# Balanced vs. Unbalanced Forces

In this unit, students analyze the phenomena of matter, forces, and energy. In this lesson, they compare the effects of unbalanced versus balanced forces on objects. They then evaluate how matter interacts with and is changed by energy, which transfers from one object or system to another. This page showcases key components of this lesson.

## Science Background for Teachers:

Science background provides teachers with more in-depth information on the topic students explore in this unit. Below is an excerpt of the science background section for the lesson on balanced and unbalanced forces.

Gravity plays a role in how long it takes a planet to orbit the sun. The closer a planet is to the sun, the stronger the gravitational pull and the faster the planet moves around the sun. Planets that are farther from the sun have a slower orbit because the gravitational pull weakens. Earth takes 365 days—one Earth year—to orbit the sun.

Here on Earth, the most massive object is Earth itself. For this reason, Earth’s gravity is the dominant force, and it pulls all objects near Earth’s surface toward the planet’s center. The pull of Earth’s gravity also keeps the space station, as well as our moon, in orbit. The space station usually takes about 90 minutes to orbit Earth because of how rapidly it travels. This is why astronauts aboard the space station see the sun rise and set 15 times every day.

Far above the space station, 384,000 kilometers (239,000 miles) above Earth, the moon also orbits Earth. The moon orbits Earth for the same reason that the planets orbit the sun. Earth is so massive that its gravity pulls the moon toward it. The moon takes about 29.5 days to orbit Earth, which equals one Earth month. These predictable motions cause patterns that we can see from Earth, and that astronauts can see from space.

Scientists who study space must consider how gravity affects objects on Earth and in space. Gravity is a force, which means that it can cause an object to move when it provides a greater pull than the other forces acting on the object. This is because unbalanced forces are needed to change an object’s motion. Unbalanced forces occur when the forces acting on an object are not equal. The sun’s gravity is so strong that it exerts an unbalanced force on the eight planets of the solar system, pulling them into orbit around it.

Here on Earth, the force of Earth’s gravity pulls you toward the center of Earth. However, the ground has its own force that pushes back with an equal and opposite force. This keeps you from sinking into the ground. The forces acting on you are balanced.

If you want to move, there must be an unbalanced force acting on you. This is because all motion requires a cause. Picture a swing. If you just sit on the swing, you won’t move. However, if someone pushes you, you will start to move. That person provided an unbalanced force. More massive objects need greater force than less massive objects to make them move.

When astronauts travel to the International Space Station, they ride in a rocket that lifts off of Earth with enough force to overcome the pull of Earth’s gravity. Rockets that are launched into space apply a tremendous amount of force. As the rocket pushes combustion exhaust downward, the exhaust pushes the rocket upward with enough force to overcome the pull of Earth’s gravity.

## Science Lesson: Discovering Balanced vs. Unbalanced Forces

In this lesson, students explore how forces act on matter, focusing on gravity, which is an attractive force between all matter. They use that knowledge to analyze how objects change their motion when acted on by an unbalanced force, and apply their analysis to motion in the solar system.

## Science Big Ideas

• Gravity is a force of attraction between all matter.
• Forces are pushes or pulls that act on an object, changing their speed, direction, or shape.
• When the forces acting on an object are balanced, the object’s motion will not change. An unbalanced force is necessary to cause movement.
• Earth’s gravity pulls everything near Earth’s surface downward toward Earth’s center with the same amount of force. This downward pull keeps you from floating off into space.
• Mass is the measure of the amount of matter that makes up a substance. Weight is the gravitational force exerted on an object by a planet or moon. Here on Earth, weight is calculated by multiplying the object’s mass by the force of Earth’s gravity.
• Earth’s gravity pulls down on both sides of a pan balance equally, so the difference between the objects on each side of the pan is due to their mass.

Discover Complete Hands-on Screens-off Core Science Curriculum for K-8 Classrooms

Prepared hands-on materials, full year grade-specific curriculum, and personalized live professional development designed to support mastery of current state science standards.

## Science Essential Questions

• Why is gravity a force?
• Why does Earth’s gravity keep you and all other objects near Earth’s surface from floating off into space?
• What is the relationship between gravity, mass, and weight?
• What is an example of an unbalanced force you’ve experienced causing movement?
• How is the force of gravity connected to the fact that astronauts lose muscle mass in space?

## Common Science Misconceptions

Misconception: Weight and mass are the same thing.
Fact: Weight and mass are two different measurements. Mass measures the amount of matter in a substance, while weight is a gravitational force exerted on an object by a planet or moon.
Misconception: If something isn’t moving, there are no forces acting on it.
Fact: Forces act on objects all the time. When all of the forces acting on an object are balanced, the object will not change its motion.

## Science Vocabulary

Cause and Effect: a relationship between events or things, where one is the result of the other

Energy :  the ability to do work (move an object, heat up an object, charge an object, etc.)

Force:  a push or pull that acts on an object, changing its speed, direction, or shape

Gravity:  a force of attraction between all matter

Mass: a measure of the amount of matter that makes up an object; measured in grams (g)

Pattern: something that happens in a regular and repeated way

Speed: the rate at which an object covers distance in a period of time; measured in meters per second (m/s)

System: a set of connected, interacting parts that form a more complex whole

Weight: a gravitational force exerted on an object by a planet or moon; measured in newtons (N)

## Lexile(R) Certified Non-Fiction Science Reading (Excerpt)

Running in Space

Sunita Williams is an astronaut who has lived on the International Space Station. She was the first person to run the Boston Marathon from space. It took her 4 hours, 23 minutes, and 10 seconds to run the 42 kilometers (26.2 miles).

There are some challenges to running in space. One challenge is that there is much less gravity on the space station than there is on Earth. Gravity is a force of attraction between all matter.

In the space station, there is much less gravity than on Earth. As a result, Sunita had to attach herself to the treadmill with bungee cords. Without the bungee cords, she would float.

On Earth, the force of gravity pulls all objects near Earth’s surface downward. A force is a push or pull that acts on an object, changing its speed, direction, or shape. Earth’s gravity keeps you from floating off into space. It is also why things thrown in the air fall back to the ground.

Mass vs. Weight

All matter has gravity, even the matter that makes up people. The reason a person doesn’t noticeably attract nearby objects is because more massive objects have more gravity. Earth’s gravity pulls on all objects on or near Earth’s surface because Earth is so massive.

The gravitational force exerted on an object by a planet or moon is called weight. It is measured in newtons (N). Here on Earth, weight is calculated by multiplying the object’s mass by the force of Earth’s gravity. The pull of gravity is nearly identical everywhere on Earth.

Non-scientists often think that an object’s weight is the same as its mass. The two are related because an object’s weight depends on its mass. However, weight also depends on the force of gravity.

When astronauts travel to the moon, their mass doesn’t change, but their weight does. This is because the moon is much less massive than Earth is. As a result, its gravitational force isn’t as strong as Earth’s.

Forces Cause Movement

When you stand on Earth’s surface, gravity pulls you down toward the center of Earth. In reaction, the ground has its own force that pushes back up with the same amount of force. This keeps you from sinking into the ground. When the forces acting on an object are equal, they are balanced.

If the ground did not push back with the same amount of force, you would fall into the ground. In this case, the forces would be unbalanced. Gravity would pull you down with a greater force than the force of the ground pushing back up.

When all of the forces acting on an object are balanced, the object will not change its motion. If you want to move, there has to be an unbalanced force acting on you. This is because all events, including motion, require a cause. Unbalanced forces cause motion.

## Hands-on Science Activity

In this lesson, students build and use pan balance models to compare an unknown mass to a known, standard mass. Students analyze how the number of paper clips on one side of the pan balance determined whether the pan balance was balanced or unbalanced. Through this investigation, students explore how gravity is an attractive force that provides an unbalanced force in the solar system, causing the objects in the solar system to move.

## Science Assessments

KnowAtom incorporates formative and summative assessments designed to make students thinking visible for deeper student-centered learning.

• Vocabulary Check
• Lab Checkpoints
• Concept Check Assessment
• Concept Map Assessment
• And More...

### See How KnowAtom Aligns to NGSS Science Standards

Discover hands-on screens-off core science curriculum for student centered K-8 classrooms. KnowAtom supports classrooms with all hands-on materials, curriculum, and professional development to support mastery of the standards.