Home · IGCSE 0625 · Motion, forces, energy · Circular motion · Practice
Extended · Practice questions · Circular motion

Turning, not speeding.

Six original Cambridge-style questions on the direction of the force, why constant speed is not constant velocity, what happens when the force is cut, and what changes the size of the force needed.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Force inward, velocity along.

01
Analysis
[2 marks]

An object moves in a circle at a constant speed. State the direction of the resultant force acting on it and what this force is called.

  • The resultant force is directed toward the centre of the circle. ✓
  • It is the centripetal force (the force keeping the object in the circle). ✓
02
Analysis
[2 marks]

Explain why an object moving in a circle at constant speed is still accelerating.

  • Velocity has both size and direction. ✓
  • The direction of motion changes continuously, so the velocity changes; a changing velocity means the object is accelerating. ✓
03
Analysis
[2 marks]

A ball is whirled on a string in a horizontal circle. The string suddenly breaks. Describe and explain the motion of the ball immediately afterwards.

  • The ball travels in a straight line, along the tangent to the circle at that point. ✓
  • With no centre-directed force, there is nothing to change its direction, so it keeps the velocity it had. ✓
04
Analysis
[2 marks]

State the angle between the velocity of the object and the resultant force on it, and use this to explain why the speed stays constant.

  • The angle is 90 degrees (the force is perpendicular to the velocity). ✓
  • A perpendicular force changes only the direction of motion, not the speed. ✓
05
Analysis
[3 marks]

State three changes that would each increase the size of the force needed to keep an object moving in a circle.

  • Increasing the speed. ✓
  • Decreasing the radius of the circle. ✓
  • Increasing the mass of the object. ✓
06
Analysis
[2 marks]

A car drives around a roundabout at a steady speed. State what provides the centripetal force and the direction in which it acts.

  • Friction between the tyres and the road provides the force. ✓
  • It acts toward the centre of the circle (the centre of the roundabout). ✓