Comparing Forces

In this unit, students learn about the tiny particles called atoms that make up all matter in the universe. In this lesson, students compare the effects of different forces on objects. This page serves to highlight the key components of this lesson.

Science Background for Teachers:

This section gives teachers more complex background information about the phenomena being explored in this unit (the Solar System).

Overcoming Gravity to Explore Space

Scientists who study space must consider how gravity affects objects on Earth and in space. For example, when scientists want to send satellites, rockets, and other spacecraft into space, they must design objects that can overcome the pull of Earth’s gravity.

The more mass an object has, the more force is needed to counter gravity’s pull and push the object beyond Earth’s gravitational reach. 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. This causes the object to move. All events, including motion, have a cause. Cause and effect describes the relationship between events or things, where one is the result of the other. For example, when you stand on the ground, you are not moving. The forces acting on you are balanced. Gravity pulls you toward the center of Earth. In reaction, the ground has its own force that pushes back. The effect of these balanced forces is that you don’t sink into the ground.

If you want to move, there must be an unbalanced force acting on you. 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 to make them move than less massive objects.

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 and beyond Earth’s gravitational field.

Supports Grade 3

Science Lesson: Comparing Forces

In this lesson, students apply their knowledge of matter, mass, and gravity to carry out an experiment and analyze the effects of unbalanced forces on the motion of a straw rocket.They discuss how unbalanced forces cause movement and how forces of different strengths affect the movement of a straw rocket.

Science Big Ideas

  • All motion is caused by unbalanced forces.
  • Gravity is a force that can cause motion.
  • A rocket needs an unbalanced force that is strong enough to overcome the force of Earth’s gravity to cause it to launch (move) into space.

Sample Unit CTA-2
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?
  • How does the force of gravity change the direction of a ball that is thrown up in the air?
  • Since gravity pulls on all objects, why don’t we all sink into the ground?
  • Why are the forces acting on you balanced when you are standing still on the ground?
  • What has to happen to cause a stationary soccer ball to move?
  • What will most likely happen if you kick the soccer ball harder?
  • Why do space rockets have engines that push hot gas downward?
  • How does the hot gas pushing downward cause a rocket to move upward?
  • What would happen if a weak force is applied to a rocket?

Common Science Misconceptions

Misconception: Only Earth has gravity.

Fact: All matter has gravity. All objects near Earth’s surface are pulled on by Earth’s gravity because Earth is the most massive object near us.

Misconception: Earth doesn't move in the solar system. 

Fact: The sun’s gravity keeps Earth and the other planets in orbit.

Misconception: If an object is at rest, no forces are acting on it.

Fact: Forces are constantly acting on objects both at rest and in motion. 

Science Vocabulary

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

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; a property of matter

Matter : everything that has mass and takes up space

Orbit: to travel in a circle around an object

Pattern : something that happens in a regular and repeated way

Planet : a body that orbits the sun, is massive enough for its own gravity to make it round, and has cleared out smaller objects around its orbit; there are eight known planets in our solar system

Scale :the size, extent, or importance (magnitude) of something relative to something else

Weight : the gravitational force exerted on an object by a planet or moon

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

Overcoming Gravity

Scientists sent Curiosity to Mars on a rocket in November 2011. The rocket launched from Cape Canaveral in Florida. More than thirteen thousand people watched it take off.

As scientists counted down, there was a loud noise. A trail of fiery gas seemed to push the rocket upward into the sky.

When a rocket takes off, an engine pushes hot gas down toward the ground. In reaction, the rocket moves up through the atmosphere.

The huge upward force is important. There has to be enough force to overcome the pull of Earth’s gravity. A force is a push or pull that acts on an object, changing its speed, direction, or shape.

Balanced & Unbalanced Forces

Forces are all around us. There are forces acting on you right now, even if you can’t feel them. Gravity is a force because it is pulling down on you.

Forces always come in pairs. For example, when you stand on the ground, gravity pulls you down toward the center of Earth. In reaction, the ground has its own force that pushes back up. This keeps you from sinking into the ground.

Forces can also be unbalanced. If the wall is not strong enough, the force of you leaning against the wall will cause the wall to collapse. This is because you applied a greater force to the wall than the wall returned.

The same thing happens when you lean against a wall. You apply a force to the wall. The wall applies the same amount of force in return. In this case, the forces are balanced.


Hands-on Science Activity

In this lesson, students conduct an experiment using a rocket launcher to see how high a straw rocket lifts off the ground when pushed on with strong and weak forces. In the experiment, students collect and analyze data on how far the straw rocket traveled upward when pushed with a strong force compared to a weak force. Students use their data to support the argument that unbalanced forces cause objects to move, while balanced forces do not change an object’s motion. They also discover that a straw rocket lifts higher off the ground when pushed by a strong force compared to a weak force.

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...

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Science Standards

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.

Download the Alignment to NGSS

Standards citation: NGSS Lead States. 2013. Next Generation Science Standards: For States, By States. Washington, DC: The National Academies Press. Neither WestEd nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of this product, and do not endorse it.