Explain inward force and tangent velocity in circular motion
Distinguish the continuously changing inward-force direction from the instantaneous direction of travel.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-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. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recognise velocity direction changes at constant speed.
- Identify perpendicular directions at a point on a circle.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Does an object turning at constant speed have constant velocity?
What you will learn
- Describe circular motion due to a perpendicular inward force.
- Identify tangent motion if the inward constraint is removed.
Turning requires a velocity change
In uniform circular motion the speed magnitude stays constant, but velocity direction changes continuously. The resultant force is inward, towards the circle's centre, perpendicular to the instantaneous direction of travel. At the rightmost point of an anticlockwise circle, velocity is upward and the force is leftward towards the centre.
At the top of that circle, velocity is leftward and force downward. The inward direction changes as the object progresses; it is not one permanently leftward or downward vector. The diagram shows four successive positions for one illustrative motion, not four simultaneous forces acting on one body at a single point.
Removing the turning force does not launch an outward velocity
If the inward constraint is removed and no other resultant acts, the object continues along its instantaneous tangent in the ideal model. At the rightmost anticlockwise position that means upward, not directly outward to the right. Its velocity just before release supplies the continuation direction.
Real released objects may also experience weight, drag or other forces, so a complete later trajectory requires those forces to be specified. The tangent result here describes the ideal no-other-resultant continuation. Arrow lengths distinguish directions only; force and velocity have different units and their drawing lengths are not compared as physical magnitudes.
Sources for this lesson
Pause and explain
The inward force is removed at the rightmost anticlockwise point, with no other resultant. Which way does the object continue?
Worked example
At the rightmost point of an anticlockwise circle, name the velocity and inward-force directions.
Show the worked solution
- The tangent points upward for anticlockwise motion.
- The centre is to the left of the object.
- Velocity is up and the inward force is left, perpendicular to it.
Answer Velocity upward; force left towards the centre.
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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.
Give velocity and force directions at the rightmost anticlockwise circular position.
Give me a hint
Locate the centre, then the tangent for the stated motion.
Compare my reasoning
- The centre is left of the point.
- Anticlockwise travel has an upward tangent there.
- Force is leftward and velocity upward.
Velocity up; force left.
Look for these in your work
- Use the stated rotation direction.
- Keep force perpendicular to velocity.
At the top of the same anticlockwise circle, identify velocity and force directions.
Give me a hint
The motion follows the tangent; force points to the centre below.
Compare my reasoning
- The anticlockwise tangent points left.
- The centre is below the top point.
- Force is downward and perpendicular to velocity.
Velocity left; force down.
Look for these in your work
- Locate tangent and centre.
- Distinguish the two vectors.
Release occurs at the rightmost anticlockwise position with no other resultant force. Describe ideal subsequent motion and reject a radial-outward claim.
Give me a hint
Use the velocity at release rather than reversing the old force arrow.
Compare my reasoning
- The release velocity is upward.
- No resultant changes it in the stated ideal model.
- Motion continues straight upward along the tangent, not radially right.
Straight upward tangent motion; not radial-outward motion.
Look for these in your work
- Retain the release velocity.
- State the no-other-resultant condition.
Common mistakes
- The inward force is not along the tangent.
- Constant speed still involves changing velocity.
- Release continuation is tangent under the stated no-other-resultant model.
What can you explain now?
At the top of an anticlockwise circle, which way is velocity and inward force?
Compare with the explanation
Velocity left; force down.
The tangent and the inward radius are perpendicular at that position.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Draw velocity and force at two circular positions, then draw the ideal tangent continuation at release.
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.