Unit 3: Forces and energy
GRADE 8 – Unit 3: Forces and energy — HIA International - Grade 8 Revision Notes & Topical Questions
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Lesson summary
GRADE 8 - Unit 3: Forces and energy — HIA INTERNATIONAL notes: Distinguish mass and weight, describe the Solar System and tides, and explain energy stores, transfers, density and thermal energy transfer.Mass = the amount of matter in an object, in kilograms; it is the same everyw…
- Strength of gravity
- Moon phases
- Solar eclipse
- Lunar eclipse
- Tidal range
- Spring tide
Section 1 of 10Gravity, weight and mass
Mass
Mass the amount of matter in an object, in kilograms; it is the same everywhere.
Weight
Weight the force of gravity on an object, in newtons (N).
Equation
- Equation weight = mass × strength of gravity (W = m × g).
Strength of gravity
- Strength of gravity about 10 N/kg on Earth, 3.7 N/kg on Mars and 1.6 N/kg on the Moon.
Astronaut
- Astronaut has the same mass on the Moon but a smaller weight.
Section 2 of 10The Solar System
1
Origin
- Origin it formed from a nebula of dust and gas pulled together by gravity.
2
Members
- Members the Sun, eight planets, dwarf planets, moons, asteroids and comets.
- (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune)
3
Orbits
- Orbits the Sun’s gravity keeps planets in orbit;
- planets further away move more slowly and take longer to orbit.
4
Vacuum
- Vacuum with no air resistance in space, planets do not slow down.
5
Moon phases
- Moon phases we see different amounts of the Moon’s sunlit half as it orbits the Earth.
6
Solar eclipse
- Solar eclipse the Moon passes between the Sun and Earth, casting a shadow on Earth.
7
Lunar eclipse
- Lunar eclipse the Earth is between the Sun and Moon, so its shadow falls on the Moon.
Section 3 of 10Tides
Cause
- Cause the gravitational pull of the Moon makes a tidal bulge in the oceans; the Sun’s pull is weaker.
Frequency
- Frequency most places have two high tides and two low tides each day.
Tidal range
Tidal range the difference between high and low water.
Spring tide
- Spring tide largest range, when the Sun, Moon and Earth are in line.
Neap tide
- Neap tide smallest range, when the Sun and Moon are at right angles.
Effects
- Effects shipping, coastal habitats, fishing and tidal energy.
Section 4 of 10Energy stores and transfers
1Law of conservation of energy
- Law of conservation of energy energy cannot be created or destroyed, only transferred from one store to another.
2Transfer
Transfer energy moving from one store to another.
3Useful and wasted
- Useful and wasted in any transfer some energy is useful and some is wasted, usually as thermal energy that spreads out.
4Efficiency
- Efficiency useful energy output ÷ total energy input × 100%.
Section 5 of 10Density
Density
Density mass per unit volume; density = mass ÷ volume.
Floating
- Floating an object less dense than water floats and a denser one sinks.
Gas density
- Gas density gases have the lowest density because their particles are far apart.
Section 6 of 10Heat and temperature
1Heat
Heat the total thermal energy of the particles in a substance, in joules.
2Temperature
Temperature the average energy of the particles.
3Bath compared with cup
- Bath compared with cup a bath of warm water at 40 °C has more thermal energy than a cup of boiling water at 100 °C because it has far more particles.
4Direction
- Direction thermal energy always transfers from a hotter object to a colder one until they reach thermal equilibrium.
5Different materials
- Different materials need different amounts of energy to raise the temperature of the same mass by 1 °C;
- water needs a lot.
6Change of state
- Change of state during melting or boiling the temperature stays constant because the energy separates the particles.
7Absolute zero
- −273 °C, the lowest possible temperature.
8Reading a thermometer
- Reading a thermometer eye level, bulb fully in the liquid and not touching the container, wait until the reading stops changing.
Section 7 of 10Transferring thermal energy
| Method | How it works | Needs particles? |
|---|---|---|
| Conduction | Vibrating particles pass energy on; metals also use free electrons | Yes; best in solids |
| Convection | Warm fluid expands, becomes less dense and rises | Yes; liquids and gases |
| Radiation | Waves that carry thermal energy | No; passes through a vacuum |
Section 8 of 10Evaporation and cooling
Mechanism
- Mechanism the highest-energy particles escape the surface, lowering the average energy of those that remain.
Result
- Result the liquid cools, e.g. sweat cools the skin.
Rate
- Rate evaporation is faster with higher temperature, larger surface area, air movement and lower humidity.
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Mass | The amount of matter in an object, in kg. |
| Weight | The force of gravity on an object, in newtons. |
| Spring tide | A tide with the largest range, when the Sun, Moon and Earth are in line. |
| Neap tide | A tide with the smallest range, when the Sun and Moon are at right angles. |
| Efficiency | Useful energy output divided by total energy input, as a percentage. |
| Density | Mass per unit volume. |
| Conduction | Transfer of thermal energy through a solid by particle vibrations. |
| Thermal equilibrium | The state in which two objects are at the same temperature. |
Section 10 of 10Exam technique
1
Using mass and weight as the same thing — mass is in kg and constant; weight is a force in N that depends on gravity.
2
Thinking energy is used up — it is transferred, and some is wasted as thermal energy.
3
Confusing solar and lunar eclipses — in a solar eclipse the Moon is between Sun and Earth.
4
Saying temperature and thermal energy are the same — thermal energy also depends on the mass.
5
Saying temperature keeps rising during melting — it stays constant while energy separates the particles.
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