A.2.25—Direction change in circular motion
- Syllabus
- First assessment 2025
- Objective
- —
- Level
- HL
Explain How Centripetal Force Changes Direction
Velocity direction changes
In circular motion, the inward centripetal acceleration changes the direction of the velocity. If speed is constant, the magnitude of velocity stays constant while its direction changes.
What happens if the inward force disappears
If the centripetal interaction is removed, the object continues along the tangent at the release point, consistent with Newton’s first law.
Maintain contact
In a vertical loop, the inward resultant must be sufficient to maintain the required radial acceleration. At the limiting contact condition, the normal force can fall to zero.
Common trap
The released object does not move along the radius; its instantaneous path is tangent to the circle.
A.2.25 Exam Analysis
Assessment in practice
1–2 marksHow it is assessed
The evidence asks for the path after a string breaks and asks why a car remains in contact with a loop.
Command terms
State / Explain
What earns marks
If the inward force disappears, state that the object leaves along the tangent because its instantaneous velocity is tangent to the circle. For loop-contact questions, set the normal force condition and compare the actual speed with the minimum required speed.
Watch for
Choosing a radial path after release or claiming that the object stops when the centripetal force is removed.
Representative question
Question 1
[Maximum number: 1]
A mass at the end of a string is swung in a horizontal circle at increasing speed until the string breaks.
The subsequent path taken by the mass is a
A
line along a radius of the circle.
B
horizontal circle.
C
curve in a horizontal plane.
D
curve in a vertical plane.
D