The shape of the lesson
By the end of the lesson, learners can
- Velocity along, force across: Watch the ball travel around the circle.
- What the force does.
- Resultant force: directed toward the centre, perpendicular to the motion.
- Speed constant, velocity not: the direction changes, so the object accelerates.
- Remove the force: the object continues in a straight line (tangent).
One picture every idea returns to
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.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, A pull that never lets you go straight.. Whirl a ball on a string and it traces a circle. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Velocity along, force across | Watch the ball travel around the circle. Project tab 7 of the lab (Circular motion) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: What the force does | What the force does. | Corners |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 13 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in What the force does, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Corners
Put one answer or viewpoint in each corner of the room. Students move to the corner they agree with, justify the choice to a partner there, then hear one voice from each corner before the reveal.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| 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. |
Circular motion: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
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.
Explain why an object moving in a circle at constant speed is still accelerating.
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.
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.
State three changes that would each increase the size of the force needed to keep an object moving in a circle.
A car drives around a roundabout at a steady speed. State what provides the centripetal force and the direction in which it acts.
Answer key and marking notes
- The resultant force is directed toward the centre of the circle. ✓
- It is the centripetal force (the force keeping the object in the circle). ✓
- Velocity has both size and direction. ✓
- The direction of motion changes continuously, so the velocity changes; a changing velocity means the object is accelerating. ✓
- 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. ✓
- The angle is 90 degrees (the force is perpendicular to the velocity). ✓
- A perpendicular force changes only the direction of motion, not the speed. ✓
- Increasing the speed. ✓
- Decreasing the radius of the circle. ✓
- Increasing the mass of the object. ✓
- Friction between the tyres and the road provides the force. ✓
- It acts toward the centre of the circle (the centre of the roundabout). ✓
1 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
A ball on a string is whirled at a constant speed in a horizontal circle. In which direction does the resultant force on the ball act?
- Aalong the direction of motion (tangent to the circle)
- Btoward the centre of the circle
- Caway from the centre of the circle
- Dthere is no resultant force because the speed is constant
Quiz key
The direction of the velocity keeps changing, so there is an acceleration toward the centre and the resultant (centripetal) force points to the centre. Option C is the common outward-force misconception; D ignores that the direction is changing even though the speed is not.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.