Gravity Fundamentals
This minimum speed must be reached by an object to break free from a planet’s gravitational pull.
Hint: It only depends on the planet’s mass and radius.
What is escape velocity?
When the orbital radius increases, the orbital velocity _____.
Hint: Think about how speed changes to balance gravity farther from Earth.
What is decreases?
What two factors affect the amount of gravitational potential energy an object has?
Hint: Think about what changes when you do work lifting an object.
What are mass and height?
Kepler's second law tells us that a planet sweeps out equal ________ in equal ________.
Hint: This law is also known as the Law of Equal Areas.
What are areas and time?
What is the approximate value of the acceleration due to gravity on Earth?
Hint: if you don’t know this, you are in trouble!
What is 9.81 m/s²?
An artificial Earth satellite in an elliptical orbit experiences its greatest centripetal acceleration at this point in its path.
Hint: This is where gravity pulls strongest and speed is highest.
What is the point closest to Earth?
This law states that the orbit of a planet is an ellipse with the Sun at one focus.
Hint: An ellipse has two foci.
What is Kepler’s First Law?
When an object falls toward Earth, its gravitational potential energy is converted into ______.
Hint: This energy grows as an object picks up speed.
What is kinetic energy?
What happens to a planet’s speed as it gets closer to the sun in its orbit?
Hint: Think about the conservation of angular momentum.
What is increases?
What is the SI unit of gravitational potential energy?
Hint: It’s the same as any other energy measurement in the metric system.
What is Joule?
According to Newton's Law of Gravitation, if the distance between two masses doubles, the force changes by this factor.
Hint: Think inverse-square.
What is ¼?
A satellite in this type of orbit appears to stay fixed above one point on Earth’s equator.
Hint: Its orbital period matches Earth’s rotation exactly.
What is a geosynchronous orbit?
What happens to gravitational potential energy as two objects move farther apart?
Hint: Think about how potential energy changes with distance in gravity.
What is it increases?
This 16th-century astronomer made the most precise naked-eye measurements of planetary positions, which provided the foundation upon which Johannes Kepler built his three laws of planetary motion.
Hint: His data was sharp, but not as sharp as the sword that took his nose.
Who is Tycho Brahe?
What is the approximate value of the gravitational constant G?
Hint: It’s a very small number
What is 6.67 × 10⁻¹¹ N·m²/kg²?
A person is hypothetically transported to the exact center of the Earth. What is the net gravitational force they experience?
Hint: Think about symmetry and what pulls in all directions equally.
What is zero?
Why can a satellite in a circular orbit be considered to be in constant free fall?
Hint: Gravity acts as centripetal force.
What is gravity constantly pulls it toward Earth?
At escape velocity, what is the total mechanical energy of an object in a gravitational field?
Hint: Think about how kinetic energy and potential energy balance each other.
What is zero?
According to Kepler's Third Law, the square of a planet's orbital period around the same star is directly proportional to the cube of the ___.
Hint: It is not the closest or farthest point.
What is semi-major axis?
What is the approximate radius of Earth?
Hint: Think of how big the distance is from Earth’s center to its surface.
What is 6.378 × 10³ km?
Imagine drilling a narrow tunnel through Earth from one side to the other and dropping an object in. Ignoring air resistance and friction, describe the motion of the object.
Hint: Springs.
What is simple harmonic motion?
How does doubling a satellite’s mass change its orbital speed or period, assuming the orbit stays the same?
Hint: Consider what factors actually influence orbital motion.
What is no effect?
Why is gravitational potential energy negative in orbital systems?
Hint: Gravitational potential energy equals the negative of the work done by gravity.
What is the zero point is at infinity?
Kepler’s Third Law relates orbital period to the semi-major axis for planets orbiting the same star. Why does the planet’s mass not affect the orbital period
Hint: Consider which mass dominates the gravitational force in the two-body system.
What is the star’s mass dominates the system?
What is the approximate mass of the Earth?
Hint: One way to solve for this is using Fc = Fg.
What is 5.97 × 10²⁴ kg?