Explain how a resultant changes speed or direction
Compare a resultant with the current velocity to describe the immediate change in motion.
Section P1.5.1 · Effects of forces
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall that velocity includes direction.
- Recognise a resultant force on one object.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Does a change of direction change velocity even at the same speed?
What you will learn
- Describe speeding up, slowing down and changing direction.
- Distinguish force direction from existing motion direction.
Compare the two directions
For an object moving east, an eastward resultant increases its eastward speed; a westward resultant initially reduces that eastward speed. A northward resultant changes the velocity direction. Describe the immediate effect using the current velocity instead of assuming every force makes an object move in its own direction instantly.
If a backward resultant persists long enough, an object can stop and later move backwards. The initial slowing description therefore applies while it is still travelling forward. Numerical acceleration and interval calculations belong to the separately labelled Extended force–acceleration lessons.
Direction change counts as velocity change
A force perpendicular to the instantaneous motion can change its direction even when the speed magnitude is maintained. For circular motion, the inward force direction must continually turn as the object moves; one fixed northward force is not automatically a force towards one fixed circle centre throughout a journey.
The diagram compares instantaneous directions rather than plotting a measured trajectory. It does not assign a numerical acceleration or prove the full later path from a single snapshot. State which information is supplied and whether your conclusion concerns the immediate change or an extended interval.
Sources for this lesson
Pause and explain
A forward-moving body has a backward resultant. Must it already be moving backwards?
Worked example
A trolley moves east. Describe the immediate effect of a westward resultant while it is still moving east.
Show the worked solution
- Current velocity is eastward.
- The resultant opposes that velocity.
- Its eastward speed decreases initially.
Answer It initially slows while still moving east.
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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 moves east and has a westward resultant. Describe its immediate motion change.
Give me a hint
Compare the resultant with the current velocity.
Compare my reasoning
- The existing motion is eastward.
- The resultant opposes that direction.
- The trolley initially slows while travelling east.
It initially slows while moving east.
Look for these in your work
- Use the current velocity.
- Separate motion and force directions.
A moving body has velocity east. Compare an eastward resultant with a northward resultant, qualitatively.
Give me a hint
One force is along the velocity and the other is sideways to it.
Compare my reasoning
- An eastward resultant increases the eastward speed.
- A northward resultant changes velocity direction.
- Do not assign a numerical rate without mass and force information.
Eastward: speeds up east; northward: changes velocity direction.
Look for these in your work
- Describe both changes.
- Avoid an unsupported numerical acceleration.
A westward resultant continues after an eastward-moving body stops. What qualitative change can follow if the conditions remain?
Give me a hint
The resultant still changes the velocity after the instant of rest.
Compare my reasoning
- The object slows eastward before stopping.
- At rest, the westward resultant remains.
- It can then gain westward velocity.
It can start moving west after stopping, if the westward resultant persists.
Look for these in your work
- Distinguish the stages.
- Retain the continued-force assumption.
Common mistakes
- Force direction is not automatically velocity direction.
- A snapshot does not establish the entire later path.
- State initially when describing braking before a possible reversal.
What can you explain now?
A northward resultant acts on an eastward-moving object. What can change?
Compare with the explanation
Its velocity direction changes.
The force is not along the existing velocity, so a direction change is involved.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Draw current velocity and resultant arrows for speeding up, braking and turning.
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.