Gravity, Mass, Energy, and The Solar System
3.1 Gravity, Weight, and Mass
Gravity as a Force:
Gravity is a force that acts between objects.
The Earth, due to its large mass (approximately 6,000,000,000,000,000,000,000,000 kg), exerts a strong gravitational force.
All objects, even small ones like pens and pencils, have gravity, but their forces are very weak and not easily noticeable.
The force of gravity acts towards the center of an object.
The strength of gravity decreases as distance from the object increases.
Weight:
Weight is defined as the force of gravity on an object.
It's measured in newtons (N).
The weight of a typical apple is approximately 1 N.
Contact Force:
When an object rests on a surface, gravity still pulls it down.
The surface pushes back with an equal and opposite force called the contact force.
The contact force supports the object and prevents it from moving through the surface, and is equal to the object's weight when the surface is stationary.
If weight exceeds the contact force, the surface may break, or the object may sink into it.
Weight vs. Mass:
Weight and mass are often confused.
Mass: the quantity of matter in an object, measured in kilograms (kg).
Weight: the force of gravity on an object, measured in newtons (N).
On Earth, the force of gravity is approximately 10 N on every 1 kg of mass.
Equation: weight (N) = mass (kg) × 10 (N/kg), abbreviated as W = m × 10
Formula Triangle:
A formula triangle can be used to solve for weight, mass, or the gravitational force (10 N/kg on Earth).
To find mass, cover 'm' in the triangle, revealing W/10, indicating that you divide the weight by 10 to get the mass.
Varying Gravitational Strength:
The gravitational force is not constant throughout the solar system.
Earth: 10 N/kg
Moon: 1.6 N/kg
Mars: 3.7 N/kg
Jupiter: 25 N/kg
To calculate weight on another planet or moon, use the equation W = m × g, where g is the gravitational strength at that location.
Mass remains constant regardless of location, while weight changes with varying gravitational strength.
3.2 Formation of the Solar System
Formation Theories:
Scientists develop theories about the formation of the solar system by:
Looking for evidence from observations and experiments.
Formulating a testable hypothesis and seeking evidence to support it.
Facts About the Solar System:
Planets orbit the Sun in the same direction.
Most planets and the Sun spin on their axes in the same direction (except Venus and Uranus).
Moons generally orbit their planets in the same direction as planets orbit the Sun.
The direction of spin of the Sun and planets (excluding Venus and Uranus) matches the direction of planetary orbits.
Planets orbit the Sun in the same plane, giving the solar system a flat appearance.
Observing Star Formation:
Scientists observe distant stars forming from clouds of dust and gas called nebulae.
Young stars are often surrounded by a flat disc of dust.
Models:
Since direct observation of star/solar system formation is impossible (timescales too long), scientists use computer models.
These models incorporate known laws of physics to predict outcomes, starting with a cloud of dust and gas.
Models predict the formation of a star surrounded by planets.
Star and Planet Formation:
Particles of dust and gas have weak gravity that pulls them together.
As particles stick together, mass increases, strengthening gravity, which attracts more dust and gas.
This forms a small ball that grows over millions of years.
If the ball becomes large enough, it gets hot enough to become a star; otherwise, it becomes a planet.
Supporting vs. Contradictory Evidence:
Most facts about the Solar System support this hypothesis.
Venus spinning in the opposite direction is contradictory evidence.
3.3 Movement in Space
The Sun's Gravity:
Objects with greater mass have greater gravity.
The Sun's mass is 330,000 times greater than Earth's, and contains more mass than all other planets combined.
Gravity on the Sun is 270 N/kg, compared to 10 N/kg on Earth.
The Sun's gravity holds all the planets in orbit.
Gravity weakens with increasing distance from the Sun.
Planetary Orbits:
Planetary orbits are nearly circular.
A force is required to keep an object moving in a circle.
The Sun's gravity provides this force, pulling planets towards it.
Without this force, planets would move in a straight line into space.
Orbital Speed:
Mercury, closest to the Sun, experiences the strongest gravitational pull and has the highest orbital speed (170,000 km/h).
Earth's average orbital speed is about 100,000 km/h.
Speed in Space:
On Earth, moving objects experience forces that slow them down, such as air resistance.
Air resistance is caused by an object pushing against air particles, and it increases with speed.
Space is a vacuum, containing very few particles.
The Juno probe reached 266,000 km/h in space, becoming the fastest human-made object, which wouldn't be possible on Earth due to air resistance.
Planets move in a vacuum, so there's no air resistance to slow them down; the only force acting on them is gravity.
3.4 Tides
What are Tides?
Tides are changes in the depth of the ocean during the day.
The tidal range is the difference in depth between high and low tides.
The largest tidal range is 16.3m in the Bay of Fundy, Canada.
Some tidal ranges are less than 1m in the Caribbean and Mediterranean seas.
Tides also cause changes in the height of land, called earth tide, with a tidal range of about 30cm.
High tides are approximately 12 hours apart, and the time between high and low tide is six hours.
Causes of Tides:
The Moon's gravity pulls on the Earth, creating a tidal force.
The Moon's gravity pulls water more easily than land.
The side of the Earth closest to the Moon experiences high tide.
The opposite side of the Earth also experiences high tide.
The Earth's rotation causes high tides to be 12 hours apart.
The Sun also exerts a tidal force, but it's weaker than the Moon's due to greater distance.
When the Sun, Moon, and Earth align, it results in a larger tidal force and greater tidal ranges.
Effects of Tides:
Harbors may only be accessible at high tide.
Coastal areas are more prone to flooding during high tides with strong winds.
The flow of water in coastal areas can be dangerous for small boats.
Tides influence food chains: birds eat shellfish exposed at low tide, and fish movements are influenced by tides.
Volcano eruptions and earthquakes may be linked to earth tides.
Tidal movements can be used to generate electricity.
3.5 Energy
Definition of Energy:
Energy is something that must be changed or transferred in order to do something.
Kinetic energy is the energy in movement.
The unit for measuring energy is the joule (J).
Energy Stores and Transfers:
Kinetic: energy stored due to movement of an object.
Chemical: energy stored in food, batteries, and chemical fuels such as wood, oil, and coal.
Thermal: heat energy stored in hot objects and transferred to colder objects.
Elastic potential: energy stored when things are stretched or squeezed to change their shape.
Gravitational potential: energy stored when an object is lifted away from a source of gravity.
Electrical: the flow of current in a circuit transfers electrical energy.
Sound: energy transferred from vibrating objects.
Light: visible energy from luminous objects (objects that give out their own light).
Storing Energy:
Energy can be stored more easily in some ways than in others.
Chemical energy (e.g., uncooked rice, coal, crude oil, batteries) is relatively easy to store for long periods.
Gravitational potential energy is also easy to store (e.g., water in a tank lifted by a pump).
Thermal energy (heat) is difficult to store for extended periods as hot objects cool down.
Kinetic energy is more difficult than chemical or gravitational potential energy to store, as moving objects eventually stop.
3.6 Changes in Energy
How Energy Changes:
Energy must be changed or transferred in order to do something.
Before energy can be changed or transferred, it is stored; when it's stored, it's not actively doing anything.
Burning wood changes chemical energy to thermal energy.
Walking upstairs changes chemical energy (from food) into kinetic energy and then into gravitational potential energy.
Power stations convert chemical energy (from gas) to thermal energy, then to kinetic energy in generators, and finally to electrical energy.
Energy changes are not always helpful and can be dangerous (typhoons, hurricanes, earthquakes, tsunamis).
Representing Energy Changes:
Processes (e.g., burning) and events (e.g., a book falling) can be represented as arrows in diagrams showing energy changes.
Examples:
A fire that burns wood: chemical → thermal.
A television: electrical → sound + light.
A book falling from a shelf: gravitational potential → kinetic.
Useful vs. Wasted Energy:
Energy changes can be useful (energy is changed in a way that we want) or result in wasted energy.
3.7 Where Does Energy Go?
Useful and Wasted Energy:
Every time energy is used, some is useful, and some is wasted.
A motorcycle converts chemical energy (fuel) into kinetic energy (movement), but also into thermal and sound energy.
Only about 25% of the fuel's chemical energy is used for movement; the other 75% is wasted.
Wasted energy is dissipated and cannot be recovered.
Dissipated energy is energy that spreads out where there is no use for it.
Thermal energy and sound cannot be gathered back into one place to be stored, changed, or transferred.
Energy Dissipation:
Every energy change or transfer results in some thermal energy being wasted and dissipated.
Even when producing thermal energy, some of it is wasted (e.g., heating water on a fire also heats the rocks, container, and air).