A FORCE is a push or a pull.

Without forces nothing would move!

Without ENERGY, there would be no forces.

Forces in the same direction are added together.

Forces in opposite directions are subtracted.


Forces can be balanced or unbalanced.

When forces are BALANCED in opposite directions, the object will either be...

1. at rest (fully stopped)

This motorcycle is parked. It is NOT speeding up. It is NOT slowing down. It is NOT changing directions.


2. moving at a CONSTANT velocity

If this motorcycle is not speeding up, not slowing down, and not changing directions, it MUST be moving at the same speed in the same direction

When forces are UNBALANCED in opposite directions the object will accelerate.

Acceleration can mean one of 3 things:

1. speed up
2. slow down
3. change direction


Play lunar lander to practice using balanced and unbalanced forces

As you play, turn on "vectors" and notice when the forces are balanced and when they are unbalanced.

Another fun game like this is Monkey lander.


Friction always opposes motion.

Objects can only move if a force is great enough to overcome friction. All motion will have some amount of friction. (There may be moments in space travel when a rocket doesn't run into anything but there is still plenty of "stuff" to run into out there)

There are 3 types of friction for you to recognize.
Static Friction
Kinetic (sliding) friction
Rolling Friction

When two objects are touching but not moving. This block can stay on the inclined plane if there is enough friction. If not it will slide down.

When one object moves across another or when two objects are moving in opposite directions.

When a cylinder like a tire rolls across a surface.


I like to think of opposite forces as arch enemies. One is always working against the other. For example, if Joker tried to pull something in Gotham city, Batman would be working to undo what was done. If Batman tries to move something one way, Joker would want to move it the opposite way.

BATMAN represents a PUSH or PULL from us.

JOKER represents FRICTION in the opposite direction




There are 4 forces that move the universe.

(listed in order of strength weakest to greatest)



(+ and -, North and South)


Just how strong are these forces ?

(NOT to be memorized, unless you REALLY want to)

If gravity had a strength of..
The "weak" force would be given a strength of...
The electromagnetic force would be given a strength of...
The STRONG force would be given a strength of...


1. Gravity - the attraction between any two masses in the universe.

It is written as an acceleration ("meters per second squared") that tells us the rate at which something will freefall towards the surface.


Mass - the amount of matter in something

(measured in grams)

Weight - the AMOUNT of gravitational FORCE between two masses.

(measured in Newtons or silly pounds)



Only two things can change the amount of gravity:
1. mass
2. Distance.

If the mass of one or both of the objects is decreased, then the gravity between them is also decreased.

If the distance between the objects is increased, then the gravity between them is decreased.


Gravity is ALWAYS a downward force.

Gravity is the attraction between two objects. Because the Earth is the closest massive thing to us, everything is attracted towards the center of the Earth.


GOBLIN represents the downward force GRAVITY.

SPIDER-MAN represents a lifting force against gravity.


When we LIFT an object, we are fighting against GRAVITY.

Lifting an object means using a force greater than the object's WEIGHT due to gravity.

Weight is the mass of an object multiplied by the gravity that it is near. It is Newton's 2nd Law of Motion and is written as...


To use our Superheroes...Spiderman does not fight the joker. They are not opposites.


When we PUSH or PULL an object we fight against FRICTION.

Gravity is always present but it is FRICTION that keeps us from pushing heavy furniture across the carpet. If we can cut down on the friction it will take less force to push or pull the object across the floor. Though gravity can play a part in the friction it is not the force we are "in battle" with.

To use our superheroes...Batman does not fight the goblin. They are not opposites.


Understanding Gravity has taken CENTURIES of discoveries.

Nicolaus Copernicus (1473 – 1543) identified that the Sun was at the center of our solar system and that the planets orbited around it. Most people in his day thought Earth was the center of the universe and that everything revolved around us. It's an easy mistake to make when you don't have a perspective from space. It certainly LOOKS like the universe revolves around the Earth from our point of view. Copernicus knew that his idea was controversial so he only had his works published after he had died.


One complete orbit around the sun is called a YEAR.

So, if you are twelve years old (and living on EARTH), you've made twelve complete trips around our sun.

Not all planets orbit at the same speed. Planets that are closer to the sun are more attracted due to gravity and orbit faster than planets that are on the outer edge of the solar system.

How old would you be if you lived on Mercury?...on Jupiter?

Calculate your age on other worlds

What does it really mean to "orbit" something?

The Space Shuttle orbits Earth when two things happen at the same time...

1) The Shuttle is attracted to the planet. We call this "falling". (green arrow)

2) The Shuttle is moving forward. It was given this velocity by being rocketed into space. (red dashed line)

The shuttle CANNOT move in two directions at once. The purple arc shows what happens when you fall to Earth WHILE moving forward.


Galileo Galilei (1564 – 1642) observed that all masses regardless of weight will fall to the Earth at the same rate. The only way to change that is with air resistance. The moon has no air, so air resistance is not a problem.

Galieo's work was also very controversial in that he added proof to the idea that the Earth was not at the center of the universe. He looked through his telescope and saw shadows on the moon's surface, phases of Venus (like we see with the Moon), and he saw moons that circled around Jupiter. All of these led him to believe that there was a force that held planets and moons together.

Legend tells of Galileo and his students dropping objects off of the Leaning Tower of Piza in Italy in order to determine if objects fall at different rates.

They found that ALL similar masses accelerated the same. The only way to slow something down is with FRICTION.

Want more proof? Watch the actual video of Apollo 15 astronauts dropping a hammer and a feather on the moon.





Johannes Kepler (1571 – 1630) discovered that objects in space don't always travel at the same rate. THE FORCE OF GRAVITY INCREASES WHEN OBJECTS ARE CLOSER TOGETHER.

As masses get closer to the Sun, they speed up. As the masses get further away from the Sun, they slow down. (This was why you would be different ages on different planets)

Not all orbits are perfect circles. In fact, most are not.

play with "my solar system" to see just how gravity works when objects are moving.



Sir Isaac Newton (1643 – 1727) discovered the relationship between mass, weight, and gravity. Whether or not the apple hit him on the head, Newton was the first to describe gravity and weight mathematically.

If we were to stand on a different planet, we would be attracted to it at a different rate than Earth. Large planets would make us heavier while smaller planets make us lighter.

Weight (or force) = mass X gravity

Calculate your weight on other worlds

A few years ago I wrote a poem called Ode to Forces. I think it makes a pretty good review guide :)


Here's some interesting information and pictures about our Solar System



3. the Electromagnetic force is the attraction and repulsion between electric charges and magnetic poles.

Opposite electric charges attract

Like electric charges repel

Opposite magnetic poles attract

Like magnetic poles repel

We can charge up an object with electrons (-) through friction. Rubbing a balloon on a sweater will move electrons and give the balloon a negative charge. Watch what happens to the electrons in the wall when you drag the balloon close to it!

Play the simulation with balloons, a sweater, and a wall.

Play ELECTRIC HOCKEY to practice attraction and repulsion of electric charges.



4. the Nuclear Strong force holds the nucleus of the atom together. It is fueled by nuclear energy.


Protons are all positive and would love to repel each other.

The nuclear strong force keeps every nucleus together.

You must have energy to exert a force. Nuclear energy (last unit) powers the strong force!

Imagine squeezing a Nerf ball. The ball "wants" to expand to its original shape but cannot because the force of your hand is too great.

The nuclear strong force is like an invisible hand that holds the nucleus (Nerf ball) together. Without it, there would only be Hydrogen in the universe. Larger elements could not exist.

Uncontrolled, this force can be used for very destructive purposes.

When controlled, this force can be used to generate electricity.

Remind yourself of this section on nuclear energy


Friction: the "force" that opposes motion

You might be thinking that there is a fifth force here. While friction definitely acts like a force it is really an ELECTROMAGNETIC force in disguise. The electrons of one group of atoms are repelling the electrons of another group of atoms. THEY WANT THEIR OWN SPACE! When they are pressed together they fight back with HEAT. Your shoe doesn't pass through the floor because the electrons of the shoe can't pass through the space of floor's electrons.