Gravity, Weight, Friction, and Air and Water Resistance

Hold an apple still in your hand. You cannot see Earth pulling it, but the pull is there. Open your hand, and the apple falls.

A pull without touching

A force is a push or a pull. Gravity is a pulling force. Earth’s gravity pulls you, a desk, a raindrop, and a flying bird toward Earth. It works through air, water, and empty space. Earth does not need to touch the apple to pull it. That makes gravity a non-contact force.

Air resistance, water resistance, and friction are contact forces. They need air, water, or another surface touching the object.

What holds an object up?

An object is supported when something holds it up. Your hand gives the apple an upward force. A table pushes up on a book. A string pulls up on a hanging yo-yo. Gravity still acts on each object.

When a cup moves past a table’s edge, the table stops supporting it. Gravity pulls it downward, and it falls. The pull did not suddenly begin when the cup left the table.

Down points toward the center

Picture Earth as a ball with children standing around its edge. Draw an arrow from each child toward Earth’s center. Each arrow shows that child’s direction of “down.”

A child in the United States and a child in Australia both drop balls toward the ground. Their arrows point in different directions on the page. Both point inward. People on the far side of Earth stay on the ground because gravity pulls them toward the center too.

A ball thrown upward also feels this downward pull. It slows as it rises, pauses at the top, then falls. A ball thrown sideways follows a curved path toward the ground.

Build an explanation from evidence

A science argument connects three parts:

• Claim: “Earth’s gravity pulls objects toward its center.”
• Evidence: “Dropped balls fall toward the ground in Ohio and Australia. A weight hanging on a string points straight down.”
• Reasoning: “The ground is toward Earth’s center in both places. These observations fit a pull toward the center.”

Evidence is something you observe or measure. Reasoning explains how it supports your claim. Saying “gravity exists because things fall” needs more detail: name the direction of the fall and connect it to Earth’s shape.

Gravity reaches beyond the ground beneath your feet. It pulls between every pair of objects that have mass. Mass means how much matter, or stuff, an object contains.

What makes the pull stronger?

You and your pencil pull on each other. That pull is much too weak to notice. Earth has enormous mass, so its pull is easy to notice. More mass gives a stronger pull. Bringing two objects closer also strengthens their pull.

Earth’s gravity weakens as you travel farther away. It does not suddenly end at the edge of the air. The Sun pulls Earth across about 150 million kilometers of space, or about 93 million miles.

Keeping worlds in their paths

Earth’s gravity keeps the Moon traveling around Earth. The Sun’s gravity keeps Earth and the other planets traveling around the Sun. Gravity also holds Earth’s air and oceans close to the planet.

The Moon’s surface pull is about one sixth as strong as Earth’s. Astronauts can make large bouncing leaps there. At the top of Jupiter’s clouds, gravity pulls about two and a half times as hard as on Earth.

Astronauts float inside the International Space Station, about 400 kilometers above Earth. That is about 250 miles. Earth’s pull there is still nearly nine tenths of its strength at the ground. The astronauts and station fall around Earth together. Their floating does not mean gravity has vanished.

Testing ideas about falling

Long ago, Aristotle taught that heavier objects always fall faster. About 400 years ago, Galileo Galilei investigated falling. He timed balls rolling down gentle ramps. Slower rolling gave him more time to measure. He reasoned that heavy and light objects fall at the same rate when air does not get in the way.

Isaac Newton connected falling apples with the Moon’s path. He explained that the same force acts in both cases. His book describing gravity appeared in 1687. A famous apple story describes what got him thinking; experiments and observations support the science. The unit of force, the newton, carries his name.

In 1971, astronaut David Scott dropped a hammer and a falcon feather on the Moon. He released them together from the same height. With no air resistance, they landed together. On Earth, air slows the feather much more.

The test checked an idea with evidence. A famous person’s name cannot settle a science question. Measurements and observations let people test explanations and improve them.

A full pencil case pulls harder on your hand than an empty one. A force meter lets you measure that pull. You may also hear it called a spring scale or a newton meter.

The spring and the unit

Inside the meter, a spring stretches when something pulls its hook. A pointer moves along a numbered scale. A harder pull stretches the spring farther.

Forces are measured in newtons, written N. A small apple with a mass of about 100 grams has a weight of about 1 N on Earth. Write “3.5 N” for a force reading, including the unit.

Get ready to measure

1. Choose a meter with a range large enough for the object. A 10 N meter suits a shoe or pencil case. A 1 N meter shows small differences in very light objects more clearly.
2. Hold the meter in the position you will use. With nothing pulling its hook, check that it reads zero. Adjust it if needed. This is zeroing.
3. Hang the object from the hook. Wait for bouncing to stop.
4. Put your eyes level with the pointer. Read straight across to the scale.
5. Record the number and N.

A reading at the end of the scale means you need a stronger meter. Overloading can stretch the spring too far and spoil future readings.

Work out the small spaces

Suppose the numbered marks are 4 N and 5 N. There are five equal spaces between them. The whole gap is 1 N, so each space is:

Each small step adds 0.2 N. Count the spaces from 4 N: 4.2 N, 4.4 N, 4.6 N. A pointer three spaces past 4 N reads 4.6 N. Counting lines instead of spaces can give the wrong step size.

Hanging or pulling sideways

A pencil case hanging still gives a reading of 2.5 N. Its weight is 2.5 N: that is gravity’s pull on it.

You can also hook the meter to a shoe. Pull it level across a table at a slow, steady speed. Now you measure a sideways pull. The meter measures force in the direction you use it; it does not only measure hanging weights.

If an empty meter starts at 0.5 N, its readings will be 0.5 N too large. Zero it before the next measurement. Read a steady pointer, since a bouncing object or a jerky pull makes the reading jump.

A rock taken to the Moon still contains the same stuff. Yet lifting it feels easier there. Its mass stays the same, but its weight changes.

How much stuff, or how much pull?

Mass is the amount of matter in an object. Measure it in grams (g) or kilograms (kg), using a balance or a kitchen scale. One kilogram contains 1,000 grams.

Weight is the force of gravity pulling on that mass. Measure it in newtons (N), using a force meter. On Earth, its direction is toward Earth’s center.

Comparing objects on Earth

At the same place, an object with more mass has more weight. Two matching backpacks hold different amounts of books. If one has twice the mass, its weight is twice as large there.

A useful benchmark on Earth is that 1 kg weighs about 10 N, more exactly about 9.8 N. A 1 kg bag of flour has that weight. A 100 g apple weighs about 1 N. The kilograms name the mass; the newtons name the pull.

Taking the rock to the Moon

Follow the rock step by step:

1. On Earth, a force meter shows its weight as 6 N.
2. You carry the whole rock to the Moon. No matter has been removed, so its mass is unchanged.
3. The Moon pulls about one sixth as hard. The meter now shows about 1 N.

Far from planets, moons, and stars, the pull can be very weak. A toolbox there still has mass. It still takes effort to start it moving or stop it. Floating does not remove its matter.

Why everyday words can confuse you

Someone might say, “I weigh 35 kilograms,” or “I weigh 77 pounds.” Everyday talk uses “weight” this way. A bathroom scale measures how hard you press on it, then displays kilograms or pounds.

In a science answer, keep the quantities clear. Say, “The book has a mass of 2 kg.” When you report its weight, use newtons. An astronaut visiting the Moon has less weight, not less body matter.

You move air aside whenever you run. Air is made of tiny particles. As you push through it, the air pushes back. This force is air resistance, also called drag.

Which way does it push?

Air resistance acts against movement through the air. A falling leaf moves down, so air resistance pushes up. A cyclist rides forward through still air, so the air pushes backward. Gravity continues pulling downward in both cases.

Air is touching the moving object. Air resistance is a contact force, even though you cannot see the contact.

What makes the push bigger?

A larger front surface pushes more air aside. Hold a broad piece of cardboard facing the air, and it feels a bigger push than its thin edge does. Shape matters too. Air flows more smoothly around some shapes than others.

Speed also matters. You hardly notice air resistance when walking slowly. On a fast bike ride, air pushes much harder against your face. Faster movement through air gives greater air resistance.

Two sheets, two different falls

Take two matching sheets of paper. Crumple one into a tight ball. Keep the other flat. Release them together from the same height.

The paper ball reaches the floor first. Work through the explanation:

1. Both sheets contain the same amount of paper, so their masses match.
2. In the same room, their weights match too.
3. The flat sheet presents a larger surface to the air.
4. Air resistance slows the flat sheet more.

Crumpling changes the shape, not the amount of paper. You do not need a heavier sheet to make a faster fall.

Heavy does not always mean faster

A feather falls slowly through air because drag has a large effect on it. Without air resistance, a feather and hammer released together land together. On Earth, a tennis ball and baseball dropped from shoulder height land at almost the same time. Air resistance has only a small effect during that short fall.

When drag helps

Air resistance slows a parachute and the spinning fall of a maple seed. A slow fall gives wind more time to carry a seed away. Moving air can also push a sail and move a boat.

For runners, cars, and airplanes, drag makes forward travel harder. Racing cyclists crouch and wear tight, smooth clothing to reduce the surface and shape pushing against air. Their legs then have less air resistance to work against.

A parachute has a wide canopy, usually made of light cloth. Lines connect it to a load, such as a person or a supply box. Opening the canopy changes how much air the parachute pushes aside.

Follow the forces

Before opening, a folded canopy presents a small surface. Once open, it pushes against much more air. The air pushes upward, slowing the falling parachute and its load.

Gravity still pulls down. Opening the parachute does not switch gravity off or reduce the load’s weight. The lines carry an upward pull from the canopy to the load. The load can keep moving down while an upward force slows its fall.

In a drawing, show gravity downward from the load. Show air resistance upward from the canopy. The open canopy needs a longer air-resistance arrow than the folded canopy at the same speed.

What changes a parachute’s fall?

For example, imagine two matching loads under matching cloth canopies. Only one canopy opens fully. Its wide surface pushes against more air, so it falls more slowly. The partly opened canopy has not changed the load’s mass.

A skydiver keeps moving down

A skydiver speeds up after jumping. With their belly facing the ground and limbs spread out, the speed can reach about 120 miles per hour, or 190 kilometers per hour. Opening the parachute greatly increases air resistance and slows the fall for landing.

In a video, a skydiver opening a parachute may seem to shoot upward. The camera person is still falling quickly. The person under the parachute falls much more slowly, so the gap changes. The parachute has not made them fly back up.

Similar ideas around you

A dandelion seed hangs below a fluffy tuft that catches air. A maple seed spins beneath its wing. Both fall slowly enough for the wind to carry them. A flying squirrel spreads skin between its legs to glide.

Parachutes also work sideways. Drag-racing cars can open one behind them to slow down. The Space Shuttle used a parachute after landing. In each case, the parachute’s drag acts against movement through air.

You want to find out whether a bigger canopy slows a parachute. A fair test lets you connect the result to the thing you changed.

Plan what changes and what stays

Your independent variable is what you change: the canopy size. Your dependent variable is what you measure: falling time. Controlled variables are the features you keep the same.

Make square canopies with sides of 10 cm, 20 cm, and 30 cm. Use the same material, load mass, string number, and string length. Release each from 2 meters using the same method. Have an adult help with high releases; keep feet on the floor and the landing area clear.

A prediction includes a reason: “The 30 cm canopy will take longest to land because it pushes against more air.”

Repeat and record

Time each size at least three times. Small changes in release and stopwatch reaction time can change readings. Repeats show how closely the results agree.

Here are example results:

The average, or mean, adds the readings and divides by their number. For the largest canopy:

Its average falling time is 2.7 s. Put one bar per canopy size on a bar chart to compare these averages.

Check an odd result

Suppose three times are 1.8 s, 2.0 s, and 0.9 s. The last differs a lot. Check what happened and repeat the drop. If the canopy failed to open, record that problem and why you left that time out. Do not remove a time just because it does not fit your prediction.

Explain the pattern

A conclusion uses both numbers and science: “The 30 cm canopy took 2.7 s on average. The 10 cm canopy took 1.2 s. The bigger canopy pushed against more air, increasing air resistance and slowing the fall.”

Canopy area grows faster than side length. A 20 cm square has an area of 400 square centimeters. A 30 cm square has an area of 900 square centimeters, more than twice as much.

Changing the load and canopy together would spoil this test. You could not tell which change caused the result. One drop alone can still be fair, but it gives little information about consistency. For a new question about load mass, change mass and keep canopy size fixed.

Walking through a pool feels harder than walking through air. Your legs must move water aside. The water pushes back against them.

A force from flowing water

Water resistance acts against an object moving through water. A swimmer, a fish, a sinking stone, and a boat on the surface all feel it. Only part of a boat needs to be in the water for water resistance to act.

Air and water are fluids: substances that flow. Resistance from either fluid is called drag. Both need contact with the object.

For the same object moving at the same speed, water resistance is much greater than air resistance. Water is much denser. A bucket of water has about 800 times the mass of the same bucket filled with air. There is much more matter to move aside.

Turn your hand to change the push

Move an open hand through water in a basin. First, lead with your wide palm. Then turn your hand and lead with its thin edge. Keep the speed about the same.

The wide palm feels more resistance. It pushes a bigger front against the water. The thin edge moves less water aside, so it feels less resistance. Moving either shape faster increases the resistance.

This explains why entering water with a wide, flat body gives a hard slap. A diver entering hands first presents a narrow front and moves through more easily.

When water resistance helps

A canoe paddle has a broad blade. You push water backward with it, and the water pushes the paddle forward. That push moves the canoe. Swim fins also provide a wide surface to push against water.

After you stop pushing a toy boat, it glides for a while. Water resistance slows it. A diver also slows after entering the water.

When it gets in the way

A ship needs energy to keep moving against drag. So does a swimmer. Smooth swim caps and close-fitting swimsuits reduce the resistance. Boat builders shape hulls to let water flow around them more easily.

Water resistance is about movement through water. It is not what holds a boat up. A boat can float still on calm water without drag slowing it. Keeping that distinction clear helps you explain what a moving boat’s resistance arrow shows.

A fish moves its body through water. A train moves through air. Both benefit from a shape that lets the fluid flow around it smoothly.

What streamlined means

A streamlined shape reduces air or water resistance. It is usually smooth, with a narrow, rounded or pointed front. Its body narrows toward the back. This narrowing is called tapering and helps the fluid close in smoothly behind it.

Streamlined does not mean light or tiny. A blue whale is huge and streamlined. A small flat square of cardboard can catch a lot of air for its size.

Shapes in nature and design

Dolphins, sharks, tuna, and swimming penguins have streamlined bodies. A diving falcon pulls its wings close. Engineers use smooth shapes on submarines, airplanes, racing cars, and high-speed trains.

Japan’s 500 Series bullet train began carrying passengers in 1997. Its long nose was shaped like a kingfisher’s beak. That design reduced noise as it entered tunnels and helped it use less electricity.

You can change your shape too. A swimmer pushes off with arms stretched above the head. A racing cyclist crouches low. Their bodies present a narrower front. Their mass has not changed just because they changed position.

Compare clay shapes fairly

Make a ball, cube, cone, flat square, and teardrop from equal masses of modeling clay. Use the same tall container of water. Release each from the same height in the water and time its trip to the bottom. Repeat each shape three times. Keep the cone pointed down and record how each shape faces the water.

These are example class results:

The teardrop arrived first. Its shape let water flow around it easily. The flat square took longest because its broad face pushed against more water. Equal clay masses help you connect the difference to shape.

Test a boat’s front

Build boats from the same material and with the same mass. Give them differently shaped fronts. Pull each the same distance using the same pulling force, then compare times. You could use the same small hanging weight to pull a string over the tray’s edge. Keep the route and water the same.

Reducing resistance is not every designer’s aim. A paddle needs a broad face. A parachute needs to catch air. Airplanes can raise panels after landing to increase drag and slow down.

Rub your hands together. They soon feel warm. Friction acts where the two surfaces touch and slide past each other.

Against sliding

Friction is a force between touching surfaces. It acts against their sliding, or their attempt to slide. A box sliding right feels friction to the left along the floor.

Friction can also act before sliding starts. Put a book on a gently tilted board. It stays in place because friction stops it from sliding down. Raise the board far enough and the book begins to slide.

Tiny rough places

Even surfaces that look smooth have tiny bumps and dips. You can see them under a microscope. These catch as surfaces press together, contributing to friction.

Rougher surfaces usually produce more friction in classroom comparisons. A sled slides easily across ice but struggles on grass. A toy car stops sooner on thick carpet than on a smooth wooden floor.

The materials matter as well as the roughness. Rubber can grip a smooth gym floor well. A smooth surface still has friction; smooth does not mean friction-free.

Pressing harder

Adding books to a box makes it press harder on the floor. It becomes harder to slide. Pressing two surfaces together harder increases friction between them.

Brake pads use this effect. Squeeze a bike’s brake lever harder and the pads press harder against the rim or disc. The greater friction slows the wheel more strongly.

Friction can push you forward

When you walk, your planted shoe pushes backward on the ground. Friction from the ground pushes the shoe forward. That helps move you ahead. On slippery ice, the shoe can slide backward because there is too little grip.

So “friction always points backward” is not a useful rule. First ask which way the surfaces are sliding or trying to slide. Friction acts against that sliding. It can stop a foot from slipping backward while pushing the walker forward.

Warmth from rubbing

Rubbing makes surfaces warm. Your hands heat up when you rub them quickly. Bike and car brakes also warm up during use. They use friction to slow moving parts, and those parts can get hot.

Friction is useful in some places and troublesome in others. Think about what a surface needs to do before choosing how to change it.

Where you need grip

Shoes need friction so you can walk and run. Tires need grip to start, turn, and stop. Your fingers grip a pencil, while friction between its tip and paper leaves a mark. Friction also helps a knot stay tied.

Bicycle brakes press pads against a wheel rim or a metal disc. Their friction slows the wheel. With rim brakes, wet rims make the pads slip more easily. The bike then needs more time and distance to stop.

Ways to increase friction

Rubber soles and handlebar grips help surfaces hold. Sand or grit on ice makes a path less slippery. Chalk helps a gymnast’s hands grip. Pressing surfaces together harder also increases friction.

Tread is a pattern of grooves and ridges on tires or soles. It matters especially on wet or soft ground. Grooves let water and mud move out, helping rubber reach the ground. Deep tractor-tire tread digs into muddy soil.

Where rubbing causes problems

Shoe soles, tires, and brake pads become thinner as friction wears them away. Moving machine parts can get hot. A stiff drawer needs a harder pull because friction resists its sliding.

A lubricant, such as oil or grease, forms a layer between moving parts. It reduces their friction. Oil helps a bicycle chain move smoothly. Wax helps skis slide, and water helps on a water slide.

Wheels, rollers, and ball bearings reduce sliding by allowing parts to roll. A box can be easier to move on rollers than dragged across a floor.

Match the change to the job

Ice skates have smooth, narrow blades for gliding. Winter boots can have deep tread or spikes for grip. Both designs suit their purpose. Removing friction everywhere would make walking, holding objects, and braking difficult.

Feeling a surface gives you a clue about friction. A measurement lets you compare it more clearly. You can test the pull needed to keep a shoe sliding or the distance a toy car rolls.

Use a force meter

Ask, “Which surface gives this shoe the most friction?” Hook a force meter to the shoe. Pull it slowly at a steady speed with the meter level. Read it while the shoe is already sliding steadily.

Change only the floor surface. Keep the same shoe, its contents, the meter, the pull direction, and speed. A tilted pull partly lifts the shoe and changes how hard it presses down. A jerk makes the reading jump. The larger starting reading is different from the steady sliding reading you are comparing.

Here are example results:

The biggest steady pull was on sandpaper. For this shoe, it gave the most friction. Tile gave the least. Repeat each measurement three times and find the average. Small differences in your pulling can change the readings.

Use a rolling car

Place a ramp at a fixed height. Release the same car from the same line without pushing. Let it roll onto each floor surface. Measure the distance from the ramp’s end to where it stops.

Example distances are 45 cm on carpet, 120 cm on wood, and 160 cm on vinyl. Here a shorter distance shows more slowing from friction. This is the opposite reading pattern from the force-meter test, where a larger force showed more friction.

Keep the ramp, release line, car, and measuring method the same. Repeat each surface. A push on one run would change the car’s starting motion and spoil the comparison.

Use a tilting board

Place a block on a board covered with a test material. Raise one end slowly. Measure its height when the block begins sliding. Keep the board and block the same for each material. A higher starting height means the surface held the block more strongly before it slipped.

Use your result

A conclusion names a measurement and explains it: “Sandpaper needed 4.5 N, while tile needed 1.0 N. The sandpaper caught against the sole more strongly. A grippy strip could help on a slippery step.”

You can also test pressing harder. Keep the surface fixed and add marbles to the same shoe. Change the mass you add, then measure the steady pull. The shoe presses harder, increasing friction.

A force is invisible, but an arrow can show its direction. A force diagram is a drawing of an object with arrows for the forces acting on it.

Give each arrow a job

Start each arrow on the object. Point it in the direction of the force. Add a label such as “gravity,” “pull,” or “friction.” Include the size in newtons when you know it.

A longer arrow means a bigger force. A 4 N arrow should be twice as long as a 2 N arrow. Arrow length shows force size, not how far the object travels.

Look for what is acting

Gravity points down, toward Earth’s center. A supporting table or floor pushes upward on an object. Air resistance acts against movement through air. Water resistance acts against movement through water. For a sliding object, friction points along the surface against the slide.

A book resting on a table still needs a downward gravity arrow. It also needs an upward arrow labeled “table pushes up.” Being still does not mean there are no forces.

Draw a sled step by step

A rope pulls a sled right across snow.

1. Draw a simple sled.
2. From the sled, draw a downward arrow labeled “gravity.”
3. Draw an upward arrow labeled “snow pushes up.”
4. Draw an arrow right along the rope labeled “pull.”
5. Draw an arrow left along the snow labeled “friction.”

Keep the arrows touching the sled. An arrow floating nearby does not clearly show which object feels the force. The labels tell you what each push or pull comes from.

Moving direction is not force direction

A falling ball moves down, but its air-resistance arrow points up. Draw gravity down too. These are different forces on the same object.

A thrown ball has no continuing “throw force” after it leaves the hand. The hand is no longer touching it. While it flies through air, gravity and air resistance act on it. When the ball rises, air resistance points down; when it falls, air resistance points up.

For friction, check the surfaces. On a sliding sled it acts against the slide. On a walking person’s planted foot it can point forward, keeping the shoe from slipping backward.

A kick starts a still soccer ball moving. A second kick can turn it. Forces can change both an object’s speed and its direction.

What can a force change?

A force can start motion, speed something up, slow it down, stop it, or turn it. Forces can change shape too, as when your hands squash modeling clay.

For the same object in matching conditions, a bigger force makes a bigger change. A hard kick changes a ball’s motion more than a gentle tap. Pressing brake pads harder slows the same wheel more strongly.

Follow an upward throw

Throw a ball straight up and let it go.

1. On the way up, gravity pulls down. The ball slows.
2. At the top, the ball stops for a moment. Gravity still pulls down.
3. On the way down, gravity makes it speed up.

Air resistance also acts during the flight. It points down while the ball rises and up while the ball falls. Its direction changes with the motion through air. Gravity’s direction stays down.

A ball dropped from higher up has farther to speed up before hitting the ground. In an ordinary short drop, it hits harder than the same ball dropped from lower down.

Follow a bike downhill

Gravity pulls on you and the bike as you travel downhill. You speed up. As you go faster, air resistance grows and pushes against your motion.

Squeezing the brakes presses pads against the rim or disc. Friction slows the wheels. Grip between tires and road helps slow the whole bike. The wheel’s rubbing and the tire’s grip have different jobs.

Why moving things stop

A toy car can roll after your hand stops pushing. On carpet it soon stops. On a smooth floor it travels farther because less friction slows it.

The car has not “used up” a supply of force. Forces are pushes and pulls acting on it now.

A toy boat glides after a push, then water resistance slows it. A hockey puck slides a long way on ice because friction is small. In space, a probe can travel for years without running its engines because there is so little resistance.