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Two teams in a tug of war on a grass field with a red ribbon on the rope hovering over the center line.

Science / All lessons / Forces and Motion

Equal and Unequal Forces

Tug of war and what the rope tells you.

Lesson 14 of 41

Try this first

Before we start

Today’s story

Equal and Unequal Forces

Picture a tug of war at a school picnic. Both teams strain. The ribbon stays right over the line. Nobody is winning.

A force is a push or a pull. When two forces on an object are equal and opposite, they are balanced. The object does not change its motion. When one force is bigger, they are unbalanced. The object speeds up, slows down or turns.

Patterns of motion come from these forces. Let us see how.

See it

Five steps. Every time.

    Hands on

    Spring Scale Tug

    25 minutes · Do this with a grown-up nearby

    You need

    • Two spring scales
    • String
    • A partner
    • Paper and pencil

    Do this

    1. Hook the two scales end to end.
    2. You and a partner each hold one scale and pull gently.
    3. Read both scales at the same time.
    4. Pull harder with one person and note the readings.
    5. Let the pull go slack and note what happens.
    6. Write what you see.
    Write or draw: A table of your pull, your partner's pull and what moved.

    Watch for: When the readings match, the scales stay still. What changes when one reading is larger?

    Safety: Pull gently so the scales do not snap back.

    Same every time

    Three rules

    Rule 1Look at both sides

    A force is part of a pair of pushes or pulls.

    Rule 2Compare the sizes

    Bigger force, bigger change.

    Rule 3Check for motion

    If nothing changes, forces are balanced.

    Grandpa Fenn’s notebook

    Say it like a scientist

    force
    A push or a pull.
    balanced
    Equal and opposite, so there is no change in motion.
    unbalanced
    Not equal, so the motion changes.
    motion
    A change in position.

    Poppy’s quiz

    Six questions

    Tap an answer. Green means right. You will see why.

    Talk about it

    Say it out loud

    • How do you know the forces were balanced?
    • Where have you seen a tug of war or a stuck truck at home?
    Did you know

    A book resting on a table has gravity pulling it down and the table pushing up with the same force.

    Someone who does this

    A physical therapist uses forces and balance to help people move better.

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