Mechanics: Forces, Motion, and Interactions

Collisions and Kinetic Energy Transfer

Things move all around us. A ball can roll. A toy car can zoom. A block can slide. When something is moving, it has moving energy. This moving energy helps it keep going.

Sometimes two things collide. A collision means they bump into each other. When they bump, the push from the bump can make things move in a new way.

When objects collide, some moving energy can go from one object to another. This is called energy transfer. You do not need to see the energy. You can tell it moved because the objects change how they move.

After a collision, an object may:

• move faster,
• move slower,
• stop, or
• change direction.

Main Idea: When moving objects bump into things, their moving energy can be shared. That sharing can change speed and direction.

What is a collision?

A collision is when two objects touch with a bump. The bump can be gentle, like two marbles tapping. The bump can be stronger, like two toy cars crashing.

In a collision, each object pushes on the other object. This push happens where they touch. That push can make one object start moving, stop moving, slow down, speed up, or turn.

What is moving energy?

Moving energy is the energy an object has because it is moving. If a ball is sitting still, it does not have moving energy. If the ball is rolling, it has moving energy.

Usually, a faster-moving object has more moving energy than a slower-moving object. You can often see this with toys. A fast toy car can push a block farther than a slow toy car.

How does energy transfer in a collision?

Let us think about a rolling ball and a block. The ball is moving, so it has moving energy. The block is still. When the ball bumps the block, some of the ball's moving energy can move into the block.

Now the block may start moving. The ball may slow down. This shows that energy was transferred from the ball to the block.

If one object gives some moving energy away, it may move less after the collision. If one object gets moving energy, it may move more after the collision.

What can happen after a collision?

• One object starts moving: A rolling marble hits a still marble. The still marble begins to roll.
• One object slows down: A toy car hits a block and moves more slowly after the bump.
• One object stops: A rolling ball bumps into something and stops.
• Objects change direction: A ball hits the wall and bounces back a different way.

Collisions can happen in different ways

1. A moving object hits a still object.

This is easy to see. The moving object shares some moving energy. The still object may start to move.

2. Two moving objects hit each other.

Both objects can change. They may both slow down. One may turn. One may stop. Both can share energy with each other.

3. A moving object hits a wall.

The wall does not roll away like a ball, but the moving object can bounce back or stop. Its motion changes because of the collision.

Worked Example 1: Ball and Block

A small ball rolls across the floor and bumps into a block that is not moving.

1. The ball is moving, so it has moving energy.
2. The block is still, so it does not have moving energy yet.
3. The ball and block collide.
4. Some moving energy goes from the ball to the block.
5. The block starts to slide a little.
6. The ball slows down.

What do we learn? The collision transferred moving energy from the ball to the block.

Worked Example 2: Toy Car and Toy Car

A fast toy car rolls into a toy car that is standing still.

1. The fast car has a lot of moving energy.
2. The still car has no moving energy yet.
3. They bump.
4. The still car begins to move.
5. The first car moves slower after the bump.

What do we learn? The fast car gave some moving energy to the still car. After the collision, both cars changed how they moved.

Worked Example 3: Ball Bounces Off a Wall

You roll a ball toward a wall. The ball hits the wall and comes back toward you.

1. The ball is moving toward the wall.
2. The ball collides with the wall.
3. The push at the wall changes the ball's direction.
4. The ball moves back the other way.

What do we learn? A collision can change direction. The object does not always keep going the same way.

Worked Example 4: Two Balls Roll Toward Each Other

Two balls roll toward each other and bump in the middle.

1. Both balls are moving, so both have moving energy.
2. They collide.
3. After the bump, they may slow down, stop, or roll in new directions.

What do we learn? When both objects are moving, both can change speed and direction in the collision.

How can we tell energy transferred?

We cannot hold moving energy in our hands, but we can look for clues. Ask:

• Did a still object start moving?
• Did a moving object slow down?
• Did it stop?
• Did it change direction?

If the motion changed after a bump, energy was transferred in the collision.

Fast and slow collisions

A faster object often causes a bigger change when it collides. For example, a fast-rolling ball may push a block farther than a slow-rolling ball.

That is because the faster ball has more moving energy to share.

We can think about it like this:

slow move \(\rightarrow\) less moving energy

fast move \(\rightarrow\) more moving energy

Simple compare idea

If Car A is faster than Car B, then:

$$\text{Car A has more moving energy than Car B}$$

This does not need numbers. We are just comparing fast and slow.

Try thinking about these

• If a rolling marble hits a still marble and the still marble starts rolling, energy moved to the still marble.
• If a ball hits a wall and comes back, the collision changed the ball's direction.
• If a toy car hits a block and stops, the collision changed the car's speed.

Important words to know

• Collision: a bump when two objects touch
• Moving energy: energy an object has when it moves
• Transfer: move from one thing to another
• Direction: the way something is moving
• Speed: how fast or slow something moves

Let’s review

When objects collide, they push on each other. This push can transfer moving energy. Because of that transfer, the objects may speed up, slow down, stop, or change direction.

You can understand a collision by watching what happens before the bump and after the bump. If motion changes, the collision changed the movement.

Brief Summary

A collision is when two objects bump into each other. Moving objects have moving energy. In a collision, moving energy can transfer from one object to another, and that can make objects move faster, slower, stop, or change direction.

Magnetism and Electromagnetism

Have you ever used a magnet on a refrigerator or played with a magnet wand? Magnets are special objects that can pull some kinds of metal. This pulling is called magnetism.

Magnetism is a force. A force is a push or a pull. Magnets can pull things without touching them first. That makes magnets very interesting!

In this lesson, you will learn:

• what a magnet is,
• what magnets can and cannot attract,
• what the two ends of a magnet do,
• and how electricity can make a magnet called an electromagnet.

1. What is a magnet?

A magnet is an object that attracts some metal things. A magnet can pick up objects like some paper clips, some nails, and some metal pins.

Not all metals are attracted to magnets. Also, many things are not metal at all, like wood, plastic, paper, rubber, and cloth. These things are usually not pulled by a magnet.

Magnets can come in different shapes. Some are long bars. Some are circles. Some are horseshoe-shaped. Even if they look different, magnets still have special pulling power.

2. Magnets have two poles

Every magnet has two ends called poles. One end is a north pole. The other end is a south pole.

These poles are important because they decide how magnets act when they come near each other.

• Opposite poles attract: north and south pull together.
• Same poles repel: north and north push away, and south and south push away.

You can think of it like this:

• N + S = pull together
• N + N = push apart
• S + S = push apart

When magnets push away from each other, that is called repel. When they pull together, that is called attract.

3. Magnetic force can work through space

A magnet does not always need to touch an object to affect it. If a paper clip is close enough, the magnet may pull it in. The space around a magnet where it can push or pull is called a magnetic field.

You do not need to memorize that big name. Just remember this: a magnet has an invisible area around it where it can act on some objects.

4. What things do magnets attract?

Magnets attract some metal objects. Here are some examples of things a magnet may attract:

• paper clips
• iron nails
• some screws
• steel objects

Here are some things magnets usually do not attract:

• wood blocks
• plastic toys
• glass marbles
• paper
• rubber bands

The best way to know is to test safely with a magnet and see what happens.

5. Strong and weak magnets

Some magnets are stronger than others. A strong magnet can pull heavier things or pull things from a little farther away. A weak magnet may only pull light objects.

If one magnet picks up 10 paper clips and another picks up 3 paper clips, the first magnet is stronger.

6. What is electromagnetism?

Now let us learn something amazing. Electricity can help make magnetism.

An electromagnet is a magnet made by using electricity. When electricity moves through a wire, it can create magnetic force.

This means a wire with electricity in it can act like a magnet. That is called electromagnetism.

7. How can we make an electromagnet?

A simple electromagnet can be made with:

• a battery,
• a wire,
• and an iron nail.

If the wire is wrapped around the nail and connected to the battery, electricity moves through the wire. Then the nail can act like a magnet for a short time.

It may be able to pick up small paper clips.

When the electricity stops, the nail may stop acting like a magnet. That is one special thing about an electromagnet: it can be turned on and off.

8. Why are electromagnets useful?

Electromagnets are useful because people can control them. They can turn them on when they want magnet power, and turn them off when they do not.

Electromagnets are used in many machines and tools. For example:

• some large cranes use electromagnets to lift metal,
• doorbells use electricity and magnets,
• some speakers use magnets too.

You do not need to know all the machine details yet. The big idea is this: electricity can create magnetism.

9. How can an electromagnet change?

An electromagnet can become stronger or weaker.

For example, it may become stronger if:

• more wire is wrapped around the nail,
• or a stronger battery is used.

This means electromagnets can be changed more easily than many regular magnets.

Worked Example 1: What will the magnet attract?

Question: A magnet is near a paper clip, a wooden block, and a rubber band. Which object will the magnet most likely attract?

Step 1: Think about which item is metal.

• paper clip = metal
• wooden block = wood
• rubber band = rubber

Step 2: Magnets attract some metal objects.

Answer: The magnet will most likely attract the paper clip.

Worked Example 2: Will the magnets attract or repel?

Question: One magnet's north pole is pointed toward another magnet's south pole. What will happen?

Step 1: Remember the rule: opposite poles attract.

Step 2: North and south are opposite poles.

Answer: The magnets will attract, or pull together.

Worked Example 3: Same poles together

Question: A north pole faces another north pole. What will happen?

Step 1: Remember the rule: same poles repel.

Step 2: North and north are the same.

Answer: The magnets will repel, or push apart.

Worked Example 4: What happens when electricity stops?

Question: A wire is wrapped around a nail and connected to a battery. The nail picks up paper clips. Then the battery is disconnected. What will most likely happen?

Step 1: The nail was acting like an electromagnet because electricity was moving through the wire.

Step 2: When the battery is disconnected, the electricity stops moving.

Answer: The nail will most likely stop acting like a magnet, so it may not pick up the paper clips anymore.

Things to remember

• A magnet is an object that attracts some metal things.
• Magnets have two poles: north and south.
• Opposite poles attract.
• Same poles repel.
• A magnetic field is the invisible area around a magnet where it can push or pull.
• An electromagnet is a magnet made with electricity.
• Electromagnets can be turned on and off.

Brief Summary

Magnetism is a force that lets magnets pull some metal objects. Magnets have two poles, called north and south. Opposite poles pull together, and same poles push apart.

Electromagnetism happens when electricity creates magnetism. A wire with electricity can make an iron nail act like a magnet. This is useful because an electromagnet can be turned on, turned off, and changed to be stronger or weaker.

Mechanical Advantage and Simple Machines

Have you ever used a ramp to move something heavy, turned a doorknob, or used a shovel to dig? If so, you have used a simple machine.

Simple machines help us do work more easily. They do not make work disappear, but they can help us by changing how much force we need, which direction we push or pull, or how far something moves.

In this lesson, you will learn what mechanical advantage means and how the six simple machines help people move, lift, split, and turn things.

What is work?

In science, work happens when a force moves an object. If you push a box and it slides across the floor, you are doing work. If you push very hard on a wall and it does not move, no work is done on the wall because it did not move.

A simple way to think about work is:

$$\text{work} = \text{force} \times \text{distance}$$

This means if you use more force or move something farther, you do more work.

What is mechanical advantage?

Mechanical advantage tells us how much a machine helps us. A machine has a mechanical advantage when it lets us use less force to move or lift something.

We can compare the force that comes out of the machine to the force we put into the machine:

$$\text{mechanical advantage} = \frac{\text{output force}}{\text{input force}}$$

If a machine has a mechanical advantage of 2, it means the machine helps make your force act like it is twice as strong. You use less force, but you usually have to move your end a longer distance.

Important idea: Simple machines do not create extra energy. They help by trading force for distance. If you use less force, you often have to move farther.

The 6 simple machines

• Lever
• Pulley
• Inclined plane
• Wedge
• Screw
• Wheel and axle

Let’s learn about each one.

1. Lever

A lever is a stiff bar that turns around a fixed point. The fixed point is called the fulcrum.

Examples of levers include:

• Seesaw
• Crowbar
• Bottle opener
• Hammer pulling a nail

A lever helps lift or move a load. When you push down on one end, the other end moves up. If the fulcrum is closer to the load, it is easier to lift the load.

This means a longer effort arm can give you more mechanical advantage.

Example: If you use a crowbar to lift a rock, the crowbar helps you lift with less force than lifting the rock with just your hands.

2. Pulley

A pulley is a grooved wheel with a rope running around it.

Pulleys can:

• Change the direction of a force
• Make lifting easier when more than one pulley is used

Examples of pulleys include:

• Flagpoles
• Window blinds
• Construction cranes

With one fixed pulley, you may still use about the same amount of force, but you can pull down instead of lifting up. Pulling down can feel easier because you can use your body weight.

With more pulleys, the load is shared, so you need less force to lift the object.

3. Inclined plane

An inclined plane is a flat surface set at a slant, like a ramp.

Examples include:

• Loading ramp
• Slide
• Road up a hill

An inclined plane helps you raise an object by moving it over a longer distance with less force. It is usually easier to push a box up a ramp than to lift it straight up.

The trade-off is that the box travels farther along the ramp.

4. Wedge

A wedge is like two inclined planes put together. A wedge is often used to split, cut, or push things apart.

Examples include:

• Knife
• Axe
• Chisel
• Doorstop

When you push a wedge forward, its shape helps force things apart. A sharp wedge can cut or split more easily.

5. Screw

A screw is an inclined plane wrapped around a pole.

Examples include:

• Jar lid threads
• Wood screws
• Light bulb base

When you turn a screw, it moves forward little by little. This helps hold things together tightly or lift something in a small space, like a car jack.

The threads on a screw help turn a twisting force into a pushing force.

6. Wheel and axle

A wheel and axle is made of a large wheel attached to a smaller rod, called an axle. When one turns, the other turns too.

Examples include:

• Doorknob
• Steering wheel
• Pizza cutter
• Bicycle wheel

This machine helps by making turning easier. A larger wheel can help you turn the smaller axle with less force.

For example, a doorknob is easier to turn than trying to turn the small rod inside the door with your fingers.

How simple machines help

Simple machines can help in different ways:

• Reduce force: You use less push or pull.
• Change direction: You pull down to lift something up.
• Increase distance: You move farther so the job feels easier.
• Help control motion: You can move things more smoothly.

Mechanical advantage in simple words

If a machine helps you use less force, it has mechanical advantage.

We can write:

$$\text{mechanical advantage} = \frac{\text{output force}}{\text{input force}}$$

Let’s say a machine helps you lift a load of 20 newtons by using only 10 newtons of force.

$$\text{mechanical advantage} = \frac{20}{10} = 2$$

This means the machine gives you a mechanical advantage of 2.

Worked Example 1: A ramp

Maria wants to move a heavy box into a truck. She can lift it straight up, or she can push it up a ramp.

Question: Why is the ramp helpful?

Answer: The ramp is an inclined plane. It helps Maria use less force because she pushes the box over a longer distance instead of lifting it straight up. The ramp gives her a mechanical advantage.

Worked Example 2: A pulley

Jayden uses a pulley to raise a flag.

Question: What does the pulley change?

Answer: The pulley changes the direction of the force. Jayden pulls down on the rope, and the flag moves up. This makes the job easier to do.

Worked Example 3: Finding mechanical advantage

A machine helps lift a 12-newton object using 4 newtons of input force.

Step 1: Write the formula.

$$\text{mechanical advantage} = \frac{\text{output force}}{\text{input force}}$$

Step 2: Put in the numbers.

$$\text{mechanical advantage} = \frac{12}{4}$$

Step 3: Solve.

$$\text{mechanical advantage} = 3$$

Answer: The machine has a mechanical advantage of 3. That means it makes the input force act three times as strong.

Worked Example 4: Choosing the best simple machine

Lena needs to split a piece of wood.

Question: Which simple machine would help most?

Answer: A wedge, such as an axe, would help most. A wedge is used to split or cut objects apart.

Comparing simple machines

1. Lever: Lifts or moves with a bar and fulcrum
2. Pulley: Uses a wheel and rope to lift or change direction
3. Inclined plane: Uses a ramp to move objects upward more easily
4. Wedge: Splits, cuts, or pushes apart
5. Screw: Holds things together or moves by turning
6. Wheel and axle: Makes turning and moving easier

Things to remember

• Simple machines make work easier, but they do not remove work completely.
• If you use less force, you often move the object a longer distance.
• Mechanical advantage tells how much a machine helps.
• Different simple machines help in different ways.

Quick check for yourself

• Is a ramp a simple machine? Yes, it is an inclined plane.
• Does a pulley only lift? No, it can also change the direction of force.
• Is a knife a wedge? Yes.
• Does a simple machine create energy? No.

Summary

Simple machines are tools that make work easier. The six simple machines are the lever, pulley, inclined plane, wedge, screw, and wheel and axle.

Mechanical advantage tells how much a machine helps by reducing the force you need. Machines can reduce force, change direction, or help you move something over a longer distance.

When you understand simple machines, you can see how everyday tools help people lift, move, cut, and build things more easily.