Connect zero resultant force with unchanged velocity
Explain rest and straight-line constant-speed motion using the resultant rather than individual forces.
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Calculate a collinear resultant.
- Distinguish rest from constant-speed motion.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the sum of 18 N east and 18 N west on one object?
What you will learn
- State what zero resultant implies for velocity.
- Avoid treating motion as proof of a non-zero resultant.
Velocity does not change when the resultant is zero
In an ordinary laboratory inertial reference frame, an object remains at rest or continues in a straight line at constant speed unless a resultant force acts. A trolley already moving east at 8 m/s with balanced 18 N driving and 18 N resisting forces keeps that velocity while those conditions hold.
A stationary object with zero resultant stays at rest under the same ideal assumptions. Zero resultant does not by itself distinguish those two starting states. You need the initial velocity, and the zero-resultant condition must persist over the interval you describe.
Balance the forces without erasing them
A book resting on a table still experiences weight downward and a support force upward. A powered vehicle at steady speed can have forward driving force balanced by resistance. Neither situation means the individual forces are absent.
A resultant changes velocity: speed, direction or both can change. Constant speed along a curved path is therefore different from constant velocity along a straight line. Also, force and motion need not point the same way; a backward resultant can slow a forward-moving object.
Sources for this lesson
Pause and explain
A body already moves straight at constant velocity with zero resultant. What happens next under unchanged conditions?
Worked example
A trolley moves east at 8 m/s while 18 N east and 18 N west are its only horizontal forces. Describe its motion while the forces stay balanced.
Show the worked solution
- Resultant = 18 − 18 = 0 N.
- There is no horizontal velocity change in this model.
- It continues straight east at 8 m/s.
Answer Straight east at 8 m/s while the resultant remains zero.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A trolley initially travels east at 8 m/s with balanced 18 N horizontal forces. Describe its subsequent ideal motion.
Give me a hint
Use the resultant and the initial velocity together.
Compare my reasoning
- The signed force sum is zero.
- Velocity is unchanged under the stated continued balance.
- It travels straight east at 8 m/s.
Straight east at 8 m/s while the forces stay balanced.
Look for these in your work
- Calculate the resultant.
- Retain the given initial velocity.
A book is at rest with 4 N weight downward and 4 N support upward. Are the forces absent, and will its velocity change?
Give me a hint
Separate individual forces from their combined effect.
Compare my reasoning
- Both weight and support act.
- Their resultant is zero.
- The stationary book remains at rest under unchanged conditions.
The forces are present and balanced; the book remains at rest.
Look for these in your work
- Name the two forces.
- Connect balance with unchanged velocity.
A student claims a car going round a bend at constant speed needs zero resultant. Explain why the straight-line condition matters.
Give me a hint
Velocity includes the changing direction around the bend.
Compare my reasoning
- The speed magnitude can stay constant.
- Direction changes, so velocity changes.
- A resultant is needed for that direction change.
The direction changes, so constant speed around a bend is not zero-resultant straight-line motion.
Look for these in your work
- Distinguish speed from velocity.
- Identify the missing straight-line condition.
Common mistakes
- Zero resultant does not mean zero velocity.
- Individual balanced forces remain present.
- Constant speed around a bend is not constant velocity.
What can you explain now?
Must a moving object have a forward resultant?
Compare with the explanation
No; straight constant-velocity motion is compatible with zero resultant.
A forward resultant is needed for a forward velocity change, not merely for an existing forward velocity.
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Make it stick
Explain one stationary and one moving zero-resultant example without saying that no forces act.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.