Forces are balanced when they cancel each other out. The object does not change how it is moving.
Balanced forces Forces that cancel out, leaving no overall push or pull.
Step 1: Something to Notice
Watch first. The explanation comes later.
Two teams pull a rope with everything they have. Nobody moves. Then one person on the left lets go, and the whole thing slides.
The forces were huge the whole time. What actually changed?
Step 2: Find Out
Set both pulls to 9. Then take one down by a single unit.
Step 1 — Predict
A pull of 9 to the left and 9 to the right. What does the block do?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 3: So Here Is Why
Now the explanation, after you have seen it happen.
Unbalanced forces
When one side is bigger, there is some force left over. That is what changes the motion.
Unbalanced forces Forces that do not cancel, leaving a push or pull left over.
It is the difference that counts
9 against 9 does nothing. 9 against 8 moves it. The size of each force on its own tells you nothing.
Balanced forces are still real
A book resting on a table has two forces on it the whole time. They cancel, but neither has stopped.
It is the difference that moves things
Two teams pulling equally hard on a rope go nowhere, even though both are pulling hard. Nothing moves because the forces cancel. Motion changes only when one side wins.
Still does not mean no forces
A book on a table has gravity pulling it down and the table pushing it up. Both forces are real and they balance, so the book stays put. Being motionless is not the same as being force-free.
Step 4: A Common Mistake
Lots of people think
“A bigger force always wins, so a big pull will always move something.”
Step 5: Balanced Does Not Mean Nothing Is Happening
The same idea somewhere new.
A book on a table has gravity pulling it down and the table pushing it up, both all the time. The forces are balanced, so it does not move — but neither force has gone away. Take the table out and you find out immediately.
Step 6: Think It Through
Practice makes it stick.
The Tug of War
Problem 1 of 2
The red team pulls with 7 and the blue team pulls with 7. How much force is left over?
units
One Person Lets Go
Problem 2 of 2
Now the blue team drops to 5 while red stays at 7. How much force is left over, and which way?
units toward red
Balanced or Not?
1 of 4
A pull of 6 left and 6 right. What is the leftover force?
2 of 4
A pull of 10 left and 3 right. What is the leftover force?
3 of 4
Which of these will move the block?
4 of 4
A book sits still on a table. Are there forces on it?
Step 7: Quick Check
Show what you know.
Question 1 of 1
Which picture shows forces that will move the block?
Step 8: Explain It in Writing
Claim, evidence, then reasoning.
The question
Using your two runs, write an argument answering the question: what makes the block move — how big the pulls are, or the difference between them?
Evidence you can use
- With 9 pulling left and 9 pulling right, every row of the table said 0 cm.
- With 8 pulling left and 9 pulling right, the block had moved 50 cm after 5 seconds.
- A pull of 9 is nearly the biggest the control goes to, and two of them moved the block nothing at all.
- The surface was the same in both runs. The only thing I changed was one pull.
- The difference between the pulls went from 0 to 1, and the block started moving.
- Forces that are equal cancel each other out, so nothing is left over.
- A book resting on a table has gravity pulling it down and the table pushing it up.
Words to usebalancedunbalanceddifferencecancelleft overthe same
Fill in all three boxes. The reasoning box is the one that matters most.
What You Found Out
- Balanced forces cancel out and nothing changes.
- Unbalanced forces leave something over, and that moves the object.
- It is the difference that matters, not how big each force is.