Lesson 26: Intro to fields and Newton's Universal law of Gravitation

Aims

Introduce the concept of fields to explain how bodies can experience a force without contact.
State Newton's Universal law and apply it to calculate the force acting on a body close to the earth.
See how Newton's law gives the correct value for the acceleration of gravity close to the earth and the period of the earth's orbit around the Sun.
Understand the concept of "field strength" as a measure of the size of a field and calculate field strengths.
Understand how field lines can be used to give a visual representation of the field.
Knowing that Field strength is a vector, add field strengths vectorially.

TOK

Newton's law is called "The Universal law" which means it applies to the Universe, this is a bold claim. Can we really claim to know that this law applies to every body in the universe?
Gravity acts between every particle of mass in the universe. This means that if a red ball on the earth is moved every particle of matter in the Universe will feel it.
Field lines do not exist but just help us to visualise the field, why do we need to be able to visualise it? What about blind people, how do they visualise fields?
Why does a ball fall? Because of gravity. What is gravity? The thing that makes the ball fall.

Notes

The myth is that Newton realised that an apple fell because of gravity but i don't think he was the first person to notice that an apple fell because it was pulled downwards. The big breakthrough was to realise that the force pulling the apple down was the same force that held the Earth in orbit around the sun (at least this is the myth that I spread).

For SL you don't have to go into much detail about Gravitational fields but to put things in context I like to go a bit deeper and include things like Cavendish's experiment and a calculation of the time period of the earths orbit.

Some students find the concept of field strength quite difficult to grasp and always seem to forget that it's force per unit mass.

Test

Gravitational fields

Gravitational fields answers

Gravitational fields mc test

Text Book

Page 181 - 185

Homework

Problems 1-5

Vocabulary list

Gravity
Universal
Field
Orbit

SMART board Notes

Resources

Orbits in Algodoo

Both IP and Algodoo are really good for showing how the force of gravity can pull objects towards the earth and provide the centripetal force to hold them in orbit. This animation is made in Algodoo which is a bit more interactive than IP allowing you to grab things and throw them around whilst the simulation is running. The first part shows that object of different mass fall with the same acceleration and objects further away have a smaller acceleration. The second part shows how the same force can hold the masses in orbit (it takes a bit of practice).

Orbits in IP

In IP you can make a mini solar system and watch the planets orbit the Sun. You can then make one of the planets the centre of the motion and see what it would look like from the earth. Its then that students can see why it was not obvious that the planets were going around the Sun. Another neat thing about using IP is that you can clearly see that the planets closest to the sun have a greater velocity.