Idea 6of 6Builds onEvery push is a pairA perfect draw
Pairs act on different things
Equal and opposite: so why don't the pair cancel?
Today you'll discover
- why two equal, opposite forces may not cancel
- the one question that sorts a pair from a draw
- what Earth's pull is paired with
Think firstThe book presses the table; the table presses back
Book on table, table on book: equal and opposite. Do they cancel?
Decide first, then press Next. Ask of each arrow which object it acts on.
Two labelled shapes, a book and a table, with both arrows of the pair drawn between them. After a prediction, the arrow acting on the table is picked out, then the arrow acting on the book, then both with the touching line marked. The last step asks for the partner of Earth's pull on the book.
See the whole ideathe big idea and the key word, in one place
The big idea
Partner forces act on two different objects, so they never cancel.
Easy to get wrongThe table's push up on the book is not the partner of Earth's pull down on the book. Both act on the book. The partner of Earth's pull is the book pulling Earth.
Key word
- action–reaction pair
- the two forces of one interaction: equal in size, opposite in direction, each acting on a different object
About this modelThe two arrows act on two different objects. They never cancel each other.
The two arrows act on two different objects. They never cancel each other.
Ready to use it?
- Sort a described set of forces into third-law partners and balanced pairs.