Idea 7of 8Builds onThe touching test
Draw every force
What belongs on a drawing of the forces on something?
Today you'll discover
- the one object every drawing is about
- where each arrow starts
- what never goes on the drawing
Think firstA crate pushed along a floor. Which arrows belong?
You draw the forces on the crate. What goes on the drawing?
Decide first, then press Next. Every arrow starts at the dot in the middle.
A box with a dot at its centre on a floor. After a prediction, arrows grow from the dot one at a time: Earth's pull down, the floor's push up with the hand's push to the right, then the floor's drag to the left. The finished drawing is named. The last step hangs the box from a cord and asks how many arrows its drawing needs.
See the whole ideathe big idea and the key word, in one place
The big idea
One object, every force on it, nothing else.
Easy to get wrongThere is no arrow for motion. A moving crate can have arrows that all cancel. The drawing shows pushes and pulls, not where the object is going.
Key word
- free-body diagram
- a drawing of one object as a dot or box with every force on it as an arrow from its centre, to scale
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?
- Draw the free-body diagram of a described object and check it with the touching test.