KO: Gravity
Subject: Science | Year: 5
Name: _________________________ Class/Set: ____________ Date: ____________
1. Key Knowledge and Core Facts
- Non-contact Force: Gravity is a force that acts between two objects even when they are not touching.
- Pulling Force: Gravity always pulls objects towards the centre of the Earth.
- Massive Objects: All objects with mass have a gravitational pull; the larger the mass, the stronger the pull.
- Planetary Scale: Earth’s gravity is so strong because the planet has a colossal amount of mass.
- Weight vs Mass: Gravity determines the weight of an object, but it does not change its mass.
- Universal Law: Gravity exists everywhere in the universe where there is matter.
- Atmosphere: Gravity holds our atmosphere in place, preventing air from escaping into space.
- Constant Force: On Earth, gravity exerts a consistent pull on all objects regardless of their appearance.
2. Key Vocabulary
- Gravity: The invisible force that pulls objects toward each other.
- Mass: The amount of 'stuff' (matter) inside an object, measured in kilograms (kg).
- Weight: The measure of the force of gravity pulling on mass, measured in Newtons (N).
- Newton (N): The standard unit of measurement for force.
- Force Meter: A piece of apparatus used to measure the magnitude of a force (also called a Newton meter).
- Air Resistance: A type of friction (drag) caused by air particles pushing against a moving object.
- Surface Area: The total area of the outside of an object; high surface area increases air resistance.
- Magnitude: The size or extent of a force.
- Acceleration: The rate at which an object increases in speed.
- Orbit: The curved path of a celestial object or spacecraft around a planet or star.
- Vacuum: A space entirely devoid of matter (like air), where air resistance cannot exist.
- Terminal Velocity: The constant speed reached when the force of gravity is balanced by air resistance.
3. Isaac Newton and Historical Context
- Sir Isaac Newton: The British scientist who first formulated the theory of universal gravitation in 1687.
- Mathematical Principles: Newton published 'Philosophiæ Naturalis Principia Mathematica', detailing how gravity works.
- The Apple Myth: Legend says an apple falling from a tree inspired his theory; he actually spent years on calculations.
- Galileo Galilei: Earlier scientist who proved that gravity pulls all objects at the same rate regardless of mass.
- Leaning Tower Experiment: Galileo supposedly dropped two different masses to show they hit the ground simultaneously.
- Planetary Motion: Newton realised the same force pulling the apple also keeps the Moon in orbit.
- Inverse Square Law: Newton discovered that the further you move from an object, the weaker its gravity becomes.
- Scientific Revolution: These discoveries moved science from guesswork to evidence-based mathematical laws.
4. Mass vs Weight: The Critical Distinction
- Mass Stability: Your mass stays the same no matter where you are in the universe.
- Weight Variability: Your weight changes depending on the strength of gravity in your location.
- Calculation: Weight is calculated by multiplying mass by the gravitational field strength (W = m × g).
- Earth's Gravity: On Earth, 1kg of mass weighs approximately 9.8 Newtons (often rounded to 10N for KS2).
- Measurement Tools: Mass is measured with balances/scales; Weight is measured with a force meter.
- Units: Mass is recorded in grams (g) or kilograms (kg); Weight is always recorded in Newtons (N).
- Space Travel: An astronaut has the same mass on the Moon but weighs much less.
- Zero Gravity: In deep space, an object has zero weight but still possesses its full mass.
5. Gravitational Pull in the Solar System
- Solar Dominance: The Sun has the most mass in our solar system, so it has the strongest gravitational pull.
- Planetary Orbit: The Sun’s gravity keeps all planets, including Earth, in their circular orbits.
- The Moon: The Earth’s gravity keeps the Moon orbiting us, preventing it from floating away.
- Tidal Force: The Moon’s gravity is strong enough to pull Earth's oceans, creating high and low tides.
- Jupiter: As the largest planet, Jupiter has a gravity pull 2.4 times stronger than Earth’s.
- The Moon’s Gravity: The Moon is smaller than Earth; its gravity is only 1/6th as strong as Earth’s.
- Spherical Shape: Gravity pulls from all sides towards the centre, which is why planets and stars are round.
- Black Holes: Objects with so much mass and gravity that even light cannot escape their pull.
6. Air Resistance and Falling Objects
- Opposing Forces: When an object falls, gravity pulls it down while air resistance pushes it up.
- Surface Area Impact: Objects with a large surface area (like a parachute) experience more air resistance.
- Streamlining: Shaped objects (like rockets) are designed to minimize air resistance to move faster.
- Vacuum Conditions: In a vacuum (no air), a hammer and a feather will fall at the exact same speed.
- Balanced Forces: When air resistance equals the force of gravity, an object stops accelerating.
- Gravity vs Friction: Air resistance is a form of friction that acts in the opposite direction to motion.
- Falling Speed: Heavier objects do not fall faster because of gravity; they fall faster only if they can overcome air resistance better.
- Terminal Velocity: A skydiver reaches about 120mph before air resistance prevents further speeding up.
7. Common Misconceptions
- "No Gravity in Space": False. Gravity is everywhere; astronauts float because they are in "free fall" around the Earth.
- "Heavy Objects Fall Faster": False. Gravity pulls all objects equally; only air resistance slows lighter/wider objects down.
- "Mass and Weight are the same": False. Mass is the amount of matter; Weight is the force of gravity on that matter.
- "Gravity is Magnetic": False. Magnetism only affects certain metals; gravity affects everything with mass.
- "Gravity only goes down": False. Gravity pulls towards the centre of mass (which, for us, is down towards Earth's core).
- "Clouds stay up because of no gravity": False. Gravity pulls on clouds, but upward air currents keep them suspended.
- "Moon has no gravity": False. The Moon has gravity, but it is weaker than Earth’s because the Moon is smaller.
- "Vacuum of space sucks things": False. Space doesn't 'suck'; objects move due to pressure differences or gravitational pull.
8. Measuring Gravity and Forces
- Newton Meters: These contain a spring that stretches further as more force (weight) is applied.
- Calibration: Scientific instruments must be calibrated to ensure 0N is recorded when no force is applied.
- Scaling: Force meters come in different sizes (e.g., 0-10N or 0-100N) depending on the object's mass.
- Accuracy: To get an accurate weight, the object must be stationary and not bouncing on the spring.
- Vertical Measurement: Weight should always be measured vertically to align with the pull of gravity.
- Gram to Newton Conversion: On Earth, 100g is approximately equal to 1 Newton of force.
- Industrial Scales: These use the same principles as handheld force meters but on a much larger, digital scale.
- Comparison: Scientists compare the weight of objects on different planets to understand gravitational field strength.
9. Working Scientifically: Gravity Investigations
- Variable Control: In a falling object experiment, you must change one variable (e.g., surface area) and keep others the same.
- Fair Testing: Ensure the height of the drop is identical for every repeat to ensure valid results.
- Force Measurement: Use a Newton meter to record the weight of different items in the classroom.
- Data Recording: Results should be recorded in a clear table with units (N) in the column headers.
- Anomalous Results: If one drop takes significantly longer, it should be investigated and re-tested.
- Prediction: Use prior knowledge to predict which parachute design will provide the most air resistance.
- Graphing: Use a bar chart to compare the weight of an object if it were on different planets.
- Conclusion: Use evidence to explain how surface area affects the time it takes for an object to fall.
10. Consolidation Tasks
- Task A: Define the difference between mass and weight in a single sentence.
- Task B: Identify which unit (kg or N) is used for measuring the pull of gravity on an object.
- Task C: Explain why a crumpled piece of paper falls faster than a flat sheet of paper.
- Task D: Name the scientist famous for his work on gravity and the year he published his main findings.
- Task E: If an object weighs 50N on Earth, calculate its approximate mass in kilograms.
⚠ TEACHER’S GUIDANCE
💡 Pedagogical Advice for Year 5
- Delivery Method: Conduct the "Galileo Drop" as a carpet session. Use a heavy ball and a crumpled piece of paper of similar size. Ask students to predict which hits first. Most will say the ball. When they hit simultaneously, use this "cognitive conflict" to introduce the concept that gravity is a constant pull.
- Oracy & Sensory Engagement: Use force meters (Newton meters) early. Let students feel the "pull" by hanging different exercise books or pencil cases. Physically feeling the spring stretch helps transition from the concrete to the abstract concept of "Newtons."
- Greater Depth: For GDS students, introduce the idea of "Field Strength." Explain that while Earth's gravity is ~10N/kg, the Moon is ~1.6N/kg. This justifies why the math changes but the "stuff" (mass) doesn't.
- Safety & Nuance Check: Ensure students do not stand on chairs or desks to drop objects. Use a metre ruler and drop from standing height only.
🔑 Answer Key & Solutions
- Task A Answer: Mass is the amount of matter in an object, whereas weight is the force of gravity pulling on that matter.
- Task B Answer: Newtons (N).
- Task C Answer: The crumpled paper has a smaller surface area, so it experiences less air resistance to oppose the pull of gravity.
- Task D Answer: Sir Isaac Newton; 1687.
- Task E Answer: 5kg (Based on the 10N = 1kg conversion used in KS2).
