Calculate collinear net forces and connect them to changes in motion.
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Method and worked examples
The resultant or net force combines forces on one object. Add forces in the same direction; subtract opposing forces. A non-zero resultant points toward the larger total. Equal opposing totals give balanced forces and zero resultant. A stationary object can remain stationary; a moving object can keep constant speed in a straight line.
An unbalanced force changes velocity: speed, direction or both. A force opposite existing motion can slow an object, without instantly stopping it. Turning requires an unbalanced force even at constant speed. Under comparable conditions, a larger resultant changes motion more strongly; greater mass is harder to accelerate with the same resultant. Keep existing motion separate from resultant-force direction.
Example 1
A cart is pulled right with 12N while friction is 5N left. Find the resultant.
12−5=7N right, so the forces are unbalanced.
Example 2
A car travels straight at constant speed. What is its resultant force?
Zero. Driving and resistive forces balance, so velocity does not change.
Watch out:Balanced forces mean no change in velocity. Constant straight-line motion is also possible.
Balanced and unbalanced forces practice | Year 7 Science | Ascent Arcade