Idea 6of 8Builds onThe touching testEarth pulls everything
A perfect draw
Two forces act on the book. Why does nothing happen?
Today you'll discover
- what two matching arrows add up to
- why still is not the same as untouched
- the word for forces that cancel
Think firstTwo arrows: one down, one up, the same length
Earth pulls down, the table pushes up, equally. What does the book do?
Decide first, then press Next. Watch the ring appear beside the book.
A box on a heavy line with two arrows of equal length from its centre, one down and one up. After a prediction, a small ring appears beside the box, marking that the two arrows add to zero. The last step hangs the box from a cord and asks how the cord's pull compares with Earth's.
See the whole ideathe big idea and the key word, in one place
The big idea
Balanced forces add to zero; the motion does not change.
Easy to get wrongBalanced is not the same as still. A crate sliding at a steady speed has balanced forces too. Balanced means no change.
Key word
- balanced
- forces on one object that add to zero; the object's velocity stays as it is
About this modelArrows are forces, not motion: a box can move with no arrow pointing its way.
Arrows are forces, not motion: a box can move with no arrow pointing its way.
Ready to use it?
- Say whether the forces on a described object are balanced, and what that means for its motion.