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. 

• 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.

• 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?

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.

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)

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.

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.