Forces as arrows, balanced forces, what unbalanced forces change, and the difference between mass and weight. Concept tutorials with a live force diagram that can also run the motion.
Representing forces, what balanced forces do (and don't) mean, what unbalanced forces change, and gravity (AC9S7U04).
A force has a size and a direction, so we draw it as an arrow. In a force diagram every force on an object gets an arrow of the right length, pointing the right way, with its name.
Balanced forces cancel out (the net force is zero), so the motion doesn't change. The object stays still or keeps moving at a steady speed. Unbalanced forces change the motion.
An unbalanced force changes an object's motion, in the direction of the net force. Speeding up, slowing down, stopping, reversing and turning are all changes in motion, and every one needs an unbalanced force.
Mass is how much matter an object has, in kilograms, and it is the same everywhere. Weight is the force of gravity on that mass, in newtons, and depends on where you are: weight = mass × g.
For the same unbalanced force, a heavier object accelerates less than a lighter one. For the same mass, a bigger unbalanced force gives a bigger acceleration. Data from a force investigation show the pattern: acceleration goes up with force and down with mass.
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