# Difference between revisions of "PhET"

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PhET simulations and resources are freely available at the PhET website (https://phet.colorado.edu/) from University of Colorado at Boulder. | PhET simulations and resources are freely available at the PhET website (https://phet.colorado.edu/) from University of Colorado at Boulder. | ||

− | The Spoken Tutorial Effort for PhET has being contributed by Dr. Snehalatha Kaliappan, Madhuri Ganapathi | + | The Spoken Tutorial Effort for PhET has being contributed by Dr. Snehalatha Kaliappan, Madhuri Ganapathi Dr. Vidya Iyer, Meenal Ghoderao and Himanshi Karwanje. |

__TOC__ | __TOC__ | ||

Line 18: | Line 18: | ||

==Basic Level== | ==Basic Level== | ||

+ | |||

'''1. Introduction to PhET Simulations''' | '''1. Introduction to PhET Simulations''' | ||

Line 44: | Line 45: | ||

+ | '''2. Graphing Lines''' | ||

− | + | * About Cartesian coordinate system | |

− | * | + | * Calculate the slope of a graphed line |

− | * | + | * Save and erase the plotted lines |

− | * | + | * Change slope and intercept for the line |

− | * | + | * Change variables in a linear equation to change the line |

− | |||

− | + | '''3. Equation Grapher''' | |

− | * | + | * About Binomial theorem |

− | * | + | * About Cartesian co-ordinate system |

+ | * Graph of line, y = bx + c, variation of coefficient b and constant c | ||

+ | * Quadratic polynomial, y = ax^2 + bx + c, variation of coefficients a, b and constant c | ||

− | + | * Prediction of shape of parabola based on its equation | |

− | |||

− | + | '''4. Trig Tour''' | |

− | * | + | * A unit circle to find trigonometric ratios |

− | * | + | * Construct right triangles inside a unit circle |

− | * | + | * Convert degrees into radians to look at periodicity of functions |

− | * | + | * Special angle values for theta (θ) |

+ | |||

+ | * Co-ordinates of points tracing trigonometric functions | ||

+ | |||

+ | * Cosine function graph | ||

+ | |||

+ | * Sine function graph | ||

+ | |||

+ | * Tangent function graph | ||

+ | |||

+ | * Amplitude of functions | ||

+ | * Vertical asymptotes of tangent function | ||

− | ''' | + | '''5. States of Matter''' |

* How to use States of Matter, an interactive PhET simulation. | * How to use States of Matter, an interactive PhET simulation. | ||

Line 94: | Line 107: | ||

− | ''' | + | '''6. Bending Light ''' |

* Explain reflection, refraction and dispersion of light. | * Explain reflection, refraction and dispersion of light. | ||

Line 106: | Line 119: | ||

* Measure intensity and velocity of light. | * Measure intensity and velocity of light. | ||

− | *View different wave patterns. | + | * View different wave patterns. |

+ | '''7. Color Vision''' | ||

− | + | * About colours of visible light (VIBGYOR) | |

− | * | + | * How colour vision works |

− | * | + | * Types of cone cells |

− | * | + | * Different wavelength-based sensitivities |

− | * | + | * Optic nerves, tracts and chiasma |

− | * | + | * Primary visual cortex |

+ | * About types of colours and mixing: | ||

+ | |||

+ | * Two or more colours | ||

+ | |||

+ | * All colours (VIBGYOR) of visible light | ||

+ | |||

+ | * About optical filters and their subtractive nature | ||

+ | |||

+ | |||

+ | '''8. Energy forms and changes''' | ||

+ | |||

+ | * Conservation of energy in real-life systems. | ||

+ | |||

+ | * About different forms of energy. | ||

+ | |||

+ | * Predict how energy flows when objects are heated or cooled. | ||

+ | |||

+ | * Compare thermal conductivity of different objects. | ||

+ | |||

+ | * Design energy systems. | ||

+ | |||

+ | * Study how energy changes from one form to another. | ||

+ | |||

+ | * Compare efficiency between fluorescent and incandescent bulbs. | ||

+ | |||

+ | * Effect of clouds on solar panel. | ||

+ | |||

+ | * Change in internal energy. | ||

+ | |||

+ | |||

+ | '''9. Energy Skate park''' | ||

+ | |||

+ | * Define law of conservation of energy | ||

+ | |||

+ | * Define Potential energy and kinetic energy | ||

+ | |||

+ | * Explanation of Energy Skate Park simulation interface | ||

+ | |||

+ | * Effect of friction on energy | ||

+ | |||

+ | * Effect of mass of skater on energy | ||

+ | |||

+ | * Effect of change in track on the potential energy | ||

+ | |||

+ | * To show pie chart and bar graph for energy changes | ||

+ | |||

+ | * Explanation about Energy vs Time graph | ||

+ | |||

+ | * Show the energy value at a particular position on the graph | ||

+ | |||

+ | * To change location and observe the energy changes | ||

+ | |||

+ | |||

+ | '''10. pH Scale''' | ||

+ | |||

+ | * Determine pH of a solution. | ||

+ | |||

+ | * Determine the acidity and basicity of a solution. | ||

+ | |||

+ | * Arrange acids and bases according to their strength. | ||

+ | |||

+ | * Effect of dilution on pH of a solution. | ||

+ | |||

+ | * Determine the concentration of hydroxide ion, hydronium ion and water at a given pH. | ||

+ | |||

+ | * Logarithmic and Linear scale. | ||

+ | |||

+ | |||

+ | '''11. Build an Atoms''' | ||

+ | |||

+ | * Build an atom using protons, neutrons and electrons. | ||

+ | |||

+ | * Identify an element and its position on the periodic table. | ||

+ | |||

+ | * Determine whether an atom is neutral or an ion. | ||

+ | |||

+ | * Predict the charge. | ||

+ | |||

+ | * Determine the mass of an atom or ion. | ||

+ | |||

+ | * Determine the stability of atom/ion created. | ||

+ | |||

+ | * Explain element symbol, charge, atomic number, mass number and isotope. | ||

+ | |||

+ | * Relate the number of protons, neutrons and electrons to atomic symbol. | ||

− | ''' | + | '''12. The Greenhouse Effect ''' |

* Effect of greenhouse gases on atmospheric temperature. | * Effect of greenhouse gases on atmospheric temperature. | ||

Line 140: | Line 239: | ||

− | '''1. | + | '''1. Curve Fitting''' |

− | * | + | * About Cartesian co-ordinate system |

− | * | + | * Understanding the concept of best fit line and equation |

− | + | ** About Correlation coefficient, r^2 | |

− | * | + | ** About reduced chi squared statistic |

− | + | ||

− | * | + | |

+ | * Looking at the degree of polynomials | ||

+ | ** 0: lines that are parallel to x or y-axes | ||

+ | ** 1: lines: looking at the concept of slope and constant | ||

+ | ** 2: quadratic polynomials | ||

+ | ** 3: cubic polynomials | ||

+ | ** 4: quartic polynomials | ||

Line 175: | Line 278: | ||

+ | '''3. Faradays Electromagnetic Lab''' | ||

− | ''' | + | * About Faraday's Electromagnetic Lab simulation interface |

+ | |||

+ | * Change in needle spacing and needle size using Options menu | ||

+ | |||

+ | * Change in magnetic field, as strength of the bar magnet varies | ||

+ | |||

+ | * About flip polarity | ||

+ | |||

+ | * Change in loop parameters to change induced EMF | ||

+ | |||

+ | * Effect of AC current source on induced EMF | ||

+ | |||

+ | * Change in speed of electrons on coil as the voltage varies | ||

+ | |||

+ | * Explain how a transformer works | ||

+ | |||

+ | * Working of a generator | ||

+ | |||

+ | |||

+ | '''4. Pendulum Lab''' | ||

+ | |||

+ | * Define simple pendulum | ||

+ | |||

+ | * Define simple harmonic motion | ||

+ | |||

+ | * Describe simple harmonic motion | ||

+ | |||

+ | * Demonstrate the oscillatory motion of a pendulum | ||

+ | |||

+ | * Measure the time taken for 10 oscillations | ||

+ | |||

+ | * Note the time taken for 10 oscillations in a tabular column | ||

+ | |||

+ | * Explain conservation of energy during the oscillations of a pendulum | ||

+ | |||

+ | * Explain the effect of velocity on the oscillations | ||

+ | |||

+ | * Compare measured and calculated values of time period at different lengths | ||

+ | |||

+ | * Demonstrate the motion of the pendulum on other celestial bodies and observe the difference | ||

+ | |||

+ | |||

+ | '''5. Fluid pressure and flow''' | ||

+ | |||

+ | * Change in pressure in the presence of atmosphere | ||

+ | |||

+ | * Change in pressure in the absence of atmosphere | ||

+ | |||

+ | * Demonstration of change in pressure when shape of the tank changes | ||

+ | |||

+ | * Demonstration of change in pressure at various depths | ||

+ | |||

+ | * How pressure changes with change in mass | ||

+ | |||

+ | * Measuring the pressure for different fluid densities | ||

+ | |||

+ | * Demonstration of Bernoulli’s Principle | ||

+ | |||

+ | * Calculating the pressure at the bottom of the tank | ||

+ | |||

+ | * Demonstration of the flow of water by changing the position of the Hose | ||

+ | |||

+ | |||

+ | '''6. Rutherford Scattering''' | ||

+ | |||

+ | * Features of Plum Pudding model | ||

+ | |||

+ | * Behaviour of alpha particles | ||

+ | |||

+ | * Explanation of Plum Pudding simulation interface | ||

+ | |||

+ | * Deflection of alpha particles in the atom | ||

+ | |||

+ | * Limitations of plum pudding atomic model | ||

+ | |||

+ | * Rutherford Scattering simulation interface | ||

+ | |||

+ | * Deflection of alpha particles in atomic and nuclear view in Rutherford scattering | ||

+ | |||

+ | * Description about alpha particles scattering | ||

+ | |||

+ | * Identify the factors that change the deflection of alpha particles | ||

+ | |||

+ | * Effect of n/p ratio on alpha particles deflection | ||

+ | |||

+ | |||

+ | '''7. Photoelectric Effect''' | ||

+ | |||

+ | * Study Photoelectric Effect. | ||

+ | |||

+ | * Determine Threshold frequency. | ||

+ | |||

+ | * Find Stopping potential and Work function. | ||

+ | |||

+ | * Study the factors affecting current and energy of electrons. | ||

+ | |||

+ | |||

+ | '''8. Models of the Hydrogen Atom''' | ||

+ | |||

+ | * Explanation of the models of the hydrogen atom simulation interface. | ||

+ | |||

+ | * Visualize different models of the hydrogen atom. | ||

+ | |||

+ | * Relation between the physical picture of the orbits and energy level diagram of an electron. | ||

+ | |||

+ | * Explain different atomic models and their limitations. | ||

+ | |||

+ | * Explanation about energy level diagram. | ||

+ | |||

+ | * Explain the experimental prediction for each atomic model. | ||

+ | |||

+ | * Explain the limitations of each model. | ||

+ | |||

+ | * About quantum numbers n, l, m. | ||

+ | |||

+ | * Determine the orbital shape and orientation using n, l and m values. | ||

+ | |||

+ | |||

+ | '''9. Radioactive Dating Game''' | ||

+ | |||

+ | * About radioactivity | ||

+ | |||

+ | * Stable and unstable nuclei | ||

+ | |||

+ | * Concept of Half-life | ||

+ | |||

+ | * Types of radioactive decay | ||

+ | ** Alpha decay | ||

+ | ** Beta decay | ||

+ | ** Gamma decay | ||

+ | |||

+ | * What is radioactive dating? (C-14 and U-238) | ||

+ | |||

+ | |||

+ | '''10. Membrane Channels''' | ||

+ | |||

+ | * About membrane channels | ||

+ | |||

+ | * About membrane channels simulation interface | ||

+ | |||

+ | * Types of channels present on the membrane | ||

+ | |||

+ | * Movement of particles through the membrane | ||

+ | |||

+ | * Rate of diffusion based on the number of channels | ||

+ | |||

+ | * Rate of diffusion based on the type of channels | ||

+ | |||

+ | |||

+ | '''11. Neuron''' | ||

+ | |||

+ | * About Neuron simulation interface | ||

+ | |||

+ | * Components of axon | ||

+ | |||

+ | * Movement of sodium and potassium ions across the membranes when the neuron is at rest | ||

+ | |||

+ | * Movement of sodium and potassium ions across the membranes when the neuron is stimulated | ||

+ | |||

+ | * Functions of leak and gated channels | ||

+ | |||

+ | * Permeability of the sodium and potassium ions across the membrane | ||

+ | |||

+ | * About resting potential | ||

+ | |||

+ | * About action potential | ||

+ | |||

+ | * Sequence of events that generates an action potential | ||

+ | |||

+ | |||

+ | '''12. Natural Selection''' | ||

+ | |||

+ | * We will look at effects on survival of a population of rabbits: | ||

+ | ** In Equator and Arctic environments | ||

+ | ** Under selection Factors, namely Wolves and Food | ||

+ | ** Mutations (dominant versus recessive) in Fur (white and brown), Teeth (long and short) and Tail (long and short) | ||

+ | |||

+ | * Look at Pedigree chart for a particular rabbit | ||

+ | |||

+ | |||

+ | '''13. Gene Expression The Basics''' | ||

* Steps involved in Protein Synthesis. | * Steps involved in Protein Synthesis. | ||

Line 189: | Line 473: | ||

− | + | '''14. Gene Machine The Lac Operon''' | |

− | ''' | + | |

* Explanation about components of lac operon. | * Explanation about components of lac operon. | ||

Line 209: | Line 492: | ||

* Explore the effects of mutations within the lac operon by adding or removing genes from DNA. | * Explore the effects of mutations within the lac operon by adding or removing genes from DNA. | ||

+ | |||

+ | |||

+ | '''15. Gravity and Solar system''' | ||

+ | |||

+ | * Explain the interface of Gravity and orbits and My solar system simulations. | ||

+ | |||

+ | * Relation between Sun, Earth, Moon and Space station. | ||

+ | |||

+ | * Determine the Earth Days. | ||

+ | |||

+ | * Define centripetal force and gravitational force. | ||

+ | |||

+ | * How gravity controls the motion of our solar system. | ||

+ | |||

+ | * Study universal law of gravitation. | ||

+ | |||

+ | * Determine the gravitational force between sun and earth. | ||

+ | |||

+ | * Effect of distance on planet's orbit and on earth days. | ||

+ | |||

+ | * Effect of mass and distance on gravitational force. | ||

+ | |||

+ | * Explain the movement of comet around the sun. |

## Revision as of 11:46, 18 November 2019

The Physics Education Technology (PhET) project is an ongoing effort to provide an extensive suite of simulations for teaching and learning subjects like physics, chemistry, biology, mathematics and earth science.

These simulations help students to comprehend concepts through use of graphics and interactive animations.

Teachers can use **PhET simulations** to create worksheets for re-enforcement of the concepts.

PhET simulations are written in Java, Flash or HTML5. This helps to run these simulations online or to downloaded to your local computer.

To encourage quantitative exploration these simulations have measurement instruments including rulers, stop watches, voltmeters, and thermometers

PhET simulations and resources are freely available at the PhET website (https://phet.colorado.edu/) from University of Colorado at Boulder.

The Spoken Tutorial Effort for PhET has being contributed by Dr. Snehalatha Kaliappan, Madhuri Ganapathi Dr. Vidya Iyer, Meenal Ghoderao and Himanshi Karwanje.

## Contents

## Basic Level

**1. Introduction to PhET Simulations**

- Open PhET webpage

- What is a PhET simulation

- Registering on the page

- Sign in on the page

- Browse activities

- How to share your activities page

- Tips for using PhET

- How to get offline access

- Choose the simulations based on subject and grade level

- Download simulations of different file formats used in PhET

- Open the downloaded simulations

**2. Graphing Lines**

- About Cartesian coordinate system

- Calculate the slope of a graphed line

- Save and erase the plotted lines

- Change slope and intercept for the line

- Change variables in a linear equation to change the line

**3. Equation Grapher**

- About Binomial theorem

- About Cartesian co-ordinate system

- Graph of line, y = bx + c, variation of coefficient b and constant c

- Quadratic polynomial, y = ax^2 + bx + c, variation of coefficients a, b and constant c

- Prediction of shape of parabola based on its equation

**4. Trig Tour**

- A unit circle to find trigonometric ratios

- Construct right triangles inside a unit circle

- Convert degrees into radians to look at periodicity of functions

- Special angle values for theta (θ)

- Co-ordinates of points tracing trigonometric functions

- Cosine function graph

- Sine function graph

- Tangent function graph

- Amplitude of functions

- Vertical asymptotes of tangent function

**5. States of Matter**

- How to use States of Matter, an interactive PhET simulation.

- Characteristics of three States of Matter: solid, liquid and gas.

- Change in behaviour of particles as pressure and temperature changes.

- Compare interactions between particles in solids, liquids and gases.

- Study the relation between temperature and Kinetic Energy of molecules.

**6. Bending Light **

- Explain reflection, refraction and dispersion of light.

- Explain refractive index with bending of light.

- How reflection and refraction varies with different material media.

- Use of optical instruments like prism and lens.

- Measure intensity and velocity of light.

- View different wave patterns.

**7. Color Vision**

- About colours of visible light (VIBGYOR)

- How colour vision works

- Types of cone cells

- Different wavelength-based sensitivities

- Optic nerves, tracts and chiasma

- Primary visual cortex

- About types of colours and mixing:

- Two or more colours

- All colours (VIBGYOR) of visible light

- About optical filters and their subtractive nature

**8. Energy forms and changes**

- Conservation of energy in real-life systems.

- About different forms of energy.

- Predict how energy flows when objects are heated or cooled.

- Compare thermal conductivity of different objects.

- Design energy systems.

- Study how energy changes from one form to another.

- Compare efficiency between fluorescent and incandescent bulbs.

- Effect of clouds on solar panel.

- Change in internal energy.

**9. Energy Skate park**

- Define law of conservation of energy

- Define Potential energy and kinetic energy

- Explanation of Energy Skate Park simulation interface

- Effect of friction on energy

- Effect of mass of skater on energy

- Effect of change in track on the potential energy

- To show pie chart and bar graph for energy changes

- Explanation about Energy vs Time graph

- Show the energy value at a particular position on the graph

- To change location and observe the energy changes

**10. pH Scale**

- Determine pH of a solution.

- Determine the acidity and basicity of a solution.

- Arrange acids and bases according to their strength.

- Effect of dilution on pH of a solution.

- Determine the concentration of hydroxide ion, hydronium ion and water at a given pH.

- Logarithmic and Linear scale.

**11. Build an Atoms**

- Build an atom using protons, neutrons and electrons.

- Identify an element and its position on the periodic table.

- Determine whether an atom is neutral or an ion.

- Predict the charge.

- Determine the mass of an atom or ion.

- Determine the stability of atom/ion created.

- Explain element symbol, charge, atomic number, mass number and isotope.

- Relate the number of protons, neutrons and electrons to atomic symbol.

**12. The Greenhouse Effect **

- Effect of greenhouse gases on atmospheric temperature.

- Explore and compare the atmosphere of Ice age and Today.

- Effect of Glass panes and Clouds on temperature.

- Interaction of photons with atmospheric gases.

- Contribution of atmospheric gases to Greenhouse Effect.

## Intermediate Level

**1. Curve Fitting**

- About Cartesian co-ordinate system

- Understanding the concept of best fit line and equation
- About Correlation coefficient, r^2
- About reduced chi squared statistic

- Looking at the degree of polynomials
- 0: lines that are parallel to x or y-axes
- 1: lines: looking at the concept of slope and constant
- 2: quadratic polynomials
- 3: cubic polynomials
- 4: quartic polynomials

**2. Projectile Motion**

- Definition of a projectile

- Definition of a trajectory and projectile motion

- Effect of gravity on Projectile motion

- Horizontal and vertical motion of a projectile

- Effect of change in initial speed, cannon's angle and height of the pedestal on the trajectory

- Effect of air resistance on projectile motion

- Change in shape of the projectile as drag coefficient changes

- Variables affecting drag force.

- Change in mass and diameter of the projectile affects projectile motion

- Effect of custom parameters on projectile motion

**3. Faradays Electromagnetic Lab**

- About Faraday's Electromagnetic Lab simulation interface

- Change in needle spacing and needle size using Options menu

- Change in magnetic field, as strength of the bar magnet varies

- About flip polarity

- Change in loop parameters to change induced EMF

- Effect of AC current source on induced EMF

- Change in speed of electrons on coil as the voltage varies

- Explain how a transformer works

- Working of a generator

**4. Pendulum Lab**

- Define simple pendulum

- Define simple harmonic motion

- Describe simple harmonic motion

- Demonstrate the oscillatory motion of a pendulum

- Measure the time taken for 10 oscillations

- Note the time taken for 10 oscillations in a tabular column

- Explain conservation of energy during the oscillations of a pendulum

- Explain the effect of velocity on the oscillations

- Compare measured and calculated values of time period at different lengths

- Demonstrate the motion of the pendulum on other celestial bodies and observe the difference

**5. Fluid pressure and flow**

- Change in pressure in the presence of atmosphere

- Change in pressure in the absence of atmosphere

- Demonstration of change in pressure when shape of the tank changes

- Demonstration of change in pressure at various depths

- How pressure changes with change in mass

- Measuring the pressure for different fluid densities

- Demonstration of Bernoulli’s Principle

- Calculating the pressure at the bottom of the tank

- Demonstration of the flow of water by changing the position of the Hose

**6. Rutherford Scattering**

- Features of Plum Pudding model

- Behaviour of alpha particles

- Explanation of Plum Pudding simulation interface

- Deflection of alpha particles in the atom

- Limitations of plum pudding atomic model

- Rutherford Scattering simulation interface

- Deflection of alpha particles in atomic and nuclear view in Rutherford scattering

- Description about alpha particles scattering

- Identify the factors that change the deflection of alpha particles

- Effect of n/p ratio on alpha particles deflection

**7. Photoelectric Effect**

- Study Photoelectric Effect.

- Determine Threshold frequency.

- Find Stopping potential and Work function.

- Study the factors affecting current and energy of electrons.

**8. Models of the Hydrogen Atom**

- Explanation of the models of the hydrogen atom simulation interface.

- Visualize different models of the hydrogen atom.

- Relation between the physical picture of the orbits and energy level diagram of an electron.

- Explain different atomic models and their limitations.

- Explanation about energy level diagram.

- Explain the experimental prediction for each atomic model.

- Explain the limitations of each model.

- About quantum numbers n, l, m.

- Determine the orbital shape and orientation using n, l and m values.

**9. Radioactive Dating Game**

- About radioactivity

- Stable and unstable nuclei

- Concept of Half-life

- Types of radioactive decay
- Alpha decay
- Beta decay
- Gamma decay

- What is radioactive dating? (C-14 and U-238)

**10. Membrane Channels**

- About membrane channels

- About membrane channels simulation interface

- Types of channels present on the membrane

- Movement of particles through the membrane

- Rate of diffusion based on the number of channels

- Rate of diffusion based on the type of channels

**11. Neuron**

- About Neuron simulation interface

- Components of axon

- Movement of sodium and potassium ions across the membranes when the neuron is at rest

- Movement of sodium and potassium ions across the membranes when the neuron is stimulated

- Functions of leak and gated channels

- Permeability of the sodium and potassium ions across the membrane

- About resting potential

- About action potential

- Sequence of events that generates an action potential

**12. Natural Selection**

- We will look at effects on survival of a population of rabbits:
- In Equator and Arctic environments
- Under selection Factors, namely Wolves and Food
- Mutations (dominant versus recessive) in Fur (white and brown), Teeth (long and short) and Tail (long and short)

- Look at Pedigree chart for a particular rabbit

**13. Gene Expression The Basics**

- Steps involved in Protein Synthesis.

- Transcription of DNA sequence to mRNA.

- Translation of mRNA to a protein sequence.

- Factors affecting Protein Synthesis.

- Average level of Green Fluorescent Protein produced in cells.

**14. Gene Machine The Lac Operon**

- Explanation about components of lac operon.

- About lactose regulation.

- Genes involved in lactose regulation.

- Roles of LacI, LacZ, and LacY in lactose regulation.

- Build a lac operon.

- Function of RNA polymerase molecule.

- Factors affecting LacZ gene regulation.

- Predict the effects on lactose metabolism when the concentration of lactose is changed.

- Explore the effects of mutations within the lac operon by adding or removing genes from DNA.

**15. Gravity and Solar system**

- Explain the interface of Gravity and orbits and My solar system simulations.

- Relation between Sun, Earth, Moon and Space station.

- Determine the Earth Days.

- Define centripetal force and gravitational force.

- How gravity controls the motion of our solar system.

- Study universal law of gravitation.

- Determine the gravitational force between sun and earth.

- Effect of distance on planet's orbit and on earth days.

- Effect of mass and distance on gravitational force.

- Explain the movement of comet around the sun.