Forces and changes in motion
Use the resultant force to explain speeding up, slowing down and steady motion.
Before you begin
- Direction and signed numbers
- Speed as distance divided by time
Combine forces with direction
A force is a push or pull and has direction as well as magnitude. Add forces acting in the same direction and subtract opposing forces along one line. The resultant is their combined effect. A force diagram should identify forces acting on the chosen object, not forces that the object exerts on something else.
Separate balanced forces from being stationary
When the resultant force is zero, an object can remain at rest or keep moving at constant velocity. A nonzero resultant changes velocity, which may mean changing speed or direction. Friction and air resistance often explain why an everyday moving object slows when a driving force is removed. Motion itself does not require a nonzero resultant.
Worked example
A trolley experiences 18 N to the right and 11 N to the left. Find the resultant.
Show the worked solution
- Choose right as the positive direction.
- Combine the forces: 18 − 11 = 7 N.
- A positive result means the resultant acts to the right.
7 N to the right.
Try it yourself
A car moves in a straight line at constant velocity. What is its resultant force?
Common mistakes
- A force direction does not by itself tell you the current direction of motion.
What can you explain now?
A box is pulled with 25 N rightward while friction is 9 N leftward. Find the resultant and predict the acceleration direction.
Compare with the explanation
16 N rightward; acceleration is rightward.
Subtract the opposing forces: 25 − 9 = 16 N. The resultant determines acceleration, although the box's current velocity could initially point differently.
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