ISS) is only slightly less than the gravity at the surface of Earth. People (and sandwiches) "float" in the ISS because they, and the station itself, are falling around Earth.

What was the great insight Newton had regarding Earth's gravity that allowed him to develop the universal law of gravitation?

Newton suspected that earths gravity is what kept the moon from hitting the earth Earth gravity may go further for the interaction of planets with the sun and the moons with the earth and other planets. he thought of the motion of celestial bodies around other celestial bodies is all due to gravity. He believed that all the bodies in the universe attract every other matter or object.

Two asteroids begin to gravitationally attract one another. If one asteroid has twice the mass of the other, which one experiences the greater force? Which one experiences the greater acceleration?

Since Newton's law of gravity says the force is proportional to the product of the two masses, both masses experience the same force. This is also consistent with Newton's third law of motion. But the asteroid with the lesser mass would feel the greater acceleration, according to Newton's second law.

If identical spacecraft were orbiting Mars and Earth at identical radii (distances), which spacecraft would be moving faster? Why?

The spacecraft moving around Earth would be moving faster. Because Earth has a greater mass, it has a greater gravitational pull on the spacecraft; so for a similar orbit, the one orbiting Earth has to be moving faster to keep from being pulled to the ground.

Why did Kepler need Tycho Brahe's data to formulate his laws?

To deduce a universal theory for motion of planets, Kepler needed data which supports heliocentric model, and Brahe was having records which deviated from the Ptolemic model and Kepler believed in Brahe. That's why Kepler was in need of Brahe's data.

According to Kepler's second law, where in a planet's orbit would it be moving fastest? Where would it be moving slowest?

When planets are closer to the sun moves faster. When the planet is farther from the sun the speed of the planet is slow.

Explain how a rocket can propel itself using Newton's third law.

When the fuel's inside the rocket burn, the exhaust gases are released Into the atmosphere. These exhaust gases are directed towards the ground, from the rocket. This produces a very large fours on the ground. Here, according to the third law of motion there will be a reaction force. Force due to the exhaust gases which cause the rocket to lift up, acts as a reaction force. And thus the rocket propels itself using the third law of motion

What is the momentum of an object whose velocity is zero? How does Newton's first law of motion include the case of an object at rest?

When the velocity of the object is zero, the momentum is also zero. Newtons' first law states that the body is at rest or in motion , it remains at rest or in motion until and unless an external force is not applied on the body. So, When a body is at rest it remains at rest.

A body moves in a perfect circular path at a constant speed. Are there forces acting in such a system. How would you know?

Yes; there must be a force pulling toward the center of the circle. According to Newton's first law, there must a force present to keep the object moving in a circle rather than a straight line.