Forces, Motion, and Mechanical Interactions

Defining Motion and Relativity

Have you ever seen a ball roll, a car drive by, or a bird fly in the sky? These are all examples of motion.

Motion means an object changes where it is. If something is in one place and then later is in a different place, it has moved.

We can tell if something moved by looking at it compared to something else that stays still. That something is called a reference point.

A reference point is a place or object we use to help us decide if something moved. A tree, a chair, a wall, or a stop sign can be a reference point if it stays still.

This is important because motion is about where something is compared to something else.

For example, if a dog is next to a tree, and later the dog is far from the tree, the dog moved. The tree helped us notice the motion.

Main Ideas to Learn

• Motion means a change in position.
• Position means where something is.
• A reference point helps us tell if an object moved.
• We look at motion over time: where it was before and where it is later.
• Sometimes one thing can look like it is moving when compared to one object, but not when compared to another.

What is position?

Position means the place where something is. A book can be on a desk. A toy can be under a chair. A child can be near a door.

If the book was on the desk and later it is on the floor, its position changed. That means the book moved.

What is time?

To know if something moved, we think about before and after. We look at where the object was at one time and where it is at a later time.

If the swing was still before and then later it was in a different place, the swing was moving.

What is relativity?

Relativity means we describe motion by comparing it to something else. In 1st grade, this means we ask, “Moving compared to what?”

Here is a simple way to think about it: if you are sitting on a bus, you may look still compared to your seat. But compared to a tree outside, you are moving. The answer depends on the reference point.

So, an object’s motion can be described in different ways depending on what you compare it to.

Good reference points are things that stay still

When we want to tell if something moved, it helps to use a reference point that is not moving. That makes it easier to compare.

• A playground pole
• A building
• A fence
• A tree

If the reference point is also moving, it can be harder to tell what is happening.

Let’s look at some examples.

Worked Example 1

A red ball is next to a chair. Then the ball rolls under the table.

1. Where was the ball first? Next to the chair.
2. Where is the ball later? Under the table.
3. Did its position change? Yes.
4. Is the ball in motion? Yes.

The ball changed position over time, so it moved.

Worked Example 2

A child is sitting in a wagon. The wagon moves across the yard.

1. Compared to the wagon, is the child moving? No. The child stays in the same spot in the wagon.
2. Compared to the tree in the yard, is the child moving? Yes. The child gets closer to and then farther from the tree.

This shows that motion depends on the reference point.

Worked Example 3

A toy car starts by the door. Later it is by the window.

1. Reference point: the door and the window.
2. First position: by the door.
3. Later position: by the window.
4. Did the toy car move? Yes.

The toy car’s position changed from one place to another.

Worked Example 4

You are sitting in your classroom. You are still in your chair.

1. Compared to your chair, are you moving? No.
2. If your class walks to the library later, compared to the classroom door, are you moving? Yes.

This helps us remember that we need to ask what we are comparing to.

How to tell if something is moving

1. Pick a reference point that stays still.
2. Look at where the object is first.
3. Look at where the object is later.
4. If its position changed, it moved.

You can think of it like this:

first place \(\rightarrow\) later place

If the places are different, there is motion.

Practice thinking

• A bird is on a fence, then on a branch. Did it move? Yes.
• A backpack stays on the same hook all day. Did it move? No.
• A student rides past the school on a bike. Compared to the school, is the student moving? Yes.
• A student sits still in the bike seat. Compared to the bike, is the student moving? No.

Things to remember

• Motion is a change in position.
• Position means where something is.
• We need a reference point to help us decide if something moved.
• We compare where something was before and later.
• An object can seem still with one reference point and moving with another.

Brief Summary

Motion means something changes its place. We can tell if it moved by comparing it to a reference point that stays still. We look at where the object is first and where it is later. If the position changes, the object is moving.

Applied Forces: Pushes and Pulls

Things move in many ways. A toy car can roll. A wagon can slide. A door can open. What makes these things move? A force does.

A force is a push or a pull. When we push or pull an object, we can make it start moving, stop moving, speed up, slow down, or change direction.

Forces are happening all around us every day. When you push a swing, that is a push. When you pull your backpack zipper, that is a pull. Pushes and pulls help objects move.

What is a push?

A push moves something away from you.

• Pushing a shopping cart
• Pushing a toy car
• Pushing a door to close it

When you use your hands, feet, or body to move something away, you are using a push.

What is a pull?

A pull moves something toward you.

• Pulling a wagon
• Pulling a drawer open
• Pulling a blanket up

When you bring something closer to you, you are using a pull.

What can pushes and pulls do?

Pushes and pulls can change how an object moves. An object may be still, or it may already be moving. A force can change that.

• Start movement: A ball sitting still can roll when you kick or push it.
• Stop movement: You can catch a rolling ball and stop it.
• Make it go faster: A harder push can make a toy car move faster.
• Make it go slower: A gentle hand on a swinging door can slow it down.
• Change direction: Hitting a ball with a bat can send it a new way.

All changes in motion need a push or a pull. If something starts moving, stops moving, or changes the way it moves, a force caused that change.

Strong pushes and gentle pushes

Not all pushes and pulls are the same. Some are strong. Some are gentle.

A strong push can move an object farther or faster. A gentle push may move it only a little.

Think about pushing a toy car.

• A small push makes it roll a little.
• A big push makes it roll more.

Heavy objects and light objects

Some objects are easy to move. Some are hard to move.

A light ball is easy to push. A full box may need a much stronger push or pull. Bigger or heavier things often need more force to move.

Push or pull? Sometimes both!

Some actions can use both a push and a pull.

Think about a door. You might push it to close it. You might pull it to open it. The same object can be moved in different ways.

Worked Example 1

A toy block is sitting still on the floor. Mia gives it a push. What happens?

Step 1: The block is not moving at first.

Step 2: Mia uses a push.

Step 3: The push makes the block start moving.

Answer: The block moves because Mia applied a push.

Worked Example 2

A wagon is behind Ben. Ben grabs the handle and brings the wagon toward himself. Is this a push or a pull?

Step 1: The wagon moves toward Ben.

Step 2: Moving something toward you is a pull.

Answer: Ben is using a pull.

Worked Example 3

A ball is rolling across the grass. Sara catches it with her hands. What did Sara's force do?

Step 1: The ball is already moving.

Step 2: Sara uses her hands to stop the ball.

Step 3: A push or pull can stop movement.

Answer: Sara's force stopped the ball.

Worked Example 4

Leo pushes one toy car gently and another toy car hard. Which car will probably go farther?

Step 1: One push is gentle. One push is strong.

Step 2: A stronger push can make an object move faster or farther.

Answer: The car with the harder push will probably go farther.

Try thinking about these at home or school:

• Do you push your chair in, or pull it out?
• When you open a drawer, do you push or pull?
• What happens when you push a ball softly? What happens when you push it hard?
• What can you pull toward you?

Important ideas to remember

• A force is a push or a pull.
• A push moves something away from you.
• A pull moves something toward you.
• Pushes and pulls can start, stop, speed up, slow down, or change the direction of movement.
• Strong forces can move objects more than gentle forces.
• Heavier objects often need a stronger push or pull.

Summary

Forces help objects move. A force is a push or a pull. When we push or pull something, we can make it move, stop, go faster, go slower, or turn. We see pushes and pulls every day when we play, open doors, move toys, and carry things.

Friction and Surface Resistance

Have you ever pushed a toy car across the floor and watched it slow down? Have you ever slid a book across a table and seen it stop? That happens because of friction.

Friction is a force that happens when two things touch. Friction makes it harder for something to move. It pushes against motion.

Another way to think about it is this: when something moves, friction tries to slow it down or stop it.

The surface an object moves on matters a lot. Some surfaces are smooth. Some surfaces are rough.

A smooth surface, like a shiny floor, has less friction. That means objects can move more easily.

A rough surface, like carpet or sandpaper, has more friction. That means objects slow down faster.

This is called surface resistance. The surface resists, or pushes against, the object's motion.

Main Ideas to Remember

• Friction happens when two things touch.
• Friction opposes motion. That means it pushes against movement.
• Rough surfaces make more friction.
• Smooth surfaces make less friction.
• More friction means an object slows down more.
• Less friction means an object can move farther.

Think About Sliding

If you slide the same block across two different surfaces, the block will not move the same way on both.

On a smooth table, the block may slide far. On a rug, the block may stop quickly. The rug is rougher, so it has more friction.

Why Does Friction Help Us?

Friction is not always bad. Friction helps us in many ways.

• Friction helps your shoes grip the ground so you can walk.
• Friction helps car tires hold the road.
• Friction helps you hold a pencil.

Without friction, things would slip too much.

Examples of Smooth and Rough Surfaces

• Smooth: tile floor, polished table, ice
• Rough: carpet, grass, sidewalk, sandpaper

Worked Example 1

Question: A toy car rolls on a smooth floor and then on a carpet. Where will it go farther?

Think: Smooth floors have less friction. Carpet is rough and has more friction.

Answer: The toy car will go farther on the smooth floor.

Worked Example 2

Question: You slide a book across a desk. Then you slide it across a towel. Which one slows the book down faster?

Think: A towel is rougher than a desk. Rougher surfaces make more friction.

Answer: The towel slows the book down faster.

Worked Example 3

Question: Which has more friction: sandpaper or a smooth plastic table?

Think: Sandpaper is very rough. A smooth plastic table is smoother.

Answer: Sandpaper has more friction.

Worked Example 4

Question: Mia says, “If a surface is rough, things move faster on it.” Is Mia correct?

Think: Rough surfaces make more friction. More friction slows things down.

Answer: No. Mia is not correct. Rough surfaces make objects move slower, not faster.

Let’s Compare

1. A ball rolling on tile
2. A ball rolling on grass

The ball on tile will usually roll farther because tile is smoother. The ball on grass will slow down sooner because grass is rougher.

Easy Way to Remember

• Smooth = less friction = moves easier
• Rough = more friction = slows down faster

You can even think of it like this:

More friction  slower movement

Less friction  easier movement

Try It in Real Life

You can test friction with a small toy car or a block.

1. Slide or roll it on a table.
2. Slide or roll it on carpet.
3. Watch where it goes farther.

You will see that the object usually goes farther on the smoother surface.

Brief Summary

Friction is a force that happens when things touch. It pushes against motion and slows objects down. Rough surfaces have more friction, so objects stop sooner. Smooth surfaces have less friction, so objects can move farther.

Gravity is a pulling force. We cannot see gravity, but we can see what it does. Gravity pulls things down toward Earth.

Have you ever dropped a pencil and watched it fall? Have you ever jumped and come back down? That is gravity at work. Gravity is always pulling on you and on other objects.

Gravity is called an invisible force. A force is a push or a pull. Gravity is a pull. It pulls objects toward the ground.

Gravity does not need to touch an object with hands or strings. It still pulls. That is why gravity is called a non-contact force. It works even when nothing is holding the object.

Main idea: When an object is in the air, gravity pulls it down.

• If you toss a ball up, gravity pulls it back down.
• If you drop a book, gravity pulls it down.
• If rain falls from clouds, gravity pulls it down.

Gravity is pulling all the time, even when things are not moving. When you stand on the floor, gravity is pulling you down, and the floor is holding you up. That is why you do not sink into the ground.

Gravity pulls everything toward the Earth. Big things and small things both fall because of gravity.

Let’s think about what we see every day.

1. An apple falls from your hand.
   The apple does not float away. Gravity pulls it down.
2. You jump.
   You go up for a moment, but then gravity pulls you back to the ground.
3. A leaf falls from a tree.
   The leaf moves down because gravity is pulling it.

Sometimes objects fall fast, and sometimes they fall slowly. But in each case, gravity is still pulling down.

Worked Example 1: Dropping a toy

You let go of a toy car from your hand. What happens next?

Step 1: The toy is in the air.

Step 2: Gravity pulls the toy down.

Answer: The toy car falls to the floor because gravity pulls it down.

Worked Example 2: Throwing a ball up

You throw a ball up into the air. Does it keep going up forever?

Step 1: The ball goes up because you pushed it.

Step 2: Gravity keeps pulling the ball down.

Step 3: The ball slows down, stops, and comes back down.

Answer: No, it does not go up forever. Gravity pulls it back down.

Worked Example 3: Picking the best answer

Question: Why does a crayon fall off a desk?

• A. The crayon wants to roll
• B. Gravity pulls it down
• C. The desk jumps away

Step 1: Think about what makes things fall.

Step 2: Gravity is the force that pulls objects toward Earth.

Answer: B. Gravity pulls it down.

Worked Example 4: Is gravity working?

A child is sitting still on a chair. Is gravity still pulling on the child?

Step 1: Gravity pulls all the time.

Step 2: Even when the child is still, gravity is pulling down.

Step 3: The chair holds the child up.

Answer: Yes. Gravity is still pulling on the child.

Things to remember about gravity:

• Gravity is a pulling force.
• Gravity is invisible.
• Gravity pulls objects toward Earth.
• Gravity works even when it does not touch the object.
• Gravity makes things fall down.

Try saying this: Gravity pulls things down.

When you see a ball drop, a fruit fall, or yourself come back down after a jump, you are seeing gravity in action.

Summary

Gravity is an invisible force that pulls objects toward the Earth. It is always working. When something falls, gravity is pulling it down. When you jump, gravity brings you back to the ground.

Patterns and Periodicity of Motion

Things around us move in many ways. Some things move straight. Some move in a circle. Some move in a zigzag. Some move back and forth.

When motion happens the same way again and again, it makes a pattern. A pattern helps us know what may happen next.

Today we will learn to look at motion, name the kind of motion we see, and tell what comes next in the pattern.

What is motion?

Motion means moving from one place to another. A ball rolling, a fan spinning, and a swing moving are all kinds of motion.

What is a pattern?

A pattern is something that repeats. If you clap like this: clap, tap, clap, tap, that is a pattern. Motion can make patterns too.

Kinds of motion

• Straight motion: moving in one straight line.
• Circular motion: moving around and around in a circle.
• Zigzag motion: moving with sharp turns, first one way, then another way.
• Back-and-forth motion: moving one way, then back the other way, again and again.

1. Straight motion

Straight motion goes in one line. A toy car rolling across the floor can move in a straight line. A child sliding down a straight slide also moves mostly in a straight path.

If something keeps going straight, we can predict that it will keep moving forward in that line.

Examples of straight motion:

• A ball rolling across the gym floor
• A train moving on a straight track
• A runner going down a straight path

2. Circular motion

Circular motion goes around a center. It makes a round path. A clock hand moves in a circle. A merry-go-round spins in a circle too.

When something moves in a circle, we can say it will keep going around and around if nothing stops it.

Examples of circular motion:

• A ceiling fan spinning
• A wheel turning
• The hands on a clock moving around

3. Zigzag motion

Zigzag motion changes direction again and again. It does not stay in one straight line. It makes pointy turns.

A bug moving this way and that way may make a zigzag path. A child running around cones can move in a zigzag too.

Examples of zigzag motion:

• A bee flying from flower to flower
• A skier going side to side
• A child weaving around toys on the floor

4. Back-and-forth motion

Back-and-forth motion moves one way and then returns. It repeats the same motion over and over. This is a very easy pattern to see.

A swing moves forward and backward. A playground swing does not go in a straight line across the yard. It goes one way, then back, one way, then back.

Examples of back-and-forth motion:

• A swing moving
• A pendulum on a clock
• A rocking chair rocking

What does periodic mean?

Periodic motion is motion that repeats in a regular way. For 1st grade, we can think of it as motion that happens again and again in the same pattern.

A swing is periodic because it goes forward, back, forward, back. A fan is periodic because it keeps turning around and around. These motions repeat.

How can we tell what kind of motion it is?

1. Look at the path. Where does it go?
2. Ask if it repeats. Does it do the same thing again?
3. Name the motion. Is it straight, circular, zigzag, or back and forth?
4. Predict what comes next. If there is a pattern, what will it do next?

Words that help us describe motion

• Straight = one line
• Round = circle
• Turn = change direction
• Repeat = do it again
• Pattern = repeats in the same way

Worked Example 1: Easy

A toy car rolls from the door to the table in one straight line.

Question: What kind of motion is this?

Answer: This is straight motion.

Why? The toy car moves in one line and does not go around, zigzag, or back and forth.

Worked Example 2: A little harder

A ceiling fan spins above the room.

Question: What kind of motion is this? Does it repeat?

Answer: This is circular motion, and yes, it repeats.

Why? The fan blades go around and around in a circle. The same motion happens again and again.

Worked Example 3: Medium

A swing moves forward, backward, forward, backward.

Question: What kind of motion is this? What will it probably do next?

Answer: This is back-and-forth motion. It will probably keep moving forward, then backward.

Why? The swing is making a repeating pattern.

We can show the pattern like this:

forward, backward, forward, backward

If we use letters, we can call forward \(A\) and backward \(B\):

\(A, B, A, B\)

The next part is:

\(A\)

Worked Example 4: Challenge

A butterfly flies left, right, left, right as it moves across the garden.

Question: What kind of motion does this look like?

Answer: This looks like zigzag motion.

Why? The butterfly keeps changing direction from one side to the other.

If the pattern is left, right, left, right, we can predict the next move may be left.

Let’s compare motions

• A rolling marble on a straight track: straight motion
• A bicycle wheel turning: circular motion
• A kite moving side to side in sharp turns: zigzag motion
• A swing at the park: back-and-forth motion

Why patterns in motion matter

Patterns help us know what may happen next. If we know a swing goes back and forth, we can tell when it will come back. If we know a fan spins in a circle, we know the blades will keep going around.

Scientists watch patterns to learn about the world. You can be a scientist too when you watch how things move.

Try it yourself

Look around your home, classroom, or playground. Can you find:

• one thing that moves straight?
• one thing that moves in a circle?
• one thing that moves in a zigzag?
• one thing that moves back and forth?

Quick check

1. A clock hand goes around and around. Is it straight, circular, zigzag, or back and forth?
2. A child runs directly to the door. What kind of motion is that?
3. A swing moves away and comes back. What kind of motion is that?
4. A bee changes direction again and again. What kind of motion is that?

Answers:

1. Circular
2. Straight
3. Back and forth
4. Zigzag

Summary

Motion means movement. Some motion is straight, some is circular, some is zigzag, and some is back and forth.

When motion repeats, it makes a pattern. A repeating motion is called periodic motion. We can watch a motion pattern and predict what will happen next.

Static electricity is a tiny kind of electricity that can build up on an object.

We cannot see static electricity, but we can see what it does. That is why we call it an invisible force.

Sometimes, when two things are rubbed together, the static electricity can make light things move. A balloon may pull on hair. A comb may pull on tiny paper bits. That is static electricity at work.

Introduction

Forces are pushes and pulls. Some forces happen when things touch. Some forces can work even when we cannot see them.

Static electricity is one of those invisible forces. It can make objects attract, which means pull closer, or sometimes repel, which means push away.

How static electricity happens

When you rub some materials together, a tiny electric charge can build up. You do not need to see the charge to know it is there. You can see the effects.

For example, if you rub a balloon on your hair, the balloon may stick to a wall or make hair stand up. The balloon has gained a charge that can pull on nearby things.

This does not happen with every object in the same way. Some materials are better at making static electricity when rubbed.

• balloons
• hair
• wool
• some plastic combs

What static electricity can do

Static electricity can make an object pull on another object without touching it first. That is why it is called an invisible force.

It often works best on objects that are very light.

• small paper pieces
• hair
• dust

If the force is strong enough, the light object moves. It may jump, lift, or stick.

Attract and repel

Attract means pull together. A charged balloon can attract hair or paper bits.

Repel means push away. Sometimes charged objects can push each other away.

So static electricity can cause two kinds of motion:

1. objects move closer
2. objects move apart

Where we notice static electricity

You may notice static electricity in everyday life.

• clothes cling together from the dryer
• hair stands up after taking off a hat
• a balloon sticks to a wall after being rubbed
• a comb picks up tiny paper pieces

Worked Example 1: Balloon and hair

A child rubs a balloon on their hair. Then some hair lifts up toward the balloon.

What happened? Rubbing helped static electricity build up on the balloon. The balloon made an invisible pull on the hair.

Answer: The balloon attracted the hair with static electricity.

Worked Example 2: Comb and paper

A plastic comb is rubbed on a sweater. Next, the comb is held near tiny paper pieces. The paper pieces move toward the comb.

What happened? Rubbing gave the comb a static charge. The charged comb pulled on the light paper pieces.

Answer: The comb used an invisible force to attract the paper.

Worked Example 3: Balloon on the wall

A balloon is rubbed on hair and then pressed lightly to a wall. The balloon stays on the wall for a little while.

What happened? The rubbed balloon had static electricity. That invisible force helped the balloon stick to the wall.

Answer: Static electricity helped the balloon stay on the wall.

Worked Example 4: Two moving things

One charged object pulls a paper bit closer. Another charged object pushes a light object away.

What do we call these two actions?

Answer: Pulling closer is attract. Pushing away is repel.

Try to think about it

If you cannot see static electricity, how do you know it is there?

You know it is there because you can see what it does.

• hair moves
• paper jumps
• a balloon sticks

We may not see the force itself, but we can see the changes it makes.

Safety note

Static electricity in these simple classroom examples is small. Still, students should only try activities with an adult or teacher.

Do not use electricity from outlets. Static electricity is different from plugging something in.

Summary

Static electricity is a tiny kind of electricity that can build up when some materials are rubbed together.

It is an invisible force because we cannot see it, but we can see its effects. It can make light objects move.

Static electricity can attract objects by pulling them closer. It can also repel objects by pushing them away.

When you see hair stand up, paper move toward a comb, or a balloon stick to a wall, you are seeing static electricity in action.