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Satellites and weightlessness

Content objective:

What are we learning and why are we learning this? Content, procedures, or skills.

Vocabulary objective

Tier II: High frequency words used across content areas. Key to understanding directions & relationships, and for making inferences.

Tier III: Low frequency, domain specific terms.

Building on what we already know

Make connections to prior knowledge. This is where we build from.

“What keeps a satellite up? Its high speed. If a satellite stopped moving, it would fall directly to Earth. But at the high speed a satellite has, it would quickly fly out into space – if it weren’t for the gravitational force of the Earth pulling it into orbit. In fact, a satellite is falling (accelerating toward Earth), but its high tangential speed keeps it from hitting Earth.

Objects in orbit are said to experience weightlessness. They do have a gravitational force acting on them, though! The satellite and all its contents are in free fall, so there is no normal force. This is what leads to the experience of weightlessness.”

  • Giancoli Physics, 6th edition

Newton’s canon: a classic Gedankenexperiment

A gedankenexperiment is a thought experiment that allows us to learn something new.

Wikipedia article, with animations, of Newton’s cannonball


Newton's canon

Newton’s cannon app

Here are some interactive apps that we can use in class.





Objects in a falling elevator

When we are accelerating up or down in an elevator, the effective force of gravity we feel is different compared to just standing on the ground.

Giancoli Physics, Chapter 5

Giancoli Physics, Chapter 5

zero-g environment in Earth orbit

When we are orbiting a planet, we are effectively weightless. Here’s what it looks like up in the ISS orbiting Earth.

Free fall International Space Station

But WHY are we weightless when in Earth orbit? Is it because there is no gravity up there? Not at all. In fact the ISS is only 200 miles above the surface of the Earth. The gravitational pull of Earth’s gravity is 90% as strong as at the surface of the Earth.

Wow, so there a lot of gravity in space, especially when so close to the Earth. We only feel weightless because we are orbiting in free fall. See Newton’s canon, above.

If the ISS stopped orbit the Earth, and just stood still in space (200 miles up) then Earth’s gravity would pull it down just like a rock falling from our hands.

How can we experience weightlessness (briefly) here on Earth? All we have to do is out ourselves in a freefall situation, like this:

Giancoli Physics, Chapter 5, Fig 5-27

Giancoli Physics, Chapter 5, Fig 5-27

Geosynchronous orbits

A geosynchronous orbit is a high Earth orbit that allows satellites to match Earth’s rotation. 22,236 miles (35,786 kilometers) above Earth’s equator, this position is a valuable spot for monitoring weather, communications and surveillance.

“Because the satellite orbits at the same speed that the Earth is turning, the satellite seems to stay in place over a single longitude, though it may drift north to south,” NASA wrote on its Earth Observatory website…. A satellite in geosynchronous orbit can see one spot of the planet almost all of the time. For Earth observation, this allows the satellite to look at how much a region changes over months or years. The drawback is the satellite is limited to a small parcel of ground; if a natural disaster happens elsewhere, for example, the satellite won’t be able to move there due to fuel requirements.

– What Is a Geosynchronous Orbit? By Elizabeth Howell




Geosynchronous prob 1

Geosynchronous prob 2
Geosynchronous prob 3
Giancoli Physics, Chapter 5 Giancoli Physics, Chapter 5

Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework
HS-PS2-1. Analyze data to support the claim that Newton’s second law of motion is a
mathematical model describing change in motion (the acceleration) of objects when
acted on by a net force.

HS-PS2-10(MA). Use free-body force diagrams, algebraic expressions, and Newton’s laws of motion to predict changes to velocity and acceleration for an object moving in one dimension in various situations

A FRAMEWORK FOR K-12 SCIENCE EDUCATION: Practices, Crosscutting Concepts, and Core Ideas
What are the predictable patterns caused by Earth’s movement in the solar system? Readings on orbits: https://www.nap.edu/read/13165/chapter/11#175

Massachusetts Science and Technology/Engineering Curriculum Framework (2006)
1. Motion and Forces. Central Concept: Newton’s laws of motion and gravitation describe and predict the motion of most objects.
1.7 Describe Newton’s law of universal gravitation in terms of the attraction between two objects, their masses, and the distance between them.
1.8 Describe conceptually the forces involved in circular motion.

SAT Physics Subject Area Test
Gravity, such as the law of gravitation, orbits, and Kepler’s laws

AP Physics Learning Objectives
Essential Knowledge E.3.1
a. The change in the kinetic energy of an object depends on the force exerted on the object and on the displacement of the object during the interval that the force is exerted.

iii. The component of the net force exerted on an object perpendicular to the direction of the displacement of the object can change the direction of the motion of the object without changing the kinetic energy of the object. This should include uniform circular motion and projectile motion.

Learning Objective 5. Orbits of planets and satellites. Students should understand the motion of an object in orbit under the influence of gravitational forces.

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