Whirl a ball on a string and it traces a circle. Its speed can stay perfectly steady, yet it is accelerating the whole time, because its direction keeps changing. The secret is a force that points sideways, always square to the motion.
An object moving in a circle at constant speed is acted on by a resultant force directed toward the centre. This force is perpendicular to the motion, so it changes the direction of the velocity without changing its size. Remove the force and the object flies off in a straight line.
Watch the ball travel around the circle. The blue arrow is its velocity, always pointing along the path (tangent). The red arrow is the resultant force, always pointing to the centre. Alter the parameters and watch how the required force responds.
Four quick checks tied to what you can see in the simulator. Each correct answer earns XP and lights this skill on your star map.
For the ball moving at constant speed in the sim, which way does the resultant force point?
The speed reading in the sim stays the same all the way round. What is still changing?
In the sim you raise the speed but keep the radius the same. The force needed to hold the circle...
If the inward force were switched off at one instant, the ball would...
Do not invent an outward force. Many candidates draw or describe a centrifugal force pushing the object outward, or treat the centripetal force as an extra force added on top of the others. In 0625, the centripetal force is the resultant of the real forces already acting (tension, gravity, friction, the normal contact force), directed toward the centre. On a free-body diagram, never add an outward arrow, and never draw the force along the direction of motion.
Unlocks once the four checks above are done. Worth more XP, written in the style of Paper 1.
A car travels at a steady speed around a circular roundabout. Which statement is correct?
A ball is whirled in a horizontal circle on a string. The string suddenly breaks. Which path does the ball take immediately afterward?
Two identical balls move in horizontal circles at the same speed. Ball X has a smaller radius than ball Y. Which needs the larger force toward the centre?
This skill is now lit gold on your star map. Keep the chain going.