3-PS2-1: Balanced and Unbalanced Forces
Also called NGSS 3-PS2-1 · Topic: Forces and Interactions · Physical Science
The Official Wording
Plan and conduct an investigation to provide evidence of the effects of balanced and unbalanced forces on the motion of an object.
Clarification statement: Examples could include an unbalanced force on one side of a ball can make it start moving; and, balanced forces pushing on a box from both sides will not produce any motion at all.
Assessment boundary: Assessment is limited to one variable at a time: number, size, or direction of forces. Assessment does not include quantitative force size, only qualitative and relative. Assessment is limited to gravity being addressed as a force that pulls objects down.
NGSS Lead States, 2013 · Read it on nextgenscience.org · checked October 15, 2026
What 3-PS2-1 Means in Plain English
Third graders plan and run tests that show what happens when forces on an object are equal and when one side wins. They push or pull an object from different sides, change one thing at a time, and watch whether it starts, stops or changes direction.
Each test changes just one thing: how many forces, how strong they are, or which way they point. Students compare forces as stronger, weaker or equal without measuring them, and gravity is simply a pull downward. This leads to magnets in 3-PS2-3.
3-PS2-1 I Can Statements
- I can tell balanced forces from unbalanced forces.
- I can predict which way an object will move when forces are unequal.
- I can plan a test that changes only one thing.
3-PS2-1 Worked Example
A tug test a class can run on a smooth floor with a box of books and two strings.
What happens to a box when two classmates pull on it from opposite sides?
Set It Up: A cardboard box holding a few library books, a string tied to each end, and a strip of tape on the floor marking where the box starts.
- Two students pull their strings gently and evenly at the same time.Box stays on the tape
- One student pulls a little harder while the other keeps the same pull.Box slides toward the harder puller
- Switch roles so the other student pulls harder.Box slides the other way
- Now only one student pulls and the other lets the string go slack.Box slides easily toward the puller
- Record each try with arrows, drawing a longer arrow for a stronger pull.Equal arrows: no motion. Unequal: motion
The box stayed put when the pulls matched and moved toward the stronger pull when they did not.
Why it works: Equal forces in opposite directions cancel out, so only an unbalanced force can start an object moving or change its motion.
Common Mistakes with 3-PS2-1
Thinking a still object has no forces
“Nobody is pulling the box, it's just sitting there.” while both strings are tight
Try this: Point out the two equal pulls on the strings. The box stays still because those pulls are balanced, not because no force is acting.
Changing the strength and the direction together
One pull gets harder and turns sideways in the same try
Try this: Keep the pulls in a straight line and change only how hard one person pulls. Test direction in a separate round.
Believing motion stops when the force runs out
“The box stopped because the pull got used up.”
Try this: Explain that the floor rubbing on the box, called friction, is a force too, and it slows the sliding box to a stop.
Where 3-PS2-1 Fits
Practice 3-PS2-1
Quick Check
- Two kids pull a wagon with equal force from opposite sides. What happens?
- One pull is stronger than the other. Which way does the object move?
- A ball sits still on a table. Are there any forces on it?