Lessons · Teaching · forces and motion
A force is a push or a pull, and balance means no change
Forces change an object's speed, direction or shape. When forces are balanced, motion does not change: a stationary object stays still and a moving one keeps going at the same speed in the same direction. Unbalanced forces cause acceleration.
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In beta. This lesson was written for Hone and has not yet been checked by a state certification officer. Practice material, not professional advice. What that means.
What it is for
'Things stop because they run out of force' is the intuition, and it is wrong: they stop because friction acts on them. Naming friction as a force rather than an absence is the correction that makes the rest of mechanics available.
How to think about it
Draw every force on the object as an arrow whose length shows its size. Then ask whether they cancel. If they cancel, the motion does not change. If they do not, the object speeds up, slows down or turns in the direction of the leftover force.
Worked example
A book resting on a table: gravity down, the table pushing up, balanced, so no movementBalanced does not mean no forces. It means they cancel.
A car at steady speed: engine forward, friction and drag back, balancedSteady motion needs balanced forces, not zero forces.
A ball slows and stops because friction acts on it, not because its force ran outForce is not a fuel an object carries.
Speed is distance divided by time, so 100 m in 20 s is 5 m/sThe one piece of arithmetic in the topic, and it is examined.
Your turn
Name the force that slows a rolling ball.
A rolling ball is slowed by
Solve one, graded on the server
The trap
Thinking a moving object must have a force pushing it along. At a steady speed the forces balance; without friction it would keep going with nothing pushing at all.