NSF-GK12 Program at Chelsea High School

Concept of gravity, contact forces, Newton’s second law, centripetal force.

Many students have a misconception about zero gravity. The general misconception is that zero gravity occurs in a spacecraft due to the far distance of the spacecraft or satellite to Earth.  Therefore, the effect of gravity at such a long radius becomes negligible.  This lesson, describe the real reason for zero gravity and why astronauts feel weightless in orbit.

The purpose of this lesson is to correct the misconception of zero gravity and explain the reason for weightlessness of objects in orbit.

1. Scale
2. Elevator
3. Calculator

Start the lecture with showing the youtube video, “A Look at Weightlessness in Action WOW”.  Then ask students what causes weightlessness?

Hand out this quiz and collect the answers.

Ask students what forces apply on a person sitting on a chair.  Draw the free-body-diagram on the board and ask students to identify the forces.  Have students to come up with an example that normal force (contact force) on a human body is zero.  Give the example of a skydiver (free fall) as a situation where the contact force on human body is zero.  At this point write the definition of weightlessness on the board.  Weightlessness is simply a sensation experienced by an individual when there are no external objects touching one's body and exerting a push or pull upon it. Weightless sensations exist when all contact forces are removed.

Make clear that these sensations are common to any situation in which a person is momentarily (or perpetually) in a state of free fall. When in free fall, the only force acting upon the body is the force of gravity - a non-contact force.  Ask students if there is no contact force, can one feel his/her weight?  Guide the discussion towards the point that the force of gravity cannot be felt without any other opposing forces; therefore, one would have no sensation of it.  Ask students if a skydiver would feel weightless when in a state of free fall. 

The teacher can even use the scale in elevator example to illustrate this concept.

Now ask students if the skydiver was placed in a box and released from an airplane, would the skydiver feel weightless. Guide the class discussion towards the fact that if the box and person are accelerating at the same rate, there will not be any contact between them.  Next, ask class the question why astronauts feel weightless in orbit.

Have students to calculate the gravitational acceleration at an altitude of 400 km above the Earth’s surface (g = 8.7 m/s/s).  Have students draw the free-body-diagram of an astronaut orbiting Earth.  Ask how many forces exert a push or pull on the astronaut.  If at least one student claims that the force of gravity on astronaut is negligible, refer the students to the value of g that they calculated, and ask them to find the weight of an astronaut with mass of 100 kg on the surface of Earth and at 100 km altitude which is the typical distance of a satellite from the Earth’s surface.  Conclude that the astronauts do have weight, gravity acts on them, and that is the only force on an astronaut in orbit.  Conclude that Earth-orbiting astronauts are weightless for the same reasons that riders of a free-falling amusement park ride or a free-falling elevator are weightless. They are weightless because there is no external contact force pushing or pulling upon their body. In each case, gravity is the only force acting upon their body. Being an action-at-a-distance force, it cannot be felt and therefore would not provide any sensation of their weight. But for certain, the orbiting astronauts weigh something; that is, there is a force of gravity acting upon their body.

In fact, if it were not for the force of gravity, the astronauts would not be orbiting in circular motion. It is the force of gravity which supplies the centripetal force requirement to allow the inward acceleration which is characteristic of circular motion. The force of gravity is the only force acting upon their body. The astronauts are in free-fall. Like the falling amusement park rider and the falling elevator rider, the astronauts and their surroundings are falling towards the Earth under the sole influence of gravity. The astronauts and all their surroundings - the space station with its contents - are falling towards the Earth without colliding into it. Their tangential velocity allows them to remain in orbital motion while the force of gravity pulls them inward.

Again, hand out the same quiz and collect the answers. The teacher can campare students' answers to the quiz before and after the lecture. Also, this questioner can be handed out as homework.

• http://science.howstuffworks.com/weightlessness.htm
• http://www.glenbrook.k12.il.us/gbssci/phys/Class/circles/u6l4d.html