Year 8 · Springboard Physics · Forces & Motion

Forces & Motion, but bitesize.

A holiday revision booklet — ten short topics, split into Motion and Forces.

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Take it one topic at a time. There are ten topics, so don't try to do them all at once. Each one is short — about 10 minutes. Do one or two a day.

Part 1 — Motion

How fast, how far, in which direction.

Topic 01 · Motion

How fast is fast?

By the end of this topic you'll know what speed is, how to calculate it, and the units we use.

Part 1Speed = how far in how long

Speed tells you how far something travels in a certain amount of time. A car going 30 metres every second is faster than one going 10 metres every second.

The standard unit of speed in science is metres per second, written as m/s. You'll also see miles per hour (mph) on car speedometers, and kilometres per hour (km/h) on road signs in many countries — but in physics class, always use m/s unless told otherwise.

Keywords for Part 1

Speed
How far something travels in a given time.
Standard unit
Metres per second (m/s).
Quick check

What is the standard unit of speed in science?

  • Amiles per hour (mph)
  • Bmetres per second (m/s)
  • Ckilometres per hour (km/h)
  • Dseconds per metre (s/m)
Show answer
B — metres per second (m/s). mph and km/h are common in everyday life, but in physics we use m/s.

Part 2The speed formula

To work out the speed of something, you need to know two things: how far it went (distance), and how long it took (time). Then:

Speed = distance ÷ time

Worked example

A lorry travels 48 m in 12 seconds. What's its speed?

Step 1. Write the formula: Speed = distance ÷ time

Step 2. Put the numbers in: Speed = 48 ÷ 12

Step 3. Calculate: Speed = 4 m/s

⚠ Watch out — units

The formula only works if distance is in metres and time is in seconds. If you're given kilometres, multiply by 1000 first (e.g. 2 km = 2000 m). If you're given minutes, multiply by 60 (e.g. 2 minutes = 120 seconds).

Quick check

A bird flies 60 m in 12 seconds. What's its speed?

  • A720 m/s
  • B72 m/s
  • C5 m/s
  • D0.2 m/s
Show answer
C — 5 m/s. Speed = distance ÷ time = 60 ÷ 12 = 5 m/s. (If you got A, you multiplied instead of divided. If you got D, you divided the wrong way round — time ÷ distance.)

Test yourself

6 questions · click to reveal each answer

  1. Write the formula for speed.
    Speed = distance ÷ time (or s = d ÷ t).
  2. What is the standard unit of distance?
    Metres (m).
  3. A student walks 10 m in 5 seconds. What is their speed?
    2 m/s. Speed = 10 ÷ 5 = 2 m/s.
  4. A car travels 100 m in 4 seconds. Calculate its speed.
    25 m/s. Speed = 100 ÷ 4 = 25 m/s.
  5. Convert 3 km into metres.
    3000 m. 1 km = 1000 m, so multiply by 1000.
  6. A snail travels 3 m in 2 minutes. Calculate its speed in m/s.
    0.025 m/s. First convert 2 minutes to seconds: 2 × 60 = 120 s. Then Speed = 3 ÷ 120 = 0.025 m/s. (Easy mistake: forgetting to convert minutes to seconds — would give the wrong answer.)
Topic 02 · Motion

Drawing a journey

Distance-time graphs show how far something has travelled at every moment. You'll learn how to read one and how to find the speed from it.

Part 1The two axes

A distance-time graph has distance on the y-axis (the vertical one) and time on the x-axis (the horizontal one).

Why this way round? Because time keeps moving forward no matter what — it makes sense to put it along the bottom. Distance is what we're tracking against time.

⚠ Watch out

Don't get the axes mixed up. Distance up. Time across. If you swap them, every reading you take from the graph will be wrong.

Quick check

On a distance-time graph, what goes on the y-axis?

  • ADistance (m)
  • BTime (s)
  • CSpeed (m/s)
  • DMass (kg)
Show answer
A — Distance (m). The y-axis is the vertical one. Distance up, time across.

Part 2What the line tells you

The shape of the line tells you what's happening:

Reading the line

Diagonal line going up
The object is moving at a constant speed. Distance keeps increasing as time passes.
Steeper diagonal
A faster speed — covering more distance in the same time.
Horizontal (flat) line
The object is stationary (not moving). Time passes but distance doesn't change.
Quick check

On a distance-time graph, the line is flat (horizontal). What's happening?

  • AThe object is moving fast
  • BThe object is moving slowly
  • CThe object is stationary (not moving)
  • DThe object is going backwards
Show answer
C — Stationary. Time keeps going (the line moves rightwards), but distance stays the same (the line stays at the same height). So the object is not moving.

Part 3Finding speed from the graph

The steepness of a diagonal line is its gradient. The gradient gives you the speed.

Speed = change in distance ÷ change in time

Worked example

A diagonal line goes from (0 s, 0 m) up to (8 s, 24 m).

Change in distance: 24 − 0 = 24 m

Change in time: 8 − 0 = 8 s

Speed: 24 ÷ 8 = 3 m/s

Test yourself

6 questions · click to reveal each answer

  1. What goes on the x-axis of a distance-time graph?
    Time (in seconds). Always.
  2. What does a horizontal (flat) line on a distance-time graph mean?
    The object is stationary — not moving. Time passes but distance doesn't change.
  3. What does a diagonal line going up mean?
    The object is moving at a constant speed. The steeper the line, the faster the object.
  4. Two diagonal lines: A is steep, B is shallow. Which represents the faster object?
    A. A steeper gradient means more distance covered in the same time — that's faster.
  5. A line goes from (0 s, 0 m) to (4 s, 8 m). What is the speed?
    2 m/s. Speed = 8 ÷ 4 = 2 m/s.
  6. A distance-time graph shows a diagonal line from 0–10 s, then a flat line from 10–20 s. Describe the journey.
    The object moved at a constant speed for 10 seconds, then stopped (was stationary) for the next 10 seconds.
Topic 03 · Motion

How fast does it look from where you are?

Speed depends on where you're watching from. This is called relative motion — and you'll learn the two simple rules.

Part 1It depends on the observer

Imagine you're on a train going 80 km/h. The person sitting next to you isn't moving — to you. But to someone standing on a platform watching your train fly past, that person is moving 80 km/h. Same person, two different speeds. That's relative motion.

Relative motion just means: how fast something is moving compared with the thing watching it.

Key idea

Relative motion
How fast one object moves compared with another. Different observers can see different speeds.

Part 2The two rules

To work out the relative speed of two objects, there are just two rules to remember:

The two rules

Same direction → SUBTRACT
If two objects move the same way, you subtract their speeds. Example: a 12 m/s car and a 10 m/s car going the same direction — the relative speed between them is 12 − 10 = 2 m/s.
Opposite directions → ADD
If two objects move toward each other (or away from each other in opposite directions), you add their speeds. Example: two cars moving toward each other at 10 m/s and 5 m/s — the relative speed is 10 + 5 = 15 m/s.

⚠ Why this works

Two trains side by side, both going 100 km/h the same way: from one train's window, the other looks still. 100 − 100 = 0. That's why "same direction → subtract" is the rule — they're cancelling each other out.

Quick check

Car A moves at 20 m/s. Car B moves at 15 m/s in the same direction. What's their relative speed?

  • A35 m/s
  • B5 m/s
  • C0 m/s
  • D300 m/s
Show answer
B — 5 m/s. Same direction, so subtract: 20 − 15 = 5 m/s. (A is wrong — that's the rule for opposite directions.)

Test yourself

6 questions · click to reveal each answer

  1. What is relative motion?
    How fast one object is moving compared with another. Different observers can see different speeds for the same object.
  2. Two trains move in the SAME direction at the same speed. How does each train's passenger see the other train?
    Stationary. Same direction at the same speed means relative speed is zero (subtract: 80 − 80 = 0).
  3. If two objects move in OPPOSITE directions, do you add or subtract their speeds?
    Add. Opposite → add. Same → subtract.
  4. Two cars approach each other. Car A travels at 12 m/s, Car B at 8 m/s. What is the relative speed between them?
    20 m/s. Opposite directions, so add: 12 + 8 = 20 m/s.
  5. A bus travels at 30 m/s. A motorbike behind it goes at 35 m/s in the same direction. From the bus driver's view, how fast does the motorbike approach?
    5 m/s. Same direction, so subtract: 35 − 30 = 5 m/s.
  6. A boy on a moving train rolls a ball forward at 2 m/s. The train moves at 20 m/s. How fast is the ball going relative to a person standing on the platform?
    22 m/s. Same direction, but here we ADD because we want the speed relative to the platform (a stationary observer). The ball's speed relative to the train (2) plus the train's speed relative to the platform (20) = 22 m/s.
Topic 04 · Motion

Speeds you should just know

Three numbers to memorise. They come up in MCQs and short-answer questions.

Part 1Walking, running, cycling

You need to remember roughly how fast a typical person walks, runs, and cycles. The numbers form a clean pattern, which makes them easier to memorise.

Memorise these three

Walking
1.5 m/s — the slowest one.
Running
3 m/s — twice walking speed.
Cycling
6 m/s — twice running speed (or four times walking speed).

The pattern that helps

Each one is roughly double the one before. Walking → ×2 → Running → ×2 → Cycling. So if you only remember "1.5, then double it twice", you've got all three.

Quick check

What's the typical speed of a runner?

  • A1.5 m/s
  • B3 m/s
  • C6 m/s
  • D10 m/s
Show answer
B — 3 m/s. 1.5 is walking. 6 is cycling. 10 is faster than most casual cyclists.

Test yourself

6 questions · click to reveal each answer

  1. What is the typical speed of walking?
    1.5 m/s.
  2. What is the typical speed of cycling?
    6 m/s.
  3. How many times faster is cycling than walking?
    4 times faster. 6 ÷ 1.5 = 4.
  4. How far would you walk in 10 seconds at typical walking speed?
    15 m. Distance = speed × time = 1.5 × 10 = 15 m.
  5. How far would a runner go in 30 seconds at a typical pace?
    90 m. Distance = 3 × 30 = 90 m.
  6. A cyclist takes 60 seconds to cross a park at typical speed. How long is the park?
    360 m. Distance = 6 × 60 = 360 m.
Part 2 — Forces

What pushes, what pulls, and what happens next.

Topic 05 · Forces

A push or a pull

You'll learn what a force is, how we measure it, and how to draw one.

Part 1What forces actually are

A force is a push or a pull that acts on an object. When you kick a ball, push a door, lift a bag — those are all forces.

Forces try to change the motion of an object. They can speed something up, slow it down, change the direction it's going in, or even change its shape.

Keywords for Part 1

Force
A push or a pull on an object.
Newton (N)
The unit we measure forces in. 1 N is roughly the weight of a small apple.
Newton meter
The piece of equipment used to measure a force.
Quick check

Which statement best describes a force?

  • AEnergy moving through an object
  • BThe speed an object moves
  • CA push or a pull on an object
  • DThe mass of an object
Show answer
C — A push or a pull on an object. Simple as that. Anything that pushes or pulls is a force.

Part 2Drawing forces — the arrow rules

We draw forces as arrows. Two things matter about the arrow:

• The direction the arrow points = the direction the force is acting in.
• The length of the arrow = the size of the force. A bigger force gets a longer arrow.

⚠ Length, not thickness

It's the length of the arrow that shows the size of the force, not how thick or fat the arrow is. A long thin arrow shows a bigger force than a short fat arrow.

Quick check

On a force arrow, what does the length show?

  • AThe size of the force
  • BThe direction of the force
  • CThe speed of the object
  • DThe mass of the object
Show answer
A — The size of the force. Direction is shown by which way the arrow points. Length is the size.

Test yourself

6 questions · click to reveal each answer

  1. What is a force?
    A push or a pull on an object.
  2. What is the unit of force?
    The Newton (N).
  3. What piece of equipment is used to measure a force?
    A newton meter.
  4. How do we show the direction of a force on a diagram?
    With an arrow — the arrow points in the direction the force is acting.
  5. How do we show the size of a force on a diagram?
    By the length of the arrow. Bigger force = longer arrow. (Not thickness.)
  6. A student draws a thick short arrow next to a thin long one. Which represents a bigger force?
    The thin long one. Length is what counts — thickness doesn't matter.
Topic 06 · Forces

The forces you need to know by name

There are about ten forces that come up in KS3 physics. Each one has a specific cause and direction.

Part 1The full list

Here are the forces you need to be able to name and recognise. Don't try to memorise them all in one go — read through, then come back later.

The ten forces

Weight (gravity)
The pull of gravity on an object. Always acts downwards. A book sitting on a table has weight pulling it down.
Normal contact force
The push from a surface on an object resting on it. Always acts away from the surface. The table pushes the book upwards.
Thrust
A driving force that pushes something forwards. From an engine, a swimmer's arms, a rocket.
Friction
A force that opposes motion when two surfaces rub together. Always acts against the direction of movement.
Air resistance
Friction with the air. Acts against the direction of motion. Bigger when you go faster.
Water resistance
Same as air resistance, but in water. Acts against motion through water.
Upthrust
The upward push from a fluid (water or air) on an object floating in it. This is why boats float.
Lift
The upward force on an aeroplane wing as air flows over it. Keeps planes in the air.
Tension
The pulling force in a stretched rope, string, or cable.
Magnetism
The force between magnets and magnetic materials. Can pull (attract) or push (repel).

Part 2Common scenarios — which forces act?

You won't be asked to list all ten in one go. You'll usually be shown a picture (a book on a table, a plane, a swimmer) and asked to name the forces acting. Here are the common ones:

Three classic scenarios

Book resting on a table: weight (down) + normal contact force (up). Balanced.

Plane flying: thrust (forwards) + air resistance (backwards) + lift (up) + weight (down). Four forces.

Boat on water: thrust (forwards) + water resistance (backwards) + upthrust (up) + weight (down). Four forces.

Quick check

A swimmer is pushing forward through water. Which force is acting against their motion?

  • ALift
  • BUpthrust
  • CWater resistance
  • DWeight
Show answer
C — Water resistance. It always acts against the direction of motion through water. Upthrust pushes up, not against forward motion.

Test yourself

6 questions · click to reveal each answer

  1. What force pulls objects towards Earth?
    Weight (or gravity). Always acts downwards.
  2. What force keeps a boat floating on water?
    Upthrust. The upward push from the water.
  3. What force keeps a plane up in the air?
    Lift. Generated by air flowing over the wings.
  4. A car is rolling along but slows down and stops. Which force caused it to stop?
    Friction (and air resistance). Both act against the direction of motion.
  5. What force is in a rope when someone pulls it tight?
    Tension. The pulling force in a stretched rope.
  6. A book sits on a table. Name the two forces acting on the book.
    Weight (pulling it down) and the normal contact force (the table pushing it up).
Topic 07 · Forces

Touch or no touch?

A simple way to sort the ten forces into two groups, depending on whether the objects need to touch.

Part 1The test

Here's the rule: do the objects have to be touching for the force to act?

If yes → it's a contact force. If no → it's a non-contact force.

The two groups

Contact forces (need to touch)
Friction · Normal contact force · Thrust · Tension · Air resistance · Water resistance · Upthrust · Lift
Non-contact forces (no touching needed)
Gravity (weight) · Magnetism · Electrostatic force

⚠ Common mix-up — air resistance

Air resistance is a contact force. It looks like nothing's touching, but actually air particles are colliding with the moving object. Particles touching = contact.

Quick check

Which of these is a non-contact force?

  • AFriction
  • BMagnetism
  • CTension
  • DAir resistance
Show answer
B — Magnetism. Two magnets can pull or push each other without touching. The other three all need contact (yes, including air resistance — air particles count as contact).

Test yourself

6 questions · click to reveal each answer

  1. What does "non-contact force" mean?
    A force that acts without the objects needing to touch.
  2. List all the non-contact forces.
    Gravity (weight), magnetism, electrostatic force. Three of them.
  3. Is friction a contact or non-contact force?
    Contact. Two surfaces have to be touching for friction to act.
  4. Is gravity a contact or non-contact force?
    Non-contact. The Earth pulls on you whether or not you're touching the ground.
  5. A skydiver is falling through the air. Which forces are acting? Which are contact, which are non-contact?
    Weight (non-contact) pulling down, and air resistance (contact) pushing up.
  6. A student says: "Air resistance must be a non-contact force, because air isn't really touching me." Are they right?
    No. Air is made of particles. As you move through air, particles collide with you — that's contact. Air resistance is a contact force.
Topic 08 · Forces

Forces always come in pairs

Every force has a partner — equal in size, opposite in direction. This is one of the trickiest ideas in KS3, but the rule itself is simple.

Part 1The rule

Whenever object A pushes (or pulls) object B, object B pushes (or pulls) back on A. The two forces are:

• Equal in size
• Opposite in direction
• Acting on two different objects.

This is called an interaction pair. You can't have a force on its own — every force is one half of a pair.

Examples to picture

You push a wall
You push the wall. The wall pushes you back with the same force. If you push harder, it pushes back harder. That's why a wall hurts your hand if you slam it.
Apple falling from tree
Earth pulls the apple down (weight). The apple pulls the Earth up with an equal and opposite force. (Yes, really — but the Earth is so massive that it doesn't visibly move.)
Two skaters push off each other
Skater A pushes Skater B to the right. Skater B pushes Skater A to the left. Equal forces, opposite directions.

⚠ The most confusing bit

Interaction pairs do not cancel each other out. They look like balanced forces, but they're not — because they act on different objects. To have balanced forces, both forces have to act on the same object.

Quick check

A lorry crashes into a small car. The lorry exerts a force of 50,000 N on the car. What force does the car exert on the lorry?

  • ALess than 50,000 N (the car is smaller)
  • BMore than 50,000 N
  • CExactly 50,000 N, opposite direction
  • DZero — only the lorry is pushing
Show answer
C — Exactly 50,000 N, opposite direction. It feels weird because the car is much smaller, but the forces in an interaction pair are always equal. The reason the car comes off worse is that the same force does more damage to a lighter object.

Part 2How to describe an interaction pair

In exam questions you'll be asked to describe an interaction pair. Use this template:

The schema

"There is a [force] on [object 1] from [object 2] [direction]."

"There is a [force] on [object 2] from [object 1] [opposite direction]."

Worked example

A penny is falling towards the floor. Describe the interaction pair between the penny and the Earth.

"There is a gravitational force on the penny from the Earth pulling it downwards."

"There is a gravitational force on the Earth from the penny pulling it upwards."

Test yourself

6 questions · click to reveal each answer

  1. How many forces are there in an interaction?
    Two. Always exactly two — that's why we call them pairs.
  2. How many objects are involved in an interaction?
    Two. Forces never act between an object and itself.
  3. How do the sizes of the two forces in an interaction pair compare?
    Equal in size, opposite in direction. Always.
  4. "If I kick a football, the football kicks me back with the same force." True or false?
    True. Equal and opposite — that's the interaction pair rule.
  5. Why don't the two forces in an interaction pair cancel each other out?
    Because they act on different objects. For forces to cancel (be balanced), they have to act on the same object.
  6. A book sits on a table. The book pushes down on the table. What does the table do?
    The table pushes up on the book with an equal force. (This is the normal contact force, and it's the partner of the book's weight pushing down on the table.)
Topic 09 · Forces

Adding up the arrows

When more than one force acts on an object, you need to work out the overall — or "resultant" — force. This topic teaches you how.

Part 1Balanced forces

If two forces are equal in size and opposite in direction, they cancel each other out. We say the forces are balanced.

When forces are balanced, the resultant force is zero. There's no overall push or pull.

← 10 N obj 10 N →

RESULTANT = 0 N · BALANCED

A book sitting on a table is the classic example. Its weight pulls it down. The table pushes back up with the same force. Balanced. Resultant = 0. The book doesn't move.

Quick check

Two forces act on a box: 5 N to the right, and 5 N to the left. What's the resultant force?

  • A10 N to the right
  • B10 N to the left
  • C0 N — the forces are balanced
  • D5 N up
Show answer
C — 0 N, balanced. Equal forces in opposite directions cancel out.

Part 2Calculating the resultant force

When forces aren't balanced, we work out the resultant force — a single force that has the same overall effect as all the forces combined.

Two simple rules

Same direction → ADD
If two forces both point the same way, add their sizes. The resultant points in that same direction.
Opposite directions → SUBTRACT
Find the difference. The resultant points in the direction of the bigger force.

Worked example 1 — Opposite directions

← 5 N obj 15 N →

RESULTANT = 10 N TO THE RIGHT

Subtract: 15 − 5 = 10 N. The bigger force is to the right, so the resultant points right.

Quick check

A box has 20 N pushing right, and 8 N pushing left. What's the resultant force?

  • A28 N to the right
  • B12 N to the right
  • C12 N to the left
  • D0 N
Show answer
B — 12 N to the right. Opposite directions, so subtract: 20 − 8 = 12. The bigger force was to the right, so the resultant points right.

Part 3Try it yourself

Now you know the rules — try playing with them. Drag arrows of different sizes onto the box, hit "Reveal resultant", and see if your prediction matches.

Set the size of each arrow, drag it onto the box, then click "Reveal resultant"

Test yourself

6 questions · click to reveal each answer

  1. What does "balanced forces" mean?
    The forces are equal in size and opposite in direction, so they cancel out and the resultant force is zero.
  2. What is a resultant force?
    A single force that has the same overall effect as all the forces acting on an object combined.
  3. Two forces of 7 N and 3 N both push to the right. What's the resultant?
    10 N to the right. Same direction → add: 7 + 3 = 10 N.
  4. A 12 N force pulls left. A 4 N force pulls right. What's the resultant?
    8 N to the left. Opposite directions → subtract: 12 − 4 = 8 N. The bigger one was to the left.
  5. A book weighs 5 N. The table pushes up with 5 N. What's the resultant force on the book?
    0 N. Equal and opposite → balanced → resultant zero.
  6. A boat weighs 800,000 N. The forces on it are balanced as it floats. What's the size and direction of upthrust?
    800,000 N upwards. If forces are balanced, upthrust must equal weight.
Topic 10 · Forces

So what happens?

The final topic. You'll learn what a resultant force actually does to an object — whether it's already moving or standing still.

Part 1When the resultant force is zero

If the resultant force on an object is zero (forces are balanced), the object's motion doesn't change:

• If it was stationary, it stays stationary.
• If it was moving, it keeps moving at the same speed in the same direction.

That second one feels weird at first. Surely something needs a force to keep moving? But no — once it's already moving, balanced forces (or no forces at all) just mean the motion carries on. What slows things down in everyday life is friction or air resistance — those are unbalanced forces.

Key phrase

Resultant force = 0
Motion stays the same. Stationary objects stay still. Moving objects keep moving at constant speed.
Quick check

A box is sliding across smooth ice with a resultant force of zero on it. What happens?

  • AIt slows down and stops
  • BIt speeds up
  • CIt keeps moving at the same speed in the same direction
  • DIt changes direction
Show answer
C — It keeps moving at the same speed in the same direction. No resultant force = no change to motion. (On smooth ice there's almost no friction to slow it down.)

Part 2When the resultant force is NOT zero

If there's a resultant force, the object's motion changes. The direction of the change depends on which way the resultant points:

The three outcomes

Resultant force in the same direction as motion
The object speeds up. (e.g. a car with engine thrust bigger than friction.)
Resultant force opposite to motion
The object slows down. (e.g. a car when you brake — friction now bigger than thrust.)
Resultant force on a stationary object
The object starts to move in the direction of the resultant force.

⚠ Constant speed = balanced forces

This catches people out. A car driving along at a steady 25 m/s isn't "force-free" — its engine is doing work against friction and air resistance. But because the engine's thrust equals the resistance, the resultant is zero, so the speed stays constant. Constant speed → balanced forces.

Quick check

A skydiver is falling and reaches a constant speed (terminal velocity). What does this tell you about the forces on them?

  • AThe forces are unbalanced
  • BWeight is bigger than air resistance
  • CThe forces are balanced (air resistance = weight)
  • DThere are no forces acting
Show answer
C — Forces are balanced. Constant speed always means balanced forces (resultant = 0). Weight pulls them down; air resistance pushes them up; the two are equal at terminal velocity.

Test yourself

6 questions · click to reveal each answer

  1. If a stationary object has a resultant force of zero, what happens?
    It stays stationary. No change to motion.
  2. If a moving object has a resultant force of zero, what happens?
    It keeps moving at the same speed in the same direction. Constant motion.
  3. A car has a thrust of 800 N forwards. Friction and air resistance total 300 N backwards. What's the resultant force, and what happens to the car?
    500 N forwards. The car speeds up. Resultant in the direction of motion = speeds up.
  4. A cyclist stops pedalling. What happens to their motion, and why?
    They slow down and stop. Without thrust, friction and air resistance are now unbalanced — both push backwards, so the resultant is backwards, and that's against their motion.
  5. A lorry travels at a constant 25 m/s. Its driving force is 800 N. What's the size of friction and air resistance combined?
    800 N. Constant speed means forces are balanced — so the backward forces must equal the forward force.
  6. Why does a parachute slow a skydiver down?
    The parachute increases air resistance. Air resistance becomes bigger than weight, so the resultant force is now upwards (against the downward motion), and the skydiver slows down.

Done. Have a brilliant holiday.

All ten topics are still here if you want to revisit anything. The force builder is fun to play with even when you're just messing around.