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Gravity, friction and fluid resistance - Science Year 5

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Introduction

A parachute changes a fall. Bicycle brakes change a wheel's motion. A boat's shape affects its movement through water. How can we explain these designs using forces rather than simply saying that they “slow down”?

You will distinguish gravity, friction, air resistance and water resistance; draw labelled force arrows; and interpret a fair-test record for two model parachutes. You will also compare an Earth classroom example with NASA's hammer-and-feather demonstration on the Moon.

Recall: Forces are pushes or pulls. Their effects can include starting motion, changing speed or direction, and changing shape. A moving object can have several forces acting on it at once. An object at rest can have forces acting on it too.

Your evidence will be a force diagram, a checked comparison of explicitly fictional repeat measurements and a design recommendation with a limitation. No human parachuting, bike-brake adjustment or high drop is part of the tasks.

Gravity and a simple falling-model diagram

Gravity does not need air or physical contact.

Gravity is an attractive force between masses. Earth's gravitational pull acts on an object near Earth. When an unsupported object is released, this pull causes it to accelerate towards Earth, unless other effects balance or alter the result. Near the surface, we describe this direction as down, towards Earth's centre.

A book resting on a table still experiences gravity. The table supplies an upward support force. Its lack of motion is not evidence that gravity has switched off. Likewise, a hand holding a model provides support before release; after release, the hand is no longer holding it up.

Start a force diagram: Draw a simple dot or box for a model just released in still air. Add a downward arrow labelled “gravity” or “weight”. As it falls through the air, add an upward arrow labelled “air resistance”. These arrows show forces acting on the model, not arrows for where the model has already travelled. Do not label the upward arrow “gravity”.

Arrows can also represent relative force size when that is known. A photograph does not give those sizes. If the model is falling at a steady speed in a simple vertical-fall case, the upward resistance balances the downward weight: balanced forces can accompany continuing motion. They do not necessarily mean an object is stationary.

Moon evidence: In 1971, Apollo 15 commander David Scott released a hammer and a feather together from approximately the same height on the Moon. With essentially no air resistance, they reached the surface together within the demonstration's observational accuracy. The Moon has gravity; the objects fell because of it. The result does not show an absence of gravity.

The original NASA film tests an important idea: without significant air resistance, objects released together from rest at the same height have the same gravitational acceleration, regardless of their mass. It does not mean that a feather and hammer must behave identically in Earth's air, or that they experience identical forces. Keep release conditions and the surrounding fluid in the explanation.

Check the claim: “A feather always lands later because it has less mass.” “Always” is contradicted by the Moon demonstration. On Earth, a feather's shape and air resistance can strongly affect its motion. Mass alone does not predict every fall time.

Friction, grip and bicycle brakes

Friction can slow sliding and provide useful grip.

Friction acts between surfaces in contact and resists their sliding past each other, or their tendency to slide. It can be useful: shoes grip a floor and tyres grip a road. “Friction is always a problem” is therefore a poor design rule.

In a bicycle rim brake, pads press against the moving rim. Friction resists the relative movement at that contact and can slow the wheel. A disc brake acts at a different contact surface, so identify the type before describing the parts. Use a diagram or teacher's model; do not adjust a working bicycle or put fingers near moving wheels for this course.

Close-up of a bicycle's rear hydraulic rim-brake assembly beside the wheel rim.

A rear rim-brake assembly. Braking involves friction where the pad contacts the moving rim; the photograph is not a brake-inspection or repair guide. Pittigrilli, CC BY-SA 4.0.

Worked explanation: “The brake deletes the force that made the bicycle go.” A better explanation identifies contact: the pads press on a moving surface, and friction opposes sliding there. At this level, explain the interaction and change in motion rather than inventing a stored quantity of “going force”.

For a fair comparison of sliding surfaces, use the same test object and a consistent method. State the outcome: perhaps a pulling force needed to keep it sliding steadily, measured by suitable school equipment. A rough appearance alone does not determine an exact friction force. Surface condition, materials and how firmly they are pressed together matter.

Directions need a context: In a simple sliding-block example, friction acts opposite the sliding of the block relative to the floor. That does not mean every friction force points backwards relative to the whole person's or bicycle's travel. Grip can help propel motion. Our braking example concerns the pad and rim's relative sliding.

Air resistance, water resistance and design

Air and water are both fluids.

A fluid is a substance that can flow. Air is a gas and water is a liquid; both can resist an object's movement through them. Air resistance and water resistance are forms of drag. In our still-air and still-water examples, resistance acts against the object's movement through that fluid.

For a downward-falling model in still air, drag acts upward while gravity acts downward. Opening a canopy can greatly increase resistance compared with a compact falling model. Gravity has not disappeared. Real parachutes can also steer and glide, so the two-arrow vertical model is useful but incomplete for every photograph or flight situation.

Design situation	Relevant interaction	A possible feature to investigate
A model falling in still air	Air resistance against movement through air	Canopy area and shape.
A model hull moving through still water	Water resistance against movement through water	Shape facing the flow and how smoothly water moves around it.
A block sliding on a table	Friction at contacting surfaces	Surface materials and conditions, using the same block and test method.

A water-resistance comparison: A more streamlined shape can reduce drag compared with a blunt shape in suitable conditions. This is a design claim to test, not a promise that any pointed object is best. Compare models under matched conditions, such as the same water depth, immersed size and steady towing speed. Measure a relevant outcome, for example the towing force in a teacher's supplied record. Different speed or depth could confound a shape comparison.

Moving more slowly because an object is heavier is not automatically evidence of higher water resistance. A floating model also experiences gravity and an upward support force from the water. Our question here concerns resistance to its horizontal movement; do not confuse it with the question of whether it floats.

Try a force-arrow explanation: Draw a hull travelling to the right through still water. Add a leftward water-resistance arrow. If a steady pull keeps it moving, include that rightward applied force. A diagram with only a motion arrow does not yet identify the forces. You can draw the vertical support and weight as a separate pair if explaining the floating model's full situation.

A shallow teacher-approved model investigation can be planned on paper. No deep water, swimming test, outdoor current, motor or electrical apparatus is needed. Actual investigation credit requires genuine observations, not a drawing of what you hoped would happen.

Interpret repeated parachute data and recommend a design

Use repeated observations to support a design choice.

These are invented teaching records, not results measured by this class. Two model parachutes are specified to have the same total mass, material, general shape, line length, release method and 1 m drop height. Only canopy area differs. Matching total mass matters because a larger piece of material could otherwise add mass as well as area.

Model	Canopy area	Fall times in seconds: trials 1, 2, 3
A	100 square centimetres	1.1, 1.2, 1.3
B	225 square centimetres	1.6, 1.8, 2.0

Worked summary: A's mean time is $(1.1 + 1.2 + 1.3) / 3 = 1.2$ seconds. B's mean is $(1.6 + 1.8 + 2.0) / 3 = 1.8$ seconds. For these records, B takes 0.6 seconds longer on average over the same drop height. Longer time here means a slower average descent, not that the model travelled a greater distance.

The changed variable is canopy area, and the measured outcome is fall time in seconds. The result supports choosing B if this small model's aim is slower descent. A plausible explanation is increased air resistance from the larger canopy in the stated setup. It is not evidence that gravity was weaker for B.

Why repeat? Times vary between trials. Small differences in release, timing and canopy behaviour can matter. Three fictional observations do not establish that the pattern must hold for every material, shape, wind condition or size. They also do not establish safe landing, stability or suitability for carrying a person.

Method review: A draft compares a small light canopy indoors with a large heavier one outdoors from a higher release point. Too much changed at once. Match total mass, height, environment and release method before attributing a difference to area. In an approved low-level classroom investigation, the teacher selects lightweight models and a clear landing area; pupils do not climb, drop heavy objects or test from windows or stairs.

A useful limitation: “B worked best” is too broad. Write, “B had the longer mean fall time in these three matched fictional trials. We have not tested stability or other conditions.” If a later trial has tangled lines, record and investigate it rather than silently replacing its result.

Final evidence: Combine the model's labelled downward weight and upward air-resistance arrows, a checked data summary, one friction example and one water-resistance example. Recommend a design feature with evidence and state an untested feature. Use the NASA comparison to explain why the fluid around the object matters.

Interactive Tasks

Quiz: Test Your Knowledge

Which force causes an unsupported object near Earth to accelerate downwards when it is released? (Earth's gravitational pull) (!The air creating gravity by pushing down) (!A force that exists only after the object touches the ground) (!Friction with a surface it is not touching)

A book rests on a table. Which explanation is correct? (Gravity still acts and the table supplies support) (!Gravity switches off whenever an object stops) (!Only the table has gravity acting on it) (!There can be no forces because the book is not moving)

For a vertically falling model in still air, which directions fit the two-force diagram? (Weight downwards and air resistance upwards) (!Weight upwards and air resistance downwards) (!Both forces necessarily point downwards) (!Both forces disappear when the canopy opens)

What does the NASA hammer-and-feather example support? (With negligible air resistance, matched releases have the same gravitational acceleration regardless of mass) (!There is no gravity on the Moon) (!A feather always falls later in every environment) (!The two objects must have identical forces because they land together)

Why can rim brakes slow a bicycle wheel? (Pads press on the moving rim and friction resists relative sliding) (!The pads remove gravity from the wheel) (!The brake turns off every force on the bicycle) (!Air resistance disappears where the pads touch)

Which statement about friction is accurate? (It can provide useful grip as well as resist sliding) (!It is always harmful to movement and never useful) (!It acts only when objects are not in contact) (!Every friction force must point backwards relative to a person's travel)

What changed between the two fictional parachute designs? (Canopy area) (!Total model mass) (!Release height) (!Number of recorded trials)

What is B's mean fictional fall time from 1.6, 1.8 and 2.0 seconds? (1.8 seconds) (!5.4 seconds) (!0.4 seconds) (!1.2 seconds)

Which conclusion fits the supplied parachute data? (B has the longer mean fall time in the stated matched trials) (!Gravity is weaker whenever a bigger canopy opens) (!B travelled a greater drop distance than A) (!The records certify B for safely carrying a person)

Which condition should be controlled when comparing water resistance of two model shapes? (Their steady towing speed) (!The shape difference that is being investigated) (!The wording of a hoped-for winning conclusion) (!The identity of the learner choosing a favourite design)

Memory Game

Gravity	Attractive force between masses
Friction	Interaction resisting relative sliding at contacting surfaces
Air resistance	Drag associated with movement through air
Water resistance	Drag associated with movement through water
Fluid	A substance that can flow, including liquids and gases
Mean time	Total recorded time divided by the number of trials

Drag and Drop

Downward model-parachute arrow	Weight due to gravity
Upward falling-model arrow in still air	Air resistance
Brake pad contacting a moving rim	Friction at the contact
Hull moving through still water	Water resistance against its movement
Book resting on a table	Gravity with an upward support force

Crossword Puzzle

GRAVITY	Attractive force between masses
FRICTION	Force interaction at surfaces resisting relative sliding
FLUID	A substance that can flow
CANOPY	The parachute part whose area differs in the fictional test
RESISTANCE	Word used for drag through air or water
REPEAT	Do another trial under the stated conditions

LearningApps

Optional extra practice. Choose an appropriate activity with your teacher. If the external collection is unavailable, use the native activities and independent tasks in this course.

Cloze Text

Open Tasks

Easy

Name forces in a context: Match a released model, a brake pad touching a moving rim and a hull moving through water with gravity, friction and water resistance. Add the interacting objects or fluid rather than only a force name.
Draw a falling-model diagram: In still air draw a downward weight arrow and an upward air-resistance arrow on a vertically falling model. Explain that arrows show forces, not its travelled path, and that sizes have not been measured.
Explain support at rest: Explain why a book on a table still experiences gravity. Check: the table provides upward support; rest does not mean absence of forces.
Read repeated fall times: List A's three fictional times and B's three fictional times with seconds. Identify the same drop height and total mass as stated controls. Label the numbers supplied, not personally measured.

Standard

Calculate the fictional means: Calculate A's mean from 1.1, 1.2, 1.3 seconds and B's from 1.6, 1.8, 2.0 seconds. Compare them. Check: 1.2 seconds and 1.8 seconds, a 0.6-second difference over the same drop height.
Explain the NASA comparison: Explain why the hammer and feather reached the lunar surface together in the stated demonstration. Include matched release, negligible air resistance and the Moon's gravity. Do not claim identical forces or identical behaviour in Earth's air.
Plan a fair canopy comparison: Identify canopy area as changed variable and fall time as outcome. Specify matched total mass, material, general shape, line length, height, environment and release method, with repeats. Explain why larger material area could otherwise change total mass too.
Explain two resistance designs: Explain braking at a rim-pad contact and a streamlined model moving through water. Distinguish contacting-surface friction from fluid drag and identify a controlled condition for a shape comparison.

Advanced

Evaluate balanced forces in motion: For a model stated to be falling at steady speed in a simple vertical case, explain balanced upward drag and downward weight. Contrast this with the mistaken claim that balanced forces always mean stationary.
Repair a confounded experiment: A small light model is dropped indoors and a large heavier model outdoors from higher up. Explain why canopy area cannot be isolated, then redesign the comparison. Use only a teacher-approved low-level lightweight model plan, not high drops.
Limit a design recommendation: Recommend B for slower descent using the fictional means and state an untested feature such as stability. Explain why the record does not certify human safety or every wind condition. Retain a hypothetical tangled-line observation for investigation.
Create a forces evidence card: Combine the labelled force model, data summary, friction and water-resistance examples, NASA comparison and limitation. Explain a genuine revision and distinguish real investigation evidence from supplied data or a drawn plan.

Learning Check

Gravity and force arrows: Explain a released object's fall near Earth and label the simple still-air vertical-parachute model.
Interpret the Moon film: What does the matched hammer-and-feather release show, and what does it not show?
Friction and water resistance: Explain useful grip, braking and a water-resistance design feature.
Calculate and recommend: Calculate the mean fall times, identify variables and give a limited recommendation.
Model and evidence limits: Explain balanced forces at steady descent and one boundary on practical claims.

Worked checks and assessment guidance

Earth's gravitational pull acts towards Earth. After support is removed, it can accelerate the object downwards. Label weight down and air resistance up in the specified falling case. Gravity also acts before release and on supported objects; a real gliding canopy needs a more complete model.
With negligible air resistance, matched releases from rest at the same height have the same gravitational acceleration regardless of mass; the objects reach the lunar surface together within observational accuracy. The Moon has gravity. The film does not require identical forces or predict identical fall times for arbitrary objects in Earth's air or with different release conditions.
Friction resists relative sliding or its tendency at contacts and can supply useful grip. Pads contacting a moving rim can slow the wheel. Water drag acts against movement through the water in our still-water case; a streamlined shape can reduce it under suitable matched conditions. Different towing speed or depth limits a shape-only comparison.
A: sum 3.6 divided by 3 = 1.2 seconds. B: sum 5.4 divided by 3 = 1.8 seconds. B takes 0.6 seconds longer on average over the same height. Canopy area changes; total mass, material, shape, lines, height, environment and release are matched. Choose B for slower descent in these fictional trials, without claiming weaker gravity or proven stability/human safety.
In the stated simple steady-speed vertical case, drag balances weight, so balanced forces can accompany motion. The photograph does not measure force size, and a real gliding canopy can have other relevant effects. A drawn plan, NASA film and supplied fictional data show knowledge; actual practical evidence requires genuine observations and any real teacher check.

Learning evidence

Keep the force diagram, checked mean comparison, design recommendation and one limitation, with a friction example and a water-resistance example. Identify the objects or fluid involved and use correct arrow directions for the stated situation. Distinguish gravity from air resistance and avoid mass-only predictions. Mark supplied times as fictional and actual investigation records as genuine. A labelled diagram with a spoken explanation can show the same reasoning as extended writing. Do not infer personal parachute safety, bike maintenance competence or performed practical work from an online quiz.

OER and Wikipedia

Department for Education: science programmes of study, Year 5 Forces. The fictional canopy comparison, calculations, diagrams to create and activities are original editorial teaching material.

NASA: The Apollo 15 Hammer-Feather Drop, for David Scott's 1971 demonstration, matched release, essentially vacuum conditions and the observational result. NASA Glenn: Terminal Velocity provides additional scientific background on weight, drag, canopy area and steady descent. Its equations and rocket context are not required practical work in this course.

BBC Terrific Scientific: Forces, teacher guide gives further primary-school context for friction, air and water resistance, labelled forces and controlled comparisons. This is a historical teaching resource, not a current live-event invitation or a requirement to submit children's images.

Parachute photograph: Skydivers at the 2007 Bex airshow, Roy Egloff, CC BY-SA 4.0, used as supplied on Commons without additional modification. Actual gliding canopies are distinguished from the simple vertical classroom model.

Brake photograph: Rear hydraulic bicycle rim brake, Pittigrilli, CC BY-SA 4.0, used without modification. The image illustrates a component; it does not certify mechanical condition or provide maintenance instructions.

Film: Apollo 15 Hammer-Feather Drop, NASA Solar System, original publisher's 83-second film. Embedded through YouTube without downloading or republishing. The text gives an independent route explaining the observation and its limits.