Balanced vs. Unbalanced Forces

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.