Conduct investigations that demonstrate forces acting at a...

SECTION 1

Historical Context & Motivation

Have you ever held two magnets near each other? You can feel them pull together or push apart—even though they never touch. This invisible push or pull is called a non-contact force (a force that acts on an object without physical contact). For thousands of years, people were amazed and puzzled by these mysterious forces.

Ancient Greek thinkers noticed that rubbing amber with fur could attract feathers. They also knew that certain rocks, called lodestones (naturally magnetic rocks), could pull iron toward them. But nobody could explain why. It took centuries of investigation before scientists figured out the rules behind these invisible forces.

~600 BCE

Ancient Observations

Greek philosopher Thales of Miletus observed that rubbed amber attracts lightweight objects. He also noted that lodestones attract iron.

1600

William Gilbert's Experiments

English scientist William Gilbert published a book separating magnetic forces from electric forces. He showed Earth itself acts like a giant magnet.

1687

Newton's Law of Gravity

Isaac Newton published his law of universal gravitation. He explained that every object with mass pulls on every other object with mass—even across empty space.

1785

Coulomb's Law

Charles-Augustin de Coulomb measured the force between electric charges. He found that it follows a pattern very similar to gravity.

1831

Faraday's Field Concept

Michael Faraday introduced the idea of invisible force fields surrounding magnets and charges. This idea helped scientists picture how forces act at a distance.

Today, scientists understand three main non-contact forces: gravitational, electric, and magnetic. The big question we'll explore is: How can we investigate and prove that these forces really act at a distance?

SECTION 2

Core Principles of Non-Contact Forces

A force is a push or a pull on an object. Some forces need touching, like kicking a ball. But non-contact forces work across a gap. There are three types you need to know.

Gravitational Force

Every object with mass (the amount of matter in an object) pulls on every other object with mass. This is why you stay on the ground. Gravity is always attractive—it only pulls, never pushes.

Electric Force

Objects with electric charge (a property of matter caused by extra or missing electrons) push or pull on each other. Opposite charges attract. Like charges repel (push away).

Magnetic Force

Magnets have a north pole and a south pole. Opposite poles attract. Like poles repel. Magnets can also attract certain metals like iron.

Force Fields

Scientists use the idea of a field (an invisible area around an object where a force can act) to explain how forces work at a distance. Every mass, charge, and magnet creates a field around itself.

Strength Depends on Distance

All three non-contact forces get stronger when objects are closer together. They get weaker when objects move farther apart. This is a key pattern you can test in an investigation.

✦ KEY TAKEAWAY

Think of non-contact forces like Wi-Fi. Your phone connects to the router without any wire. The signal is strongest when you are close to the router and weaker when you walk to the other side of the house. Non-contact forces work the same way—the effect is real even though nothing is touching, and distance matters a lot.

SECTION 3

Visualizing Forces at a Distance

The diagram below shows the three main non-contact forces and the invisible fields they create. Notice how arrows represent the direction each force pushes or pulls. Fields spread out in all directions from the source object.

This diagram compares the three non-contact forces. Gravitational force always pulls objects together. Electric force can attract or repel depending on charge. Magnetic force can attract or repel depending on pole orientation.

Look at the arrows in the diagram. They show the direction of each force. Gravity always points toward the larger mass. Electric and magnetic forces can point toward or away from each other. The dashed circle around Earth represents the gravitational field—the invisible region where gravity can pull on other objects.

🔬 Anchoring Phenomenon

When you rub a balloon on your hair and hold it near small pieces of paper, the paper jumps up to the balloon—even though the balloon never touches the paper! This everyday event is evidence of an electric force acting at a distance. Throughout this lesson, you will learn how to design investigations to study this and other non-contact forces.

SECTION 4

How Non-Contact Forces Work

Scientists explain non-contact forces using the concept of force fields. A field is an invisible area around an object where a force can act on another object. You cannot see a field, but you can observe its effects.

How Distance Affects Force Strength

All three non-contact forces follow the same important pattern: as distance increases, the force decreases. Scientists call this an inverse relationship (when one value goes up, the other goes down). If you double the distance between two magnets, the magnetic force drops to about one-fourth of what it was.

GRAVITATIONAL FORCE (SIMPLIFIED IDEA)

Force gets weaker as distance gets larger

If you move two objects twice as far apart, the gravitational pull between them drops to one-fourth (¼) of its original strength. Three times the distance means one-ninth (¹⁄₉) of the original force.

GENERAL PATTERN

F ∝ 1 / d²

F = force (how strong the push or pull is). d = distance between the two objects. The symbol ∝ means "is proportional to." This tells us force decreases rapidly as distance increases.

How Size and Strength Affect Force

Distance is not the only factor. For gravity, more mass means a stronger pull. For electric forces, more charge means a stronger push or pull. For magnetic forces, a stronger magnet creates a stronger force. In your investigations, you can change these variables to see how the force changes.

🔗 Crosscutting Concept: Cause and Effect

When you change the distance between two magnets (the cause), the strength of the force changes (the effect). In science, we always look for cause-and-effect relationships. A good investigation changes only one variable at a time to see its effect clearly.

SECTION 5

Designing Investigations for Non-Contact Forces

To show that forces act at a distance, you need to design a fair test (an experiment where only one variable changes at a time). Below is a diagram showing three investigation setups you could build in a classroom.

Three classroom investigations for non-contact forces. Investigation 1 tests how distance affects the number of paper clips a magnet can lift. Investigation 2 tests how a charged balloon attracts paper at different distances. Investigation 3 tests how mass affects gravitational force using a spring scale.

Planning a Fair Test

Every good investigation has three types of variables. The independent variable (the thing you change on purpose) could be distance or mass. The dependent variable (the thing you measure) is usually the strength of the force. The controlled variables (things you keep the same) include the type of magnet, the size of paper, or the spring scale used.

Variables for three non-contact force investigations

Investigation	Independent Variable	Dependent Variable	Controlled Variables
Magnet & Paper Clips	Distance between magnet and clips	Number of paper clips picked up	Same magnet, same clips, same surface
Charged Balloon	Distance between balloon and paper	Whether paper pieces move toward balloon	Same balloon, same paper, same rubbing time
Spring Scale & Mass	Mass of the hanging object	Force reading on spring scale (in Newtons)	Same spring scale, same location

🧪 Science & Engineering Practice: Planning Investigations

Real scientists follow the same process you are learning. They ask a question, identify variables, collect data, and look for patterns. When you plan an investigation about non-contact forces, you are practicing what scientists and engineers do every day.

SECTION 6

Worked Example: Magnet Investigation

Let's walk through a complete investigation step by step. Imagine you are testing how distance affects the strength of a magnet's pull on paper clips.

How Does Distance Affect Magnetic Force?

Step 1 — Write Your QuestionStart with a clear, testable question: "How does increasing the distance between a bar magnet and a pile of paper clips change the number of clips the magnet can pick up?"

Step 2 — Identify Your VariablesIndependent variable: Distance between the magnet and the paper clips (1 cm, 2 cm, 3 cm, 4 cm, 5 cm). Dependent variable: Number of paper clips picked up. Controlled: Same magnet, same type of clips, same surface.

Step 3 — Collect DataRun three trials at each distance and record results in a data table. Trial 1 at 1 cm: 12 clips. Trial 2: 11 clips. Trial 3: 12 clips.

Step 4 — Calculate the AverageAverage at 1 cm = (12 + 11 + 12) ÷ 3 = 35 ÷ 3 ≈ 11.7 clips. Repeat this for every distance.

Average at 1 cm ≈ 11.7 paper clips

Step 5 — Analyze the DataSuppose your averages are: 1 cm = 11.7, 2 cm = 7.3, 3 cm = 4.0, 4 cm = 1.7, 5 cm = 0.3. The data shows a clear pattern: as distance increases, the number of clips decreases. This is evidence that magnetic force gets weaker with distance.

Conclusion: Magnetic force acts at a distance and gets weaker as distance increases.

Sample data: Number of paper clips picked up at each distance

Distance (cm)	Trial 1	Trial 2	Trial 3	Average
1	12	11	12	11.7
2	8	7	7	7.3
3	4	4	4	4.0
4	2	1	2	1.7
5	1	0	0	0.3

SECTION 7

Comparing the Three Non-Contact Forces

Gravitational, electric, and magnetic forces all act at a distance. But they have important differences. The table below compares their key features.

Comparison of the three non-contact forces

Feature	Gravitational	Electric	Magnetic
What causes it?	Mass	Electric charge	Magnetic poles or moving charges
Attract or repel?	Attract only	Both attract and repel	Both attract and repel
Affected by distance?	Yes—weaker with more distance	Yes—weaker with more distance	Yes—weaker with more distance
Relative strength	Very weak (need huge masses)	Very strong	Strong (related to electric force)
Everyday example	Apple falling from a tree	Static cling on clothes from a dryer	Fridge magnets sticking to a door
Easy to investigate?	Harder—gravity is weak at small scales	Yes—rub a balloon!	Yes—use bar magnets

✦ KEY TAKEAWAY

All three non-contact forces share the same pattern: they get weaker with distance. Think of it like the volume of a speaker. When you stand right next to it, the sound is loud. As you walk away, it fades. Non-contact forces "fade" with distance in a similar way, but they follow a specific mathematical rule (the inverse-square relationship).

SECTION 8

Connection to Advanced Ideas

In middle school, you observe non-contact forces and measure their effects. In high school and beyond, you will learn to calculate these forces precisely using mathematical equations. Here is a preview of how the ideas connect.

How middle school concepts connect to high school physics

What You Learn Now	What Comes Next
Gravity pulls all objects with mass toward each other.	Newton's Law of Universal Gravitation: F = G × m₁ × m₂ / d²
Electric charges attract or repel at a distance.	Coulomb's Law: F = k × q₁ × q₂ / d²
Forces get weaker with more distance.	The inverse-square law applies to gravity, electricity, light, and sound.
Fields are invisible areas around objects.	Field theory: Electric, magnetic, and gravitational fields are described with vectors and field lines.

The investigations you do now build the foundation. When you see that magnetic force weakens with distance, you are discovering the same pattern that Newton and Coulomb described with their famous equations. Your data and observations are real science!

🔗 Crosscutting Concept: Patterns

One of the most powerful ideas in science is that patterns repeat across different systems. The same inverse-square pattern shows up in gravity, electricity, magnetism, and even how light spreads out from a lamp. Recognizing patterns helps scientists predict how the world works.

SECTION 9

Practice Problems

PROBLEM 1 — CONCEPTUAL

A student holds a magnet 2 cm above a pile of paper clips and picks up 8 clips. Then the student holds the magnet 6 cm above the same pile and picks up only 1 clip. Which statement best explains this observation? A) The magnet lost its magnetism. B) The paper clips became heavier. C) Magnetic force decreases as distance increases. D) Gravity pulled the paper clips down harder the second time.

PROBLEM 2 — BASIC CALCULATION

A student collects this data on paper clips picked up by a magnet: • Trial 1: 10 clips • Trial 2: 12 clips • Trial 3: 11 clips What is the average number of clips picked up? A) 10 B) 11 C) 12 D) 33

PROBLEM 3 — INTERMEDIATE

Maria wants to investigate whether the type of magnet affects how many paper clips it can attract at a distance. She uses three different magnets (bar, horseshoe, disc) and holds each one 3 cm above a pile of clips. Which variable should she keep the same across all three tests? A) The type of magnet B) The number of clips she starts with in the pile C) The number of clips picked up D) The type of force being tested

PROBLEM 4 — APPLIED

A student rubs a balloon on her sweater and holds it near a stream of water from a faucet. The water bends toward the balloon even though the balloon never touches the water. The student then moves the balloon farther away and the water bends less. Which type of force is acting, and what evidence supports this? A) Gravitational force, because water always falls down. B) Contact force, because the balloon must be touching air molecules. C) Electric force, because the charged balloon attracts the neutral water without touching it and the effect weakens with distance. D) Magnetic force, because water contains metal ions.

PROBLEM 5 — CRITICAL THINKING

Two students are debating. Alex says: "Gravity is not a non-contact force because the atmosphere connects everything on Earth—objects are always touching air." Jordan says: "Gravity is a non-contact force because it works even in the vacuum of space where there is no air at all." Which student is correct and why? A) Alex is correct because air transmits all forces on Earth. B) Jordan is correct because gravity works between any two masses regardless of what is between them. C) Both are correct because gravity works differently on Earth than in space. D) Neither is correct because gravity only acts on contact.

SUMMARY

Lesson Summary

Non-contact forces are pushes or pulls that act on objects without touching them. The three main types are gravitational force (caused by mass, always attracts), electric force (caused by charge, can attract or repel), and magnetic force (caused by magnetic poles, can attract or repel). All three forces create invisible fields around objects and get weaker with increasing distance.

You can investigate these forces by designing fair tests that change only one variable at a time. By recording data across multiple trials and calculating averages, you can find patterns such as the inverse relationship between distance and force strength. These investigations connect to the crosscutting concepts of Cause and Effect and Patterns, which scientists use across all areas of science.