The shape of the lesson
By the end of the lesson, learners can
- Add the arrows, find the winner: Set the forces acting on the block and read off the resultant.
- Combining forces on a line: When forces act along the same straight line, add those pointing one way and subtract those pointing the other.
- Zero resultant: no change in motion (at rest, or constant velocity).
- Non-zero resultant: acceleration in the direction of the resultant.
- Constant speed always means balanced forces, never a leftover force.
One picture every idea returns to
If forces are balanced, the resultant force is zero, and the object stays at rest or continues at a steady speed in a straight line. If forces are unbalanced, the resultant force is not zero, and the object accelerates.
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, When the pushing wins.. A single object usually has several forces acting on it at once. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Add the arrows, find the winner | Set the forces acting on the block and read off the resultant. Project tab 2 of the lab (Resultant forces) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Combining forces on a line | When forces act along the same straight line, add those pointing one way and subtract those pointing the other. Model the worked example Worked example on the board, one line at a time. | Think-Pair-Share |
| 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, 14 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 Combining forces on a line, 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.
Think-Pair-Share
Pose the question, give thirty seconds of silent thinking, then a minute to compare with a partner before a few pairs share with the class. The silent minute is the part that cannot be cut.
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 |
|---|---|
| The greatest trap is believing an object needs a forward resultant force just to keep moving. | It does not. A car cruising at a steady 30 m/s has a resultant force of zero, because the engine push balances the air resistance. A forward resultant force is only needed to accelerate and change the speed, not to maintain it. |
Resultant forces: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
State three different effects that a force can have on an object.
Two forces act on an object along the same line: 30 N and 20 N.
(a)Find the resultant when they act in the same direction. (b)Find the resultant when they act in opposite directions.A box is pushed across the floor with a force of 40 N and moves at a constant velocity. State the size of the friction force, the resultant force, and explain how you know.
A car travels down a hill at a steady speed. A student says, "It is moving, so there must be a resultant force acting on it." Explain why the student is wrong.
A van has a forward driving force of 5000 N and a total resistive force of 3500 N. Calculate the resultant force and state the effect on the van's motion.
An object rests on a table with balanced forces acting on it. One of the horizontal forces is then removed. Describe what happens to the object's motion and why.
Answer key and marking notes
- Change its shape (stretch, compress or bend it). ✓
- Change its speed (speed it up or slow it down). ✓
- Change its direction of motion. ✓
- (a) 30 + 20 = 50 N (in that direction). ✓
- (b) 30 − 20 = 10 N (in the direction of the 30 N force). ✓
- Friction = 40 N. ✓
- Resultant force = 0 N. ✓
- Constant velocity means no acceleration, so the forces are balanced; friction must equal the push. ✓
- Steady speed means constant velocity, which means no acceleration. ✓
- So the forces are balanced and the resultant force is zero; a resultant is needed only to change the motion, not to maintain it. ✓
The van accelerates (speeds up) in the forward direction. ✓
- There is now a non-zero resultant force (the remaining force is unopposed). ✓
- The object accelerates in the direction of that remaining force. ✓
2 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 box slides across a floor at constant velocity in a straight line. What does this tell you about the forces on the box?
- AThere are no forces acting on the box.
- BThe forces are balanced, giving zero resultant force.
- CThere is a resultant force in the direction of motion.
- DThe driving force is larger than friction.
Two horizontal forces act on a crate: 30 N to the right and 18 N to the left. What is the resultant force?
- A12 N to the left
- B12 N to the right
- C48 N to the right
- D540 N to the right
Quiz key
Constant velocity means no acceleration, so the resultant force is zero and the forces balance. Option A is wrong because a push and friction both act, they simply cancel. C and D would both produce acceleration.
30 − 18 = 12 N to the right (toward the larger force). Option A has the direction backwards; C adds the forces; D multiplies them.
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.