17 October Science 7 Newton’s 3rd Law, Momentum, Conservation of Momentum, Rocket Design and Experiment

Newton’s Laws of Motion

1st Law is a starting point, an assumption about mass and motion.

Anything that has mass tends to keep its velocity (constant speed in a straight line). Anything that has mass tends to resist changing its velocity. This is taken as a property of mass. Newton did not know why this is the case, but he was able to discern the pattern.

2nd Law is about what is necessary for a mass to change its velocity. The cause of any change in speed and/or direction is called a force. The force is proportional to the size of the mass and the rate of change of velocity. The 2nd Law is often written F = ma. This form allows calculations and predictions about the size of forces and the relation to mass and the rate of change of velocity (acceleration).

3rd Law establishes symmetry. Every force gives rise to a second force, equal in size but opposite in direction.

(Many students are misled when the third law is expressed: For every¬†“action” there is an equal and opposite “reaction.” This may be because people include far more things in the idea of what action and reaction mean. It is best to stick to “force.”)

We will do a brief experiment / demonstration with the 3rd Law. Then we will introduce a new quantity: Momentum.

Momentum can be measured. It is mass x velocity or mv . Remember velocity is speed with the direction of motion specified. This velocity is instantaneous–see our last class.

You could think that the 1st Law is about momentum being constant and the second law is about momentum changing. This is one of the great cross-cutting concepts in science–what stays the same and what changes.

Momentum can be transferred. The total momentum of a system is conserved. This suggests how deeply the property of inertia is ingrained in the material world. The symmetry of the 3rd law is definitely related to this Conservation of Momentum and this fact that momentum can be transferred. The Law of Conservation of Momentum is important in describing how rockets work.

Newton’s view of motion is a beautiful mental structure that accounts for much that is observed about motion in the world and beyond. Later Einstein (and others) saw some inadequacies and developed a new and more comprehensive structure to explain and predict (such a structure is called a scientific theory). Nevertheless, Newton’s scheme is still taught (see us!) and used (pilots and air traffic control are vivid examples) today–hundreds of years after Newton.

Pick a partner, discuss your own theories of how a rocket launches and flies. Include both sketches and words. Be prepared to share. Design a rocket–drawing and words–that you could make from the plastic bottles. Plan a simple experiment you could try (everyone will be guaranteed 2 launch attempts). Explain the rationale of your experiment and what you think will happen. Be sure to include how you will make the observations. Discuss what you are trying to test.

See the Bill Nye episode on Momentum.


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