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A force is something one thing does to another, never something an object has on its own.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will describe any force as a short sentence with two things in it — what pushes or pulls, and what is pushed or pulled — say when the force begins and when it is over, and measure its size in newtons.
You push doors open and pull drawers out all day without thinking about it. You also already know that things do not start moving on their own — if a cup slides across a table, somebody or something moved it. This lesson gives that everyday knowledge its proper name and one strict rule.
You also know how to read a scale: find two printed numbers, count the spaces, work out what one mark is worth, then count along. You will use that to measure how big a force is.
| Term | What it means |
|---|---|
| Force | A push or a pull that one thing does to another. |
| Push | A force that sends the other thing away from the one pushing. |
| Pull | A force that brings the other thing toward the one pulling. |
| Interaction | Something that happens between two things, not inside one. |
| Contact force | A force between two things that are touching. |
| Non-contact force | A force across a gap, like a magnet's pull or gravity. |
| Newton (N) | The unit a force is measured in. |
| Spring scale | A tool with a spring and a scale that measures a force in newtons. |
A force is a push or a pull, and it is never something an object has on its own. It is something one thing does to another.
So a force always has two ends. The hand pushes the door. The magnet pulls the paper clip. The rope pulls the sled. If you cannot name both ends, you have not finished describing the force.
A force also has a beginning and an end in time. Before the shoe touches the ball there is no push. While they are touching, there is. Once they part, the push is over — even though the ball keeps flying.
And a force has a size. A gentle tap and a hard kick are both pushes, but one is much bigger. Scientists measure the size of a force in newtons, written N, with a tool called a spring scale.
Another way: action
Put a book on the table and push it. Now take your hand away. The push stopped when your hand left, and the book stopped soon after. Nothing about the book changed; what changed is whether your hand was doing something to it.
Another way: steps
Every force in this course can be written as one short sentence with two things in it. Here are four:
| What happens | The sentence |
|---|---|
| a drawer is opened | the hand pulls the drawer |
| a doorbell rings | the finger pushes the button |
| a paper clip jumps up | the magnet pulls the paper clip |
| a boat is rowed | the oar pushes the water |
Notice that the last one sounds odd until you say it properly. The rower does not push the boat forward with their hands at all — the oar pushes the water backward. Getting both ends named is what makes that visible.
Most forces happen when two things touch: a hand on a door, a shoe on a ball. These are called contact forces. But some forces reach across a gap, with nothing touching at all. These are non-contact forces.
The rule still holds for every one of them: two things, and one acts on the other. The Earth pulls the ball. The magnet pulls the clip. Even when there is a gap, a force is never something one object has by itself.
A spring scale has a hook at the bottom, a spring inside, and a scale printed on the side. When you pull on the hook, the spring stretches, and a pointer moves along the scale. The harder the pull, the further it moves.
The scale is marked in newtons (N). One newton is about the pull of a small apple hanging from the hook. A backpack full of books might pull with about 50 N, and pulling a wagon along a sidewalk might take 20 N.
Read a spring scale exactly as you read any other scale. Find two printed numbers, count the spaces between them, and work out what one small mark is worth before you count along. If the numbers go up in twos with four spaces between them, each mark is half a newton.
When you push on a wall, the wall pushes back on your hand. You can feel it: your hand is squashed. When a dog pulls on its leash, the leash pulls back on the dog. Every force has a partner that acts the other way, on the other thing.
This is another reason a force cannot belong to one object. If it did, how could the wall push back? The push is between your hand and the wall, and each one acts on the other. When you jump off a skateboard, you push the board backward and the board pushes you forward, which is why the board rolls away behind you. Scientists found that the two forces in a pair are always the same size, which you will study in a later course.
A force can change how something moves, or change its shape. Here are the things a push or a pull can do:
| A force can | Example |
|---|---|
| start something moving | a hand pushes a swing |
| speed it up | a foot pushes a scooter along |
| slow it down or stop it | a glove stops a baseball |
| change its direction | a bat sends a ball the other way |
| change its shape | hands squash a ball of clay |
Notice what is not on the list: keeping something moving. A ball rolling across the floor after the push has ended does not need a force to keep going. It slows down because something else, the floor, is rubbing on it.
Before you hand in any answer about a force, run it through three checks.
A sentence that passes all three is a proper description of a force. It is the kind of sentence scientists and engineers write, and it is the kind of sentence every lesson in the rest of this course will ask you for.
A child pulls a wagon full of groceries home along the sidewalk. Say the force properly: the handle pulls the wagon, and the child's hand pulls the handle. A pull, because the wagon comes toward the child.
If you put a spring scale between the handle and the child's hand, you can measure it. On a smooth sidewalk the pull might be about 20 N. On grass the wheels sink in and the ground rubs more, and the pull goes up to about 40 N, twice as much. Up a hill it is more again.
Nothing about the wagon changed between the sidewalk and the grass. What changed was what the wagon was touching. That is the whole lesson in one trip: a force is about two things acting on each other, and when one of the things changes, the force changes too. You will study the rubbing force, friction, in two lessons' time.
A tower crane on a construction site lifts steel beams, pallets of bricks and tubs of concrete to the top of a new building. Its long cable ends in a hook, and the cable pulls the load upward. At the same time, the Earth pulls the load downward: that is the load's weight.
Crane operators must know how big these forces are. A pallet of bricks can weigh about 10,000 N, which is ten kilonewtons. Every crane has a sign showing the biggest load it may lift at each distance from its tower, and a sensor on the cable measures the pull the way a giant spring scale would. If the pull gets too big, an alarm sounds and the crane stops.
Notice that both forces on the load have two ends: the cable and the load, and the Earth and the load. Engineers who design cranes describe every force this way, because a force with only one end named cannot be measured or planned for.
The most common sentence children write is the ball has force. It feels right, because the ball is the thing moving and the shoe is long gone.
But force is not stuff, and it is not stored anywhere. A force is an event between two things: the shoe pressed on the ball for a fraction of a second and that was all of it. What the ball carries away afterward is movement, not force.
The test is simple. Every time you write the word force, check that your sentence has two things in it. If it has one, the sentence is wrong — and it is the same mistake that later turns friction into a property of sandpaper and energy into a liquid inside a battery. Catching it now saves arguing with it in every lesson that follows in this course.
Find the thing doing it.
$\text{the dog}$
It strains forward and the leash goes tight.
Find the thing it is done to.
$\text{the leash}$
The leash is what gets tugged.
Ask which way it goes.
$\text{the leash moves toward the dog}$
Toward the one acting means a pull.
Say the sentence with both ends.
$\text{the dog pulls the leash}$
Two things, and one acts on the other.
Test the one-thing version.
$\text{the leash has a pull in it?}$
Wrong: take the dog away and there is nothing.
Find what started it.
$\text{a hand rolled the ball}$
Something acted on it: it did not start by itself.
Name both ends of that force.
$\text{the hand pushes the ball}$
A push, because the ball went away from the hand.
Check whether they still touch.
$\text{the hand has let go}$
Here is the twist: the two things have parted.
Decide if the push goes on.
$\text{no: the push is over}$
A force lasts only while the two things act on each other.
Say what the ball has now.
$\text{movement, not force}$
Movement is not force.
Explain why it slows.
$\text{the floor rubs the ball}$
A new force, with two new ends: the floor and the ball.
Name both ends of the force.
$\text{the spring scale pulls the wagon}$
The scale sits between your hand and the wagon.
Read the printed numbers.
$10\ \text{N and } 20\ \text{N}$
The pointer sits between them.
Find the gap between them.
$20 - 10 = 10\ \text{N}$
The small marks share ten newtons.
Count the spaces in the gap.
$4 \text{ marks make } 5 \text{ spaces}$
Always one more space than marks.
Find one mark's worth.
$10 \div 5 = 2\ \text{N}$
Each space is two newtons.
Count along from the number.
$10 + 3 \times 2 = 16\ \text{N}$
The pointer was three marks past 10.
Say the answer as a sentence.
$\text{the scale pulls the wagon with } 16\ \text{N}$
A force has two ends and a size.
Find the thing doing it.
$\text{the crane's cable}$
It is what reaches the crate.
Find the thing it is done to.
Say the sentence with both ends.
Think about a dog tugging on its leash. Is the force a push or a pull?
Complete the worked solution: a spring scale is numbered every $10$ N, with four small marks between the numbers. A backpack hangs from it, and the pointer is $3$ small marks past $10$ N. How hard does the backpack pull down on the scale?
Count the spaces between numbers.
$4 \text{ marks make}$ a spaces
There is always one more space than there are marks.
Find one mark's worth.
$10\ \text{N} \div \text{spaces} =$ b N
The ten newtons are shared equally.
Count along and add.
$10 + 3 \times \text{one mark} =$ c N
Each mark past the number adds one mark's worth.
In a boy closing a door, which is which?
| the boy's hand | the door | |
|---|---|---|
| does the pushing or pulling | ||
| is pushed or pulled |
A class wrote four sentences about a kicked soccer ball. Mark the one that describes the force properly.
This task has no paper form; do it on a device.
Put these four moments in the order they happen when a shoe kicks a ball.
Number the steps in order (write the number in the box):
For each action, say whether the force is a push or a pull.
| Action | Push or pull? |
|---|---|
| a boy closing a door | |
| a magnet lifting a paper clip | |
| a crane raising a crate |
Complete the row for a dog tugging on its leash: what does the pushing or pulling, what it is done to, and which of the two it is.
| does the pushing or pulling | is pushed or pulled | push or pull (one word) | |
|---|---|---|---|
| a dog tugging on its leash |
A rope is tied to the front of this sled and someone is pulling it to the right. Click the arrow that stands for the rope's pull.
This task has no paper form; do it on a device.
A student hooks a spring scale to a toy wagon and pulls it along the floor. The scale is numbered every $2$ N, with three small marks between the numbers. The pointer is $2$ small marks past $6$ N. How hard is the pull?
Answer: unit: N / kN
A small sailboat crosses a lake on a windy day. Nothing is touching the boat except the water and the air. What is pushing it along?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
For each action, say whether the force is a push or a pull.
| Action | Push or pull? |
|---|---|
| a girl opening a drawer | |
| a boy closing a door | |
| a magnet lifting a paper clip |
You can name both ends of a force and say whether it is a push or a pull. Tell someone why the ball has a force in it is the wrong way to say it.
17. Your turn: a crane lifts a crate. Name both ends., step 2
$\text{the crate}$
It is what gets lifted.
17. Your turn: a crane lifts a crate. Name both ends., step 3
$\text{the cable pulls the crate}$
A pull, because the crate comes toward the cable.