SNIPPET LESSON PLAN: INERTIAL FORCES (NEWTON’S FIRST LAW OF MOTION, THE CENTRIFUGAL FORCE & ARTIFICIAL GRAVITY)
Subject: Science/Physics — Inertia
Ages: 12+; Middle School to High School
Length: Film Clips: 3 Clips of approximately 9 minutes.
Using the Snippet in Class:
Students will have a visual impression of inertia, the centrifugal force and artificial gravity in space travel.
Few movies staged in space have attempted or achieved such a degree of accuracy in their showing of weightlessness and artificial gravity created using the centrifugal force as 2001: A Space Odyssey.
The scenes show the exterior and interior of a rotating space station.
There are four fundamental forces in nature: gravity, electromagnetism and the strong and weak nuclear forces. However, in every day life it appears that we experience other forces that are none of the above, such as inertial forces. These are part of Newton’s three laws of motion. Newton’s first law describes inertia and states that “Every object in a state of uniform motion or at rest tends to remain in that state of motion unless an external force is applied to it.” Thus, when we try to accelerate an object, it will resist, trying to keep its previous state, which is either sitting still, or moving at a certain constant velocity (speed and direction).
Any change in the motion of an object requires a force and is called acceleration. (Newton´s second and most universal law of motion: “The acceleration of a body is directly proportional to, and in the same direction as, the net force acting on the body, and inversely proportional to its mass. Thus, F = ma, where F is the net force acting on the object, m is the mass of the object and a is the acceleration of the object.”)
From the point of view of the object being accelerated (or anything sitting on it), the resistance to change is felt as a force in the opposite direction to the force that is being applied to accelerate it. Thus, in a car starting to go forward after having stopped at a traffic light the passengers will feel being pushed into the seats, i.e. a force pushing them in the opposite direction to the forward acceleration applied by stepping on the gas.
When something is made to move more slowly there is also acceleration, but it is a negative one. So, if the brakes are applied in a vehicle, the force is acting in the direction opposite of the motion of the car, i.e. backwards. The resistance to this force will then push the passengers forward. We have all experienced this in a car that is slowing down and it is the reason why we wear seat belts – if the deceleration is sudden and extreme, as in an accident, the forward push could actually make someone not wearing a seat belt fly through the glass windshield.
The two examples mentioned so far are about changes to speed, but there is another change that can be made to the motion of an object: it can be made to change direction. This kind of acceleration, which causes the moving objects to describe a curve, is called centripetal, from the Latin words centrum (center) and petere (move towards), as it seems to push the object to the imaginary center of a curve. Again, there is a resistance against the centripetal force in the opposite direction, i.e. towards the outside of that curve, called centrifugal force (fugere: to escape from). Sitting in a car we have all also experienced this, feeling being pushed towards the outside of the curve.
Now, there is yet another level this can be taken to. If an object is sitting on a rotating disk and is moved either towards or away from its center, its forward speed is actually changed: closer to the center it will be smaller; further away from the center, the speed will be higher. So, effectively, a change in motion has happened, and this change will have its corresponding resistance. This one bears the name of the physicist who was able to identify it as such: Coriolis. It may seem a lesser known force, but it can be easily experienced by anyone trying to walk across a carousel: they will be pushed sideways. It has also large scale consequences on our planet: imaginarily observed from above the north or south poles, cloud masses can be clearly seen moving either towards or away from the center of the rotation of the Earth (the axis of the rotation). The clouds are then pushed sideways and form the huge cyclones and anticyclones of the weather of Earth. For a clip which will show students how the Coriolis effect works, see The Coriolis Effect from PBS Nova.
All these forces that are felt in reaction to a change in the motion of an object are called inertial forces. They are also known as fictitious forces, because no one is actually exerting them: the real forces are those the inertial forces appear to be reacting against. The inertial forces themselves can only be felt by or on the accelerated objects, and only represent resistance to change. However, for those being pushed around by them, they do feel very real!
On the rotating “centrifuge” in 2001: A Space Odyssey, the centrifugal force is used to mimic the gravity on Earth. We see one of the movie characters running along the outer wall of this room, showing that artificial gravity has indeed been achieved, allowing him to jog as if on Earth. But in order to replace the gravity of the Earth, it has to achieve the right strength, and it so happens that the strength of centrifugal force depends on the velocity at which the object is rotating (stronger for faster rotation) and, for a given rotational velocity, the distance from the center, as further out a fixed rotational velocity implies moving at faster speeds, and faster speeds will yield stronger outward (centrifugal) forces, as we all know from when a curve is taken too fast. Even this is taken into account accurately in the film, when one member of the crew accesses the centrifuge through the hub at the center and is seen to be weightless until he actually gains distance from the center by climbing “down” the ladder.
So, for the runner to feel a centrifugal force that is equal to the gravity of the Earth, the rotating room needs to be rotating at a certain speed. And then the centrifugal force will only be felt to be equal to the gravity of the Earth at a certain distance from the center, which in the case of the spaceship in 2001: A Space Odyssey, is the outer wall of the room. Only that part of the spaceship would therefore feel “Earth-like” in terms of gravity. In the giant wheel shaped Space Station V, the rotation is also mimicking gravity in all rooms located on the outer rim, as the lights (and other sequences in the film) show that it is there where life on the station actually takes place; but this will not be the case in the corridors along the radii or in areas nearer the centre.
Is this a solution that will be applied in the future in order to mimic Earth-like conditions in space? There are various arguments that lead one to think that this will actually not be the case, such as the very large size that would be required of a space station for the centrifugal force to approach the force of gravity on earth and the fact that many research projects send equipment and people into space in order to experiment with weightlessness. Thus, artificial gravity would have no purpose.
1. Read the Helpful Background section of this Snippet Lesson Plan.
2. Be familiar with the location of the clips on the DVD, check for accuracy of the minute and second locations of the clips on the DVD, and practice getting quickly from one film clip to the other. Cue the DVD to the beginning of the first clip. Note that clips 2 and 3 are contiguous.
1. Show Clips #1 and #2. Clip #1 is relatively long and has dialog only in the last minute. Teachers can speak over it imparting some of the issues in the Helpful Background section.
2. Ask students to describe how the artificial gravity in Clip #2 was attained and ask them to predict what would happen if the astronaut was closer to the center of the room.
3. Once the students have expressed their ideas, explain the situation in detail, introducing the concept of centrifugal force and the dependence of the force on the distance from the center.
4. Use Clip #3 as an example and, if desired, as an exercise to estimate the size of the station using its period of rotation and knowing that this period needs to give a value of centrifugal force at the rim that is equal to the gravity of the Earth. The period can be measured timing it in the film.
5. Continue the lesson addressing inertial forces in general and specifically the Centrifugal Force and artificial gravity.
Describe in writing [or for the class] using diagrams, the relationship between the centripetal and centrifugal forces. Identify the one which is a fictitious force. Describe fictitious and real forces and explain the differences.
Describe the Euler force and give an example.
Some arguments for why such space stations are not in the current plans of the Space Agencies
On the Coriolis force and its effect on the atmosphere:
The Coriolis Effect from National Geographic; with an animation;
Article on the Coriolis Effect from WeatherOnLine containing text and interesting diagrams;
a YouTube video containing a good experimental demonstration of the Coriolis effect
This Snippet Lesson Plan was written by Erik Stengler, Ph.D., and James Frieden. It was last revised on 12/31/13.