The shape of the lesson
By the end of the lesson, learners can
- More force, or less mass: Set the resultant force and the mass, then launch the trolley and read its acceleration.
- From forces to acceleration.
- Find the resultant first: combine all forces (with direction) before using F = ma.
- Three forms: F = ma, a = F ÷ m, m = F ÷ a.
- No resultant, no acceleration: balanced forces mean constant velocity, not necessarily rest.
One picture every idea returns to
The resultant force on an object equals its mass times its acceleration: F = ma. The acceleration is in the same direction as the resultant force, larger for a bigger force and smaller for a bigger mass.
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, Force sets the acceleration.. Push two trolleys with the same force and the lighter one speeds up faster. | Whole class, then pairs |
| 5 to 17 min | Teach and check: More force, or less mass | Set the resultant force and the mass, then launch the trolley and read its acceleration. Project tab 1 of the lab (Newton's second law) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: From forces to acceleration | From forces to acceleration. Model the worked example Worked example on the board, one line at a time. | Rally Coach |
| 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 From forces to acceleration, 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.
Rally Coach
Pairs share one worksheet. One partner solves the first question aloud while the other coaches, checking each line and offering a prompt rather than the answer; they swap roles for the next question.
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 most costly error is putting the driving or pushing force straight into F = ma and ignoring friction and drag. | Always work out the resultant force first by combining all the forces. And if an object moves at constant velocity, the resultant force is zero, so the acceleration is zero, even though it is still moving. |
Newton’s second law (F = ma): worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
State Newton's second law as an equation, define each symbol, and give the unit of each quantity.
A resultant force acts on a 1500 kg car, giving it an acceleration of 2.0 m/s². Calculate the size of the resultant force.
A resultant force of 12 N acts on a 4.0 kg trolley. Calculate its acceleration.
A resultant force of 200 N gives an object an acceleration of 5.0 m/s². Calculate the mass of the object.
A 4.0 kg box is pushed along the floor with a forward force of 50 N. Friction provides a backward force of 30 N.
(a) Calculate the resultant force on the box.
(b) Calculate its acceleration.
A car of mass 1200 kg has a forward driving force of 3000 N. The total resistive force (friction and air resistance) is 1200 N.
(a) Calculate the car's acceleration.
(b) The car speeds up. State and explain what happens to its acceleration as the resistive force grows.
Answer key and marking notes
- F = ma, where F is the resultant force (N), m is the mass (kg) and a is the acceleration (m/s²). ✓
- A resultant force of 1 N gives a 1 kg mass an acceleration of 1 m/s². ✓
- F = ma = 1500 × 2.0
3000 N
- a = F ÷ m = 12 ÷ 4.0
a = 3.0 m/s²
- m = F ÷ a = 200 ÷ 5.0
m = 40 kg
(a) resultant = 50 − 30 = 20 N forward ✓
(b) a = F ÷ m = 20 ÷ 4.0a = 5.0 m/s²
use the resultant 20 N, not the 50 N push
- (b) As the car speeds up the resistive force increases, so the resultant force gets smaller. ✓
- Since a = F ÷ m, a smaller resultant means a smaller acceleration; it falls towards zero at top speed. ✓
3 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 resultant force of 24 N acts on a 3.0 kg trolley. What is its acceleration?
- A0.13 m/s²
- B8.0 m/s²
- C21 m/s²
- D72 m/s²
A car of mass 1500 kg accelerates at 2.0 m/s². What resultant force is needed?
- A0.0013 N
- B750 N
- C1500 N
- D3000 N
A constant resultant force acts on an object. If the object's mass is doubled while the force is kept the same, what happens to its acceleration?
- Ait doubles
- Bit halves
- Cit stays the same
- Dit becomes four times larger
Quiz key
a = F ÷ m = 24 ÷ 3.0 = 8.0 m/s². Option A inverts the formula; D multiplies force by mass.
F = ma = 1500 × 2.0 = 3000 N. Option B divides mass by acceleration; C forgets to multiply by the acceleration.
Since a = F ÷ m, doubling the mass halves the acceleration. Option A treats mass and acceleration as directly proportional, which is the common error.
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.